diff --git a/libstdc++-v3/include/Makefile.am b/libstdc++-v3/include/Makefile.am
index 99eec558116..15c0ad8dd03 100644
--- a/libstdc++-v3/include/Makefile.am
+++ b/libstdc++-v3/include/Makefile.am
@@ -119,6 +119,7 @@ bits_headers = \
 	${bits_srcdir}/charconv.h \
 	${bits_srcdir}/codecvt.h \
 	${bits_srcdir}/concept_check.h \
+	${bits_srcdir}/cow_string.h \
 	${bits_srcdir}/cpp_type_traits.h \
 	${bits_srcdir}/deque.tcc \
 	${bits_srcdir}/enable_special_members.h \
diff --git a/libstdc++-v3/include/bits/basic_string.h b/libstdc++-v3/include/bits/basic_string.h
index 9911d4deb72..b61fe05efcf 100644
--- a/libstdc++-v3/include/bits/basic_string.h
+++ b/libstdc++-v3/include/bits/basic_string.h
@@ -48,9 +48,13 @@
 # include <string_view>
 #endif
 
+#if ! _GLIBCXX_USE_CXX11_ABI
+# include "cow_string.h"
+#else
 namespace std _GLIBCXX_VISIBILITY(default)
 {
 _GLIBCXX_BEGIN_NAMESPACE_VERSION
+_GLIBCXX_BEGIN_NAMESPACE_CXX11
 
 #ifdef __cpp_lib_is_constant_evaluated
 // Support P1032R1 in C++20 (but not P0980R1 yet).
@@ -61,8 +65,6 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 #elif __cplusplus > 201703L
 #endif
 
-#if _GLIBCXX_USE_CXX11_ABI
-_GLIBCXX_BEGIN_NAMESPACE_CXX11
   /**
    *  @class basic_string basic_string.h <string>
    *  @brief  Managing sequences of characters and character-like objects.
@@ -3104,2954 +3106,13 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
       template<typename, typename, typename> friend class basic_stringbuf;
     };
 _GLIBCXX_END_NAMESPACE_CXX11
-#else  // !_GLIBCXX_USE_CXX11_ABI
-  // Reference-counted COW string implentation
+_GLIBCXX_END_NAMESPACE_VERSION
+} // namespace std
+#endif  // _GLIBCXX_USE_CXX11_ABI
 
-  /**
-   *  @class basic_string basic_string.h <string>
-   *  @brief  Managing sequences of characters and character-like objects.
-   *
-   *  @ingroup strings
-   *  @ingroup sequences
-   *
-   *  @tparam _CharT  Type of character
-   *  @tparam _Traits  Traits for character type, defaults to
-   *                   char_traits<_CharT>.
-   *  @tparam _Alloc  Allocator type, defaults to allocator<_CharT>.
-   *
-   *  Meets the requirements of a <a href="tables.html#65">container</a>, a
-   *  <a href="tables.html#66">reversible container</a>, and a
-   *  <a href="tables.html#67">sequence</a>.  Of the
-   *  <a href="tables.html#68">optional sequence requirements</a>, only
-   *  @c push_back, @c at, and @c %array access are supported.
-   *
-   *  @doctodo
-   *
-   *
-   *  Documentation?  What's that?
-   *  Nathan Myers <ncm@cantrip.org>.
-   *
-   *  A string looks like this:
-   *
-   *  @code
-   *                                        [_Rep]
-   *                                        _M_length
-   *   [basic_string<char_type>]            _M_capacity
-   *   _M_dataplus                          _M_refcount
-   *   _M_p ---------------->               unnamed array of char_type
-   *  @endcode
-   *
-   *  Where the _M_p points to the first character in the string, and
-   *  you cast it to a pointer-to-_Rep and subtract 1 to get a
-   *  pointer to the header.
-   *
-   *  This approach has the enormous advantage that a string object
-   *  requires only one allocation.  All the ugliness is confined
-   *  within a single %pair of inline functions, which each compile to
-   *  a single @a add instruction: _Rep::_M_data(), and
-   *  string::_M_rep(); and the allocation function which gets a
-   *  block of raw bytes and with room enough and constructs a _Rep
-   *  object at the front.
-   *
-   *  The reason you want _M_data pointing to the character %array and
-   *  not the _Rep is so that the debugger can see the string
-   *  contents. (Probably we should add a non-inline member to get
-   *  the _Rep for the debugger to use, so users can check the actual
-   *  string length.)
-   *
-   *  Note that the _Rep object is a POD so that you can have a
-   *  static <em>empty string</em> _Rep object already @a constructed before
-   *  static constructors have run.  The reference-count encoding is
-   *  chosen so that a 0 indicates one reference, so you never try to
-   *  destroy the empty-string _Rep object.
-   *
-   *  All but the last paragraph is considered pretty conventional
-   *  for a C++ string implementation.
-  */
-  // 21.3  Template class basic_string
-  template<typename _CharT, typename _Traits, typename _Alloc>
-    class basic_string
-    {
-      typedef typename __gnu_cxx::__alloc_traits<_Alloc>::template
-	rebind<_CharT>::other _CharT_alloc_type;
-      typedef __gnu_cxx::__alloc_traits<_CharT_alloc_type> _CharT_alloc_traits;
-
-      // Types:
-    public:
-      typedef _Traits					    traits_type;
-      typedef typename _Traits::char_type		    value_type;
-      typedef _Alloc					    allocator_type;
-      typedef typename _CharT_alloc_traits::size_type	    size_type;
-      typedef typename _CharT_alloc_traits::difference_type difference_type;
-#if __cplusplus < 201103L
-      typedef typename _CharT_alloc_type::reference	    reference;
-      typedef typename _CharT_alloc_type::const_reference   const_reference;
-#else
-      typedef value_type&				    reference;
-      typedef const value_type&				    const_reference;
-#endif
-      typedef typename _CharT_alloc_traits::pointer	    pointer;
-      typedef typename _CharT_alloc_traits::const_pointer   const_pointer;
-      typedef __gnu_cxx::__normal_iterator<pointer, basic_string>  iterator;
-      typedef __gnu_cxx::__normal_iterator<const_pointer, basic_string>
-                                                            const_iterator;
-      typedef std::reverse_iterator<const_iterator>	const_reverse_iterator;
-      typedef std::reverse_iterator<iterator>		    reverse_iterator;
-
-    protected:
-      // type used for positions in insert, erase etc.
-      typedef iterator __const_iterator;
-
-    private:
-      // _Rep: string representation
-      //   Invariants:
-      //   1. String really contains _M_length + 1 characters: due to 21.3.4
-      //      must be kept null-terminated.
-      //   2. _M_capacity >= _M_length
-      //      Allocated memory is always (_M_capacity + 1) * sizeof(_CharT).
-      //   3. _M_refcount has three states:
-      //      -1: leaked, one reference, no ref-copies allowed, non-const.
-      //       0: one reference, non-const.
-      //     n>0: n + 1 references, operations require a lock, const.
-      //   4. All fields==0 is an empty string, given the extra storage
-      //      beyond-the-end for a null terminator; thus, the shared
-      //      empty string representation needs no constructor.
-
-      struct _Rep_base
-      {
-	size_type		_M_length;
-	size_type		_M_capacity;
-	_Atomic_word		_M_refcount;
-      };
-
-      struct _Rep : _Rep_base
-      {
-	// Types:
-	typedef typename __gnu_cxx::__alloc_traits<_Alloc>::template
-	  rebind<char>::other _Raw_bytes_alloc;
-
-	// (Public) Data members:
-
-	// The maximum number of individual char_type elements of an
-	// individual string is determined by _S_max_size. This is the
-	// value that will be returned by max_size().  (Whereas npos
-	// is the maximum number of bytes the allocator can allocate.)
-	// If one was to divvy up the theoretical largest size string,
-	// with a terminating character and m _CharT elements, it'd
-	// look like this:
-	// npos = sizeof(_Rep) + (m * sizeof(_CharT)) + sizeof(_CharT)
-	// Solving for m:
-	// m = ((npos - sizeof(_Rep))/sizeof(CharT)) - 1
-	// In addition, this implementation quarters this amount.
-	static const size_type	_S_max_size;
-	static const _CharT	_S_terminal;
-
-	// The following storage is init'd to 0 by the linker, resulting
-        // (carefully) in an empty string with one reference.
-        static size_type _S_empty_rep_storage[];
-
-        static _Rep&
-        _S_empty_rep() _GLIBCXX_NOEXCEPT
-        { 
-	  // NB: Mild hack to avoid strict-aliasing warnings.  Note that
-	  // _S_empty_rep_storage is never modified and the punning should
-	  // be reasonably safe in this case.
-	  void* __p = reinterpret_cast<void*>(&_S_empty_rep_storage);
-	  return *reinterpret_cast<_Rep*>(__p);
-	}
-
-        bool
-	_M_is_leaked() const _GLIBCXX_NOEXCEPT
-        {
-#if defined(__GTHREADS)
-          // _M_refcount is mutated concurrently by _M_refcopy/_M_dispose,
-          // so we need to use an atomic load. However, _M_is_leaked
-          // predicate does not change concurrently (i.e. the string is either
-          // leaked or not), so a relaxed load is enough.
-          return __atomic_load_n(&this->_M_refcount, __ATOMIC_RELAXED) < 0;
-#else
-          return this->_M_refcount < 0;
-#endif
-        }
-
-        bool
-	_M_is_shared() const _GLIBCXX_NOEXCEPT
-	{
-#if defined(__GTHREADS)
-          // _M_refcount is mutated concurrently by _M_refcopy/_M_dispose,
-          // so we need to use an atomic load. Another thread can drop last
-          // but one reference concurrently with this check, so we need this
-          // load to be acquire to synchronize with release fetch_and_add in
-          // _M_dispose.
-          return __atomic_load_n(&this->_M_refcount, __ATOMIC_ACQUIRE) > 0;
-#else
-          return this->_M_refcount > 0;
-#endif
-        }
-
-        void
-	_M_set_leaked() _GLIBCXX_NOEXCEPT
-        { this->_M_refcount = -1; }
-
-        void
-	_M_set_sharable() _GLIBCXX_NOEXCEPT
-        { this->_M_refcount = 0; }
-
-	void
-	_M_set_length_and_sharable(size_type __n) _GLIBCXX_NOEXCEPT
-	{
-#if _GLIBCXX_FULLY_DYNAMIC_STRING == 0
-	  if (__builtin_expect(this != &_S_empty_rep(), false))
-#endif
-	    {
-	      this->_M_set_sharable();  // One reference.
-	      this->_M_length = __n;
-	      traits_type::assign(this->_M_refdata()[__n], _S_terminal);
-	      // grrr. (per 21.3.4)
-	      // You cannot leave those LWG people alone for a second.
-	    }
-	}
-
-	_CharT*
-	_M_refdata() throw()
-	{ return reinterpret_cast<_CharT*>(this + 1); }
-
-	_CharT*
-	_M_grab(const _Alloc& __alloc1, const _Alloc& __alloc2)
-	{
-	  return (!_M_is_leaked() && __alloc1 == __alloc2)
-	          ? _M_refcopy() : _M_clone(__alloc1);
-	}
-
-	// Create & Destroy
-	static _Rep*
-	_S_create(size_type, size_type, const _Alloc&);
-
-	void
-	_M_dispose(const _Alloc& __a) _GLIBCXX_NOEXCEPT
-	{
-#if _GLIBCXX_FULLY_DYNAMIC_STRING == 0
-	  if (__builtin_expect(this != &_S_empty_rep(), false))
-#endif
-	    {
-	      // Be race-detector-friendly.  For more info see bits/c++config.
-	      _GLIBCXX_SYNCHRONIZATION_HAPPENS_BEFORE(&this->_M_refcount);
-              // Decrement of _M_refcount is acq_rel, because:
-              // - all but last decrements need to release to synchronize with
-              //   the last decrement that will delete the object.
-              // - the last decrement needs to acquire to synchronize with
-              //   all the previous decrements.
-              // - last but one decrement needs to release to synchronize with
-              //   the acquire load in _M_is_shared that will conclude that
-              //   the object is not shared anymore.
-	      if (__gnu_cxx::__exchange_and_add_dispatch(&this->_M_refcount,
-							 -1) <= 0)
-		{
-		  _GLIBCXX_SYNCHRONIZATION_HAPPENS_AFTER(&this->_M_refcount);
-		  _M_destroy(__a);
-		}
-	    }
-	}  // XXX MT
-
-	void
-	_M_destroy(const _Alloc&) throw();
-
-	_CharT*
-	_M_refcopy() throw()
-	{
-#if _GLIBCXX_FULLY_DYNAMIC_STRING == 0
-	  if (__builtin_expect(this != &_S_empty_rep(), false))
-#endif
-            __gnu_cxx::__atomic_add_dispatch(&this->_M_refcount, 1);
-	  return _M_refdata();
-	}  // XXX MT
-
-	_CharT*
-	_M_clone(const _Alloc&, size_type __res = 0);
-      };
-
-      // Use empty-base optimization: http://www.cantrip.org/emptyopt.html
-      struct _Alloc_hider : _Alloc
-      {
-	_Alloc_hider(_CharT* __dat, const _Alloc& __a) _GLIBCXX_NOEXCEPT
-	: _Alloc(__a), _M_p(__dat) { }
-
-	_CharT* _M_p; // The actual data.
-      };
-
-    public:
-      // Data Members (public):
-      // NB: This is an unsigned type, and thus represents the maximum
-      // size that the allocator can hold.
-      ///  Value returned by various member functions when they fail.
-      static const size_type	npos = static_cast<size_type>(-1);
-
-    private:
-      // Data Members (private):
-      mutable _Alloc_hider	_M_dataplus;
-
-      _CharT*
-      _M_data() const _GLIBCXX_NOEXCEPT
-      { return  _M_dataplus._M_p; }
-
-      _CharT*
-      _M_data(_CharT* __p) _GLIBCXX_NOEXCEPT
-      { return (_M_dataplus._M_p = __p); }
-
-      _Rep*
-      _M_rep() const _GLIBCXX_NOEXCEPT
-      { return &((reinterpret_cast<_Rep*> (_M_data()))[-1]); }
-
-      // For the internal use we have functions similar to `begin'/`end'
-      // but they do not call _M_leak.
-      iterator
-      _M_ibegin() const _GLIBCXX_NOEXCEPT
-      { return iterator(_M_data()); }
-
-      iterator
-      _M_iend() const _GLIBCXX_NOEXCEPT
-      { return iterator(_M_data() + this->size()); }
-
-      void
-      _M_leak()    // for use in begin() & non-const op[]
-      {
-	if (!_M_rep()->_M_is_leaked())
-	  _M_leak_hard();
-      }
-
-      size_type
-      _M_check(size_type __pos, const char* __s) const
-      {
-	if (__pos > this->size())
-	  __throw_out_of_range_fmt(__N("%s: __pos (which is %zu) > "
-				       "this->size() (which is %zu)"),
-				   __s, __pos, this->size());
-	return __pos;
-      }
-
-      void
-      _M_check_length(size_type __n1, size_type __n2, const char* __s) const
-      {
-	if (this->max_size() - (this->size() - __n1) < __n2)
-	  __throw_length_error(__N(__s));
-      }
-
-      // NB: _M_limit doesn't check for a bad __pos value.
-      size_type
-      _M_limit(size_type __pos, size_type __off) const _GLIBCXX_NOEXCEPT
-      {
-	const bool __testoff =  __off < this->size() - __pos;
-	return __testoff ? __off : this->size() - __pos;
-      }
-
-      // True if _Rep and source do not overlap.
-      bool
-      _M_disjunct(const _CharT* __s) const _GLIBCXX_NOEXCEPT
-      {
-	return (less<const _CharT*>()(__s, _M_data())
-		|| less<const _CharT*>()(_M_data() + this->size(), __s));
-      }
-
-      // When __n = 1 way faster than the general multichar
-      // traits_type::copy/move/assign.
-      static void
-      _M_copy(_CharT* __d, const _CharT* __s, size_type __n) _GLIBCXX_NOEXCEPT
-      {
-	if (__n == 1)
-	  traits_type::assign(*__d, *__s);
-	else
-	  traits_type::copy(__d, __s, __n);
-      }
-
-      static void
-      _M_move(_CharT* __d, const _CharT* __s, size_type __n) _GLIBCXX_NOEXCEPT
-      {
-	if (__n == 1)
-	  traits_type::assign(*__d, *__s);
-	else
-	  traits_type::move(__d, __s, __n);	  
-      }
-
-      static void
-      _M_assign(_CharT* __d, size_type __n, _CharT __c) _GLIBCXX_NOEXCEPT
-      {
-	if (__n == 1)
-	  traits_type::assign(*__d, __c);
-	else
-	  traits_type::assign(__d, __n, __c);	  
-      }
-
-      // _S_copy_chars is a separate template to permit specialization
-      // to optimize for the common case of pointers as iterators.
-      template<class _Iterator>
-        static void
-        _S_copy_chars(_CharT* __p, _Iterator __k1, _Iterator __k2)
-        {
-	  for (; __k1 != __k2; ++__k1, (void)++__p)
-	    traits_type::assign(*__p, *__k1); // These types are off.
-	}
-
-      static void
-      _S_copy_chars(_CharT* __p, iterator __k1, iterator __k2) _GLIBCXX_NOEXCEPT
-      { _S_copy_chars(__p, __k1.base(), __k2.base()); }
-
-      static void
-      _S_copy_chars(_CharT* __p, const_iterator __k1, const_iterator __k2)
-      _GLIBCXX_NOEXCEPT
-      { _S_copy_chars(__p, __k1.base(), __k2.base()); }
-
-      static void
-      _S_copy_chars(_CharT* __p, _CharT* __k1, _CharT* __k2) _GLIBCXX_NOEXCEPT
-      { _M_copy(__p, __k1, __k2 - __k1); }
-
-      static void
-      _S_copy_chars(_CharT* __p, const _CharT* __k1, const _CharT* __k2)
-      _GLIBCXX_NOEXCEPT
-      { _M_copy(__p, __k1, __k2 - __k1); }
-
-      static int
-      _S_compare(size_type __n1, size_type __n2) _GLIBCXX_NOEXCEPT
-      {
-	const difference_type __d = difference_type(__n1 - __n2);
-
-	if (__d > __gnu_cxx::__numeric_traits<int>::__max)
-	  return __gnu_cxx::__numeric_traits<int>::__max;
-	else if (__d < __gnu_cxx::__numeric_traits<int>::__min)
-	  return __gnu_cxx::__numeric_traits<int>::__min;
-	else
-	  return int(__d);
-      }
-
-      void
-      _M_mutate(size_type __pos, size_type __len1, size_type __len2);
-
-      void
-      _M_leak_hard();
-
-      static _Rep&
-      _S_empty_rep() _GLIBCXX_NOEXCEPT
-      { return _Rep::_S_empty_rep(); }
-
-#if __cplusplus >= 201703L
-      // A helper type for avoiding boiler-plate.
-      typedef basic_string_view<_CharT, _Traits> __sv_type;
-
-      template<typename _Tp, typename _Res>
-	using _If_sv = enable_if_t<
-	  __and_<is_convertible<const _Tp&, __sv_type>,
-		 __not_<is_convertible<const _Tp*, const basic_string*>>,
-		 __not_<is_convertible<const _Tp&, const _CharT*>>>::value,
-	  _Res>;
-
-      // Allows an implicit conversion to __sv_type.
-      static __sv_type
-      _S_to_string_view(__sv_type __svt) noexcept
-      { return __svt; }
-
-      // Wraps a string_view by explicit conversion and thus
-      // allows to add an internal constructor that does not
-      // participate in overload resolution when a string_view
-      // is provided.
-      struct __sv_wrapper
-      {
-	explicit __sv_wrapper(__sv_type __sv) noexcept : _M_sv(__sv) { }
-	__sv_type _M_sv;
-      };
-
-      /**
-       *  @brief  Only internally used: Construct string from a string view
-       *          wrapper.
-       *  @param  __svw  string view wrapper.
-       *  @param  __a  Allocator to use.
-       */
-      explicit
-      basic_string(__sv_wrapper __svw, const _Alloc& __a)
-      : basic_string(__svw._M_sv.data(), __svw._M_sv.size(), __a) { }
-#endif
-
-    public:
-      // Construct/copy/destroy:
-      // NB: We overload ctors in some cases instead of using default
-      // arguments, per 17.4.4.4 para. 2 item 2.
-
-      /**
-       *  @brief  Default constructor creates an empty string.
-       */
-      basic_string()
-#if _GLIBCXX_FULLY_DYNAMIC_STRING == 0
-      _GLIBCXX_NOEXCEPT
-      : _M_dataplus(_S_empty_rep()._M_refdata(), _Alloc())
-#else
-      : _M_dataplus(_S_construct(size_type(), _CharT(), _Alloc()), _Alloc())
-#endif
-      { }
-
-      /**
-       *  @brief  Construct an empty string using allocator @a a.
-       */
-      explicit
-      basic_string(const _Alloc& __a)
-      : _M_dataplus(_S_construct(size_type(), _CharT(), __a), __a)
-      { }
-
-      // NB: per LWG issue 42, semantics different from IS:
-      /**
-       *  @brief  Construct string with copy of value of @a str.
-       *  @param  __str  Source string.
-       */
-      basic_string(const basic_string& __str)
-      : _M_dataplus(__str._M_rep()->_M_grab(_Alloc(__str.get_allocator()),
-					    __str.get_allocator()),
-		    __str.get_allocator())
-      { }
-
-      // _GLIBCXX_RESOLVE_LIB_DEFECTS
-      // 2583. no way to supply an allocator for basic_string(str, pos)
-      /**
-       *  @brief  Construct string as copy of a substring.
-       *  @param  __str  Source string.
-       *  @param  __pos  Index of first character to copy from.
-       *  @param  __a  Allocator to use.
-       */
-      basic_string(const basic_string& __str, size_type __pos,
-		   const _Alloc& __a = _Alloc());
-
-      /**
-       *  @brief  Construct string as copy of a substring.
-       *  @param  __str  Source string.
-       *  @param  __pos  Index of first character to copy from.
-       *  @param  __n  Number of characters to copy.
-       */
-      basic_string(const basic_string& __str, size_type __pos,
-		   size_type __n);
-      /**
-       *  @brief  Construct string as copy of a substring.
-       *  @param  __str  Source string.
-       *  @param  __pos  Index of first character to copy from.
-       *  @param  __n  Number of characters to copy.
-       *  @param  __a  Allocator to use.
-       */
-      basic_string(const basic_string& __str, size_type __pos,
-		   size_type __n, const _Alloc& __a);
-
-      /**
-       *  @brief  Construct string initialized by a character %array.
-       *  @param  __s  Source character %array.
-       *  @param  __n  Number of characters to copy.
-       *  @param  __a  Allocator to use (default is default allocator).
-       *
-       *  NB: @a __s must have at least @a __n characters, &apos;\\0&apos;
-       *  has no special meaning.
-       */
-      basic_string(const _CharT* __s, size_type __n,
-		   const _Alloc& __a = _Alloc())
-      : _M_dataplus(_S_construct(__s, __s + __n, __a), __a)
-      { }
-
-      /**
-       *  @brief  Construct string as copy of a C string.
-       *  @param  __s  Source C string.
-       *  @param  __a  Allocator to use (default is default allocator).
-       */
-#if __cpp_deduction_guides && ! defined _GLIBCXX_DEFINING_STRING_INSTANTIATIONS
-      // _GLIBCXX_RESOLVE_LIB_DEFECTS
-      // 3076. basic_string CTAD ambiguity
-      template<typename = _RequireAllocator<_Alloc>>
-#endif
-      basic_string(const _CharT* __s, const _Alloc& __a = _Alloc())
-      : _M_dataplus(_S_construct(__s, __s ? __s + traits_type::length(__s) :
-				 __s + npos, __a), __a)
-      { }
-
-      /**
-       *  @brief  Construct string as multiple characters.
-       *  @param  __n  Number of characters.
-       *  @param  __c  Character to use.
-       *  @param  __a  Allocator to use (default is default allocator).
-       */
-      basic_string(size_type __n, _CharT __c, const _Alloc& __a = _Alloc())
-      : _M_dataplus(_S_construct(__n, __c, __a), __a)
-      { }
-
-#if __cplusplus >= 201103L
-      /**
-       *  @brief  Move construct string.
-       *  @param  __str  Source string.
-       *
-       *  The newly-created string contains the exact contents of @a __str.
-       *  @a __str is a valid, but unspecified string.
-       */
-      basic_string(basic_string&& __str)
-#if _GLIBCXX_FULLY_DYNAMIC_STRING == 0
-      noexcept // FIXME C++11: should always be noexcept.
-#endif
-      : _M_dataplus(std::move(__str._M_dataplus))
-      {
-#if _GLIBCXX_FULLY_DYNAMIC_STRING == 0
-	__str._M_data(_S_empty_rep()._M_refdata());
-#else
-	__str._M_data(_S_construct(size_type(), _CharT(), get_allocator()));
-#endif
-      }
-
-      /**
-       *  @brief  Construct string from an initializer %list.
-       *  @param  __l  std::initializer_list of characters.
-       *  @param  __a  Allocator to use (default is default allocator).
-       */
-      basic_string(initializer_list<_CharT> __l, const _Alloc& __a = _Alloc())
-      : _M_dataplus(_S_construct(__l.begin(), __l.end(), __a), __a)
-      { }
-
-      basic_string(const basic_string& __str, const _Alloc& __a)
-      : _M_dataplus(__str._M_rep()->_M_grab(__a, __str.get_allocator()), __a)
-      { }
-
-      basic_string(basic_string&& __str, const _Alloc& __a)
-      : _M_dataplus(__str._M_data(), __a)
-      {
-	if (__a == __str.get_allocator())
-	  {
-#if _GLIBCXX_FULLY_DYNAMIC_STRING == 0
-	    __str._M_data(_S_empty_rep()._M_refdata());
-#else
-	    __str._M_data(_S_construct(size_type(), _CharT(), __a));
-#endif
-	  }
-	else
-	  _M_dataplus._M_p = _S_construct(__str.begin(), __str.end(), __a);
-      }
-#endif // C++11
-
-      /**
-       *  @brief  Construct string as copy of a range.
-       *  @param  __beg  Start of range.
-       *  @param  __end  End of range.
-       *  @param  __a  Allocator to use (default is default allocator).
-       */
-      template<class _InputIterator>
-        basic_string(_InputIterator __beg, _InputIterator __end,
-		     const _Alloc& __a = _Alloc())
-	: _M_dataplus(_S_construct(__beg, __end, __a), __a)
-	{ }
-
-#if __cplusplus >= 201703L
-      /**
-       *  @brief  Construct string from a substring of a string_view.
-       *  @param  __t   Source object convertible to string view.
-       *  @param  __pos The index of the first character to copy from __t.
-       *  @param  __n   The number of characters to copy from __t.
-       *  @param  __a   Allocator to use.
-       */
-      template<typename _Tp, typename = _If_sv<_Tp, void>>
-	basic_string(const _Tp& __t, size_type __pos, size_type __n,
-		     const _Alloc& __a = _Alloc())
-	: basic_string(_S_to_string_view(__t).substr(__pos, __n), __a) { }
-
-      /**
-       *  @brief  Construct string from a string_view.
-       *  @param  __t  Source object convertible to string view.
-       *  @param  __a  Allocator to use (default is default allocator).
-       */
-      template<typename _Tp, typename = _If_sv<_Tp, void>>
-	explicit
-	basic_string(const _Tp& __t, const _Alloc& __a = _Alloc())
-	: basic_string(__sv_wrapper(_S_to_string_view(__t)), __a) { }
-#endif // C++17
-
-      /**
-       *  @brief  Destroy the string instance.
-       */
-      ~basic_string() _GLIBCXX_NOEXCEPT
-      { _M_rep()->_M_dispose(this->get_allocator()); }
-
-      /**
-       *  @brief  Assign the value of @a str to this string.
-       *  @param  __str  Source string.
-       */
-      basic_string&
-      operator=(const basic_string& __str)
-      { return this->assign(__str); }
-
-      /**
-       *  @brief  Copy contents of @a s into this string.
-       *  @param  __s  Source null-terminated string.
-       */
-      basic_string&
-      operator=(const _CharT* __s) 
-      { return this->assign(__s); }
-
-      /**
-       *  @brief  Set value to string of length 1.
-       *  @param  __c  Source character.
-       *
-       *  Assigning to a character makes this string length 1 and
-       *  (*this)[0] == @a c.
-       */
-      basic_string&
-      operator=(_CharT __c) 
-      { 
-	this->assign(1, __c); 
-	return *this;
-      }
-
-#if __cplusplus >= 201103L
-      /**
-       *  @brief  Move assign the value of @a str to this string.
-       *  @param  __str  Source string.
-       *
-       *  The contents of @a str are moved into this string (without copying).
-       *  @a str is a valid, but unspecified string.
-       */
-      basic_string&
-      operator=(basic_string&& __str)
-      _GLIBCXX_NOEXCEPT_IF(allocator_traits<_Alloc>::is_always_equal::value)
-      {
-	// NB: DR 1204.
-	this->swap(__str);
-	return *this;
-      }
-
-      /**
-       *  @brief  Set value to string constructed from initializer %list.
-       *  @param  __l  std::initializer_list.
-       */
-      basic_string&
-      operator=(initializer_list<_CharT> __l)
-      {
-	this->assign(__l.begin(), __l.size());
-	return *this;
-      }
-#endif // C++11
-
-#if __cplusplus >= 201703L
-      /**
-       *  @brief  Set value to string constructed from a string_view.
-       *  @param  __svt An object convertible to  string_view.
-       */
-      template<typename _Tp>
-	_If_sv<_Tp, basic_string&>
-	operator=(const _Tp& __svt)
-	{ return this->assign(__svt); }
-
-      /**
-       *  @brief  Convert to a string_view.
-       *  @return A string_view.
-       */
-      operator __sv_type() const noexcept
-      { return __sv_type(data(), size()); }
-#endif // C++17
-
-      // Iterators:
-      /**
-       *  Returns a read/write iterator that points to the first character in
-       *  the %string.  Unshares the string.
-       */
-      iterator
-      begin() // FIXME C++11: should be noexcept.
-      {
-	_M_leak();
-	return iterator(_M_data());
-      }
-
-      /**
-       *  Returns a read-only (constant) iterator that points to the first
-       *  character in the %string.
-       */
-      const_iterator
-      begin() const _GLIBCXX_NOEXCEPT
-      { return const_iterator(_M_data()); }
-
-      /**
-       *  Returns a read/write iterator that points one past the last
-       *  character in the %string.  Unshares the string.
-       */
-      iterator
-      end() // FIXME C++11: should be noexcept.
-      {
-	_M_leak();
-	return iterator(_M_data() + this->size());
-      }
-
-      /**
-       *  Returns a read-only (constant) iterator that points one past the
-       *  last character in the %string.
-       */
-      const_iterator
-      end() const _GLIBCXX_NOEXCEPT
-      { return const_iterator(_M_data() + this->size()); }
-
-      /**
-       *  Returns a read/write reverse iterator that points to the last
-       *  character in the %string.  Iteration is done in reverse element
-       *  order.  Unshares the string.
-       */
-      reverse_iterator
-      rbegin() // FIXME C++11: should be noexcept.
-      { return reverse_iterator(this->end()); }
-
-      /**
-       *  Returns a read-only (constant) reverse iterator that points
-       *  to the last character in the %string.  Iteration is done in
-       *  reverse element order.
-       */
-      const_reverse_iterator
-      rbegin() const _GLIBCXX_NOEXCEPT
-      { return const_reverse_iterator(this->end()); }
-
-      /**
-       *  Returns a read/write reverse iterator that points to one before the
-       *  first character in the %string.  Iteration is done in reverse
-       *  element order.  Unshares the string.
-       */
-      reverse_iterator
-      rend() // FIXME C++11: should be noexcept.
-      { return reverse_iterator(this->begin()); }
-
-      /**
-       *  Returns a read-only (constant) reverse iterator that points
-       *  to one before the first character in the %string.  Iteration
-       *  is done in reverse element order.
-       */
-      const_reverse_iterator
-      rend() const _GLIBCXX_NOEXCEPT
-      { return const_reverse_iterator(this->begin()); }
-
-#if __cplusplus >= 201103L
-      /**
-       *  Returns a read-only (constant) iterator that points to the first
-       *  character in the %string.
-       */
-      const_iterator
-      cbegin() const noexcept
-      { return const_iterator(this->_M_data()); }
-
-      /**
-       *  Returns a read-only (constant) iterator that points one past the
-       *  last character in the %string.
-       */
-      const_iterator
-      cend() const noexcept
-      { return const_iterator(this->_M_data() + this->size()); }
-
-      /**
-       *  Returns a read-only (constant) reverse iterator that points
-       *  to the last character in the %string.  Iteration is done in
-       *  reverse element order.
-       */
-      const_reverse_iterator
-      crbegin() const noexcept
-      { return const_reverse_iterator(this->end()); }
-
-      /**
-       *  Returns a read-only (constant) reverse iterator that points
-       *  to one before the first character in the %string.  Iteration
-       *  is done in reverse element order.
-       */
-      const_reverse_iterator
-      crend() const noexcept
-      { return const_reverse_iterator(this->begin()); }
-#endif
-
-    public:
-      // Capacity:
-      ///  Returns the number of characters in the string, not including any
-      ///  null-termination.
-      size_type
-      size() const _GLIBCXX_NOEXCEPT
-      { return _M_rep()->_M_length; }
-
-      ///  Returns the number of characters in the string, not including any
-      ///  null-termination.
-      size_type
-      length() const _GLIBCXX_NOEXCEPT
-      { return _M_rep()->_M_length; }
-
-      ///  Returns the size() of the largest possible %string.
-      size_type
-      max_size() const _GLIBCXX_NOEXCEPT
-      { return _Rep::_S_max_size; }
-
-      /**
-       *  @brief  Resizes the %string to the specified number of characters.
-       *  @param  __n  Number of characters the %string should contain.
-       *  @param  __c  Character to fill any new elements.
-       *
-       *  This function will %resize the %string to the specified
-       *  number of characters.  If the number is smaller than the
-       *  %string's current size the %string is truncated, otherwise
-       *  the %string is extended and new elements are %set to @a __c.
-       */
-      void
-      resize(size_type __n, _CharT __c);
-
-      /**
-       *  @brief  Resizes the %string to the specified number of characters.
-       *  @param  __n  Number of characters the %string should contain.
-       *
-       *  This function will resize the %string to the specified length.  If
-       *  the new size is smaller than the %string's current size the %string
-       *  is truncated, otherwise the %string is extended and new characters
-       *  are default-constructed.  For basic types such as char, this means
-       *  setting them to 0.
-       */
-      void
-      resize(size_type __n)
-      { this->resize(__n, _CharT()); }
-
-#if __cplusplus >= 201103L
-#pragma GCC diagnostic push
-#pragma GCC diagnostic ignored "-Wdeprecated-declarations"
-      ///  A non-binding request to reduce capacity() to size().
-      void
-      shrink_to_fit() noexcept
-      { reserve(); }
-#pragma GCC diagnostic pop
-#endif
-
-      /**
-       *  Returns the total number of characters that the %string can hold
-       *  before needing to allocate more memory.
-       */
-      size_type
-      capacity() const _GLIBCXX_NOEXCEPT
-      { return _M_rep()->_M_capacity; }
-
-      /**
-       *  @brief  Attempt to preallocate enough memory for specified number of
-       *          characters.
-       *  @param  __res_arg  Number of characters required.
-       *  @throw  std::length_error  If @a __res_arg exceeds @c max_size().
-       *
-       *  This function attempts to reserve enough memory for the
-       *  %string to hold the specified number of characters.  If the
-       *  number requested is more than max_size(), length_error is
-       *  thrown.
-       *
-       *  The advantage of this function is that if optimal code is a
-       *  necessity and the user can determine the string length that will be
-       *  required, the user can reserve the memory in %advance, and thus
-       *  prevent a possible reallocation of memory and copying of %string
-       *  data.
-       */
-      void
-      reserve(size_type __res_arg);
-
-      /// Equivalent to shrink_to_fit().
-#if __cplusplus > 201703L
-      [[deprecated("use shrink_to_fit() instead")]]
-#endif
-      void
-      reserve();
-
-      /**
-       *  Erases the string, making it empty.
-       */
-#if _GLIBCXX_FULLY_DYNAMIC_STRING == 0
-      void
-      clear() _GLIBCXX_NOEXCEPT
-      {
-	if (_M_rep()->_M_is_shared())
-	  {
-	    _M_rep()->_M_dispose(this->get_allocator());
-	    _M_data(_S_empty_rep()._M_refdata());
-	  }
-	else
-	  _M_rep()->_M_set_length_and_sharable(0);
-      }
-#else
-      // PR 56166: this should not throw.
-      void
-      clear()
-      { _M_mutate(0, this->size(), 0); }
-#endif
-
-      /**
-       *  Returns true if the %string is empty.  Equivalent to 
-       *  <code>*this == ""</code>.
-       */
-      _GLIBCXX_NODISCARD bool
-      empty() const _GLIBCXX_NOEXCEPT
-      { return this->size() == 0; }
-
-      // Element access:
-      /**
-       *  @brief  Subscript access to the data contained in the %string.
-       *  @param  __pos  The index of the character to access.
-       *  @return  Read-only (constant) reference to the character.
-       *
-       *  This operator allows for easy, array-style, data access.
-       *  Note that data access with this operator is unchecked and
-       *  out_of_range lookups are not defined. (For checked lookups
-       *  see at().)
-       */
-      const_reference
-      operator[] (size_type __pos) const _GLIBCXX_NOEXCEPT
-      {
-	__glibcxx_assert(__pos <= size());
-	return _M_data()[__pos];
-      }
-
-      /**
-       *  @brief  Subscript access to the data contained in the %string.
-       *  @param  __pos  The index of the character to access.
-       *  @return  Read/write reference to the character.
-       *
-       *  This operator allows for easy, array-style, data access.
-       *  Note that data access with this operator is unchecked and
-       *  out_of_range lookups are not defined. (For checked lookups
-       *  see at().)  Unshares the string.
-       */
-      reference
-      operator[](size_type __pos)
-      {
-        // Allow pos == size() both in C++98 mode, as v3 extension,
-	// and in C++11 mode.
-	__glibcxx_assert(__pos <= size());
-        // In pedantic mode be strict in C++98 mode.
-	_GLIBCXX_DEBUG_PEDASSERT(__cplusplus >= 201103L || __pos < size());
-	_M_leak();
-	return _M_data()[__pos];
-      }
-
-      /**
-       *  @brief  Provides access to the data contained in the %string.
-       *  @param __n The index of the character to access.
-       *  @return  Read-only (const) reference to the character.
-       *  @throw  std::out_of_range  If @a n is an invalid index.
-       *
-       *  This function provides for safer data access.  The parameter is
-       *  first checked that it is in the range of the string.  The function
-       *  throws out_of_range if the check fails.
-       */
-      const_reference
-      at(size_type __n) const
-      {
-	if (__n >= this->size())
-	  __throw_out_of_range_fmt(__N("basic_string::at: __n "
-				       "(which is %zu) >= this->size() "
-				       "(which is %zu)"),
-				   __n, this->size());
-	return _M_data()[__n];
-      }
-
-      /**
-       *  @brief  Provides access to the data contained in the %string.
-       *  @param __n The index of the character to access.
-       *  @return  Read/write reference to the character.
-       *  @throw  std::out_of_range  If @a n is an invalid index.
-       *
-       *  This function provides for safer data access.  The parameter is
-       *  first checked that it is in the range of the string.  The function
-       *  throws out_of_range if the check fails.  Success results in
-       *  unsharing the string.
-       */
-      reference
-      at(size_type __n)
-      {
-	if (__n >= size())
-	  __throw_out_of_range_fmt(__N("basic_string::at: __n "
-				       "(which is %zu) >= this->size() "
-				       "(which is %zu)"),
-				   __n, this->size());
-	_M_leak();
-	return _M_data()[__n];
-      }
-
-#if __cplusplus >= 201103L
-      /**
-       *  Returns a read/write reference to the data at the first
-       *  element of the %string.
-       */
-      reference
-      front()
-      {
-	__glibcxx_assert(!empty());
-	return operator[](0);
-      }
-
-      /**
-       *  Returns a read-only (constant) reference to the data at the first
-       *  element of the %string.
-       */
-      const_reference
-      front() const noexcept
-      {
-	__glibcxx_assert(!empty());
-	return operator[](0);
-      }
-
-      /**
-       *  Returns a read/write reference to the data at the last
-       *  element of the %string.
-       */
-      reference
-      back()
-      {
-	__glibcxx_assert(!empty());
-	return operator[](this->size() - 1);
-      }
-
-      /**
-       *  Returns a read-only (constant) reference to the data at the
-       *  last element of the %string.
-       */
-      const_reference
-      back() const noexcept
-      {
-	__glibcxx_assert(!empty());
-	return operator[](this->size() - 1);
-      }
-#endif
-
-      // Modifiers:
-      /**
-       *  @brief  Append a string to this string.
-       *  @param __str  The string to append.
-       *  @return  Reference to this string.
-       */
-      basic_string&
-      operator+=(const basic_string& __str)
-      { return this->append(__str); }
-
-      /**
-       *  @brief  Append a C string.
-       *  @param __s  The C string to append.
-       *  @return  Reference to this string.
-       */
-      basic_string&
-      operator+=(const _CharT* __s)
-      { return this->append(__s); }
-
-      /**
-       *  @brief  Append a character.
-       *  @param __c  The character to append.
-       *  @return  Reference to this string.
-       */
-      basic_string&
-      operator+=(_CharT __c)
-      { 
-	this->push_back(__c);
-	return *this;
-      }
-
-#if __cplusplus >= 201103L
-      /**
-       *  @brief  Append an initializer_list of characters.
-       *  @param __l  The initializer_list of characters to be appended.
-       *  @return  Reference to this string.
-       */
-      basic_string&
-      operator+=(initializer_list<_CharT> __l)
-      { return this->append(__l.begin(), __l.size()); }
-#endif // C++11
-
-#if __cplusplus >= 201703L
-      /**
-       *  @brief  Append a string_view.
-       *  @param __svt The object convertible to string_view to be appended.
-       *  @return  Reference to this string.
-       */
-      template<typename _Tp>
-	_If_sv<_Tp, basic_string&>
-	operator+=(const _Tp& __svt)
-	{ return this->append(__svt); }
-#endif // C++17
-
-      /**
-       *  @brief  Append a string to this string.
-       *  @param __str  The string to append.
-       *  @return  Reference to this string.
-       */
-      basic_string&
-      append(const basic_string& __str);
-
-      /**
-       *  @brief  Append a substring.
-       *  @param __str  The string to append.
-       *  @param __pos  Index of the first character of str to append.
-       *  @param __n  The number of characters to append.
-       *  @return  Reference to this string.
-       *  @throw  std::out_of_range if @a __pos is not a valid index.
-       *
-       *  This function appends @a __n characters from @a __str
-       *  starting at @a __pos to this string.  If @a __n is is larger
-       *  than the number of available characters in @a __str, the
-       *  remainder of @a __str is appended.
-       */
-      basic_string&
-      append(const basic_string& __str, size_type __pos, size_type __n = npos);
-
-      /**
-       *  @brief  Append a C substring.
-       *  @param __s  The C string to append.
-       *  @param __n  The number of characters to append.
-       *  @return  Reference to this string.
-       */
-      basic_string&
-      append(const _CharT* __s, size_type __n);
-
-      /**
-       *  @brief  Append a C string.
-       *  @param __s  The C string to append.
-       *  @return  Reference to this string.
-       */
-      basic_string&
-      append(const _CharT* __s)
-      {
-	__glibcxx_requires_string(__s);
-	return this->append(__s, traits_type::length(__s));
-      }
-
-      /**
-       *  @brief  Append multiple characters.
-       *  @param __n  The number of characters to append.
-       *  @param __c  The character to use.
-       *  @return  Reference to this string.
-       *
-       *  Appends __n copies of __c to this string.
-       */
-      basic_string&
-      append(size_type __n, _CharT __c);
-
-#if __cplusplus >= 201103L
-      /**
-       *  @brief  Append an initializer_list of characters.
-       *  @param __l  The initializer_list of characters to append.
-       *  @return  Reference to this string.
-       */
-      basic_string&
-      append(initializer_list<_CharT> __l)
-      { return this->append(__l.begin(), __l.size()); }
-#endif // C++11
-
-      /**
-       *  @brief  Append a range of characters.
-       *  @param __first  Iterator referencing the first character to append.
-       *  @param __last  Iterator marking the end of the range.
-       *  @return  Reference to this string.
-       *
-       *  Appends characters in the range [__first,__last) to this string.
-       */
-      template<class _InputIterator>
-        basic_string&
-        append(_InputIterator __first, _InputIterator __last)
-        { return this->replace(_M_iend(), _M_iend(), __first, __last); }
-
-#if __cplusplus >= 201703L
-      /**
-       *  @brief  Append a string_view.
-       *  @param __svt The object convertible to string_view to be appended.
-       *  @return  Reference to this string.
-       */
-      template<typename _Tp>
-	_If_sv<_Tp, basic_string&>
-	append(const _Tp& __svt)
-	{
-	  __sv_type __sv = __svt;
-	  return this->append(__sv.data(), __sv.size());
-	}
-
-      /**
-       *  @brief  Append a range of characters from a string_view.
-       *  @param __svt The object convertible to string_view to be appended
-       *               from.
-       *  @param __pos The position in the string_view to append from.
-       *  @param __n   The number of characters to append from the string_view.
-       *  @return  Reference to this string.
-       */
-      template<typename _Tp>
-        _If_sv<_Tp, basic_string&>
-	append(const _Tp& __svt, size_type __pos, size_type __n = npos)
-	{
-	  __sv_type __sv = __svt;
-	  return append(__sv.data()
-	      + std::__sv_check(__sv.size(), __pos, "basic_string::append"),
-	      std::__sv_limit(__sv.size(), __pos, __n));
-	}
-#endif // C++17
-
-      /**
-       *  @brief  Append a single character.
-       *  @param __c  Character to append.
-       */
-      void
-      push_back(_CharT __c)
-      { 
-	const size_type __len = 1 + this->size();
-	if (__len > this->capacity() || _M_rep()->_M_is_shared())
-	  this->reserve(__len);
-	traits_type::assign(_M_data()[this->size()], __c);
-	_M_rep()->_M_set_length_and_sharable(__len);
-      }
-
-      /**
-       *  @brief  Set value to contents of another string.
-       *  @param  __str  Source string to use.
-       *  @return  Reference to this string.
-       */
-      basic_string&
-      assign(const basic_string& __str);
-
-#if __cplusplus >= 201103L
-      /**
-       *  @brief  Set value to contents of another string.
-       *  @param  __str  Source string to use.
-       *  @return  Reference to this string.
-       *
-       *  This function sets this string to the exact contents of @a __str.
-       *  @a __str is a valid, but unspecified string.
-       */
-      basic_string&
-      assign(basic_string&& __str)
-      noexcept(allocator_traits<_Alloc>::is_always_equal::value)
-      {
-	this->swap(__str);
-	return *this;
-      }
-#endif // C++11
-
-      /**
-       *  @brief  Set value to a substring of a string.
-       *  @param __str  The string to use.
-       *  @param __pos  Index of the first character of str.
-       *  @param __n  Number of characters to use.
-       *  @return  Reference to this string.
-       *  @throw  std::out_of_range if @a pos is not a valid index.
-       *
-       *  This function sets this string to the substring of @a __str
-       *  consisting of @a __n characters at @a __pos.  If @a __n is
-       *  is larger than the number of available characters in @a
-       *  __str, the remainder of @a __str is used.
-       */
-      basic_string&
-      assign(const basic_string& __str, size_type __pos, size_type __n = npos)
-      { return this->assign(__str._M_data()
-			    + __str._M_check(__pos, "basic_string::assign"),
-			    __str._M_limit(__pos, __n)); }
-
-      /**
-       *  @brief  Set value to a C substring.
-       *  @param __s  The C string to use.
-       *  @param __n  Number of characters to use.
-       *  @return  Reference to this string.
-       *
-       *  This function sets the value of this string to the first @a __n
-       *  characters of @a __s.  If @a __n is is larger than the number of
-       *  available characters in @a __s, the remainder of @a __s is used.
-       */
-      basic_string&
-      assign(const _CharT* __s, size_type __n);
-
-      /**
-       *  @brief  Set value to contents of a C string.
-       *  @param __s  The C string to use.
-       *  @return  Reference to this string.
-       *
-       *  This function sets the value of this string to the value of @a __s.
-       *  The data is copied, so there is no dependence on @a __s once the
-       *  function returns.
-       */
-      basic_string&
-      assign(const _CharT* __s)
-      {
-	__glibcxx_requires_string(__s);
-	return this->assign(__s, traits_type::length(__s));
-      }
-
-      /**
-       *  @brief  Set value to multiple characters.
-       *  @param __n  Length of the resulting string.
-       *  @param __c  The character to use.
-       *  @return  Reference to this string.
-       *
-       *  This function sets the value of this string to @a __n copies of
-       *  character @a __c.
-       */
-      basic_string&
-      assign(size_type __n, _CharT __c)
-      { return _M_replace_aux(size_type(0), this->size(), __n, __c); }
-
-      /**
-       *  @brief  Set value to a range of characters.
-       *  @param __first  Iterator referencing the first character to append.
-       *  @param __last  Iterator marking the end of the range.
-       *  @return  Reference to this string.
-       *
-       *  Sets value of string to characters in the range [__first,__last).
-      */
-      template<class _InputIterator>
-        basic_string&
-        assign(_InputIterator __first, _InputIterator __last)
-        { return this->replace(_M_ibegin(), _M_iend(), __first, __last); }
-
-#if __cplusplus >= 201103L
-      /**
-       *  @brief  Set value to an initializer_list of characters.
-       *  @param __l  The initializer_list of characters to assign.
-       *  @return  Reference to this string.
-       */
-      basic_string&
-      assign(initializer_list<_CharT> __l)
-      { return this->assign(__l.begin(), __l.size()); }
-#endif // C++11
-
-#if __cplusplus >= 201703L
-      /**
-       *  @brief  Set value from a string_view.
-       *  @param __svt The source object convertible to string_view.
-       *  @return  Reference to this string.
-       */
-      template<typename _Tp>
-	_If_sv<_Tp, basic_string&>
-	assign(const _Tp& __svt)
-	{
-	  __sv_type __sv = __svt;
-	  return this->assign(__sv.data(), __sv.size());
-	}
-
-      /**
-       *  @brief  Set value from a range of characters in a string_view.
-       *  @param __svt  The source object convertible to string_view.
-       *  @param __pos  The position in the string_view to assign from.
-       *  @param __n  The number of characters to assign.
-       *  @return  Reference to this string.
-       */
-      template<typename _Tp>
-        _If_sv<_Tp, basic_string&>
-        assign(const _Tp& __svt, size_type __pos, size_type __n = npos)
-	{
-	  __sv_type __sv = __svt;
-	  return assign(__sv.data()
-	      + std::__sv_check(__sv.size(), __pos, "basic_string::assign"),
-	      std::__sv_limit(__sv.size(), __pos, __n));
-	}
-#endif // C++17
-
-      /**
-       *  @brief  Insert multiple characters.
-       *  @param __p  Iterator referencing location in string to insert at.
-       *  @param __n  Number of characters to insert
-       *  @param __c  The character to insert.
-       *  @throw  std::length_error  If new length exceeds @c max_size().
-       *
-       *  Inserts @a __n copies of character @a __c starting at the
-       *  position referenced by iterator @a __p.  If adding
-       *  characters causes the length to exceed max_size(),
-       *  length_error is thrown.  The value of the string doesn't
-       *  change if an error is thrown.
-      */
-      void
-      insert(iterator __p, size_type __n, _CharT __c)
-      {	this->replace(__p, __p, __n, __c);  }
-
-      /**
-       *  @brief  Insert a range of characters.
-       *  @param __p  Iterator referencing location in string to insert at.
-       *  @param __beg  Start of range.
-       *  @param __end  End of range.
-       *  @throw  std::length_error  If new length exceeds @c max_size().
-       *
-       *  Inserts characters in range [__beg,__end).  If adding
-       *  characters causes the length to exceed max_size(),
-       *  length_error is thrown.  The value of the string doesn't
-       *  change if an error is thrown.
-      */
-      template<class _InputIterator>
-        void
-        insert(iterator __p, _InputIterator __beg, _InputIterator __end)
-        { this->replace(__p, __p, __beg, __end); }
-
-#if __cplusplus >= 201103L
-      /**
-       *  @brief  Insert an initializer_list of characters.
-       *  @param __p  Iterator referencing location in string to insert at.
-       *  @param __l  The initializer_list of characters to insert.
-       *  @throw  std::length_error  If new length exceeds @c max_size().
-       */
-      void
-      insert(iterator __p, initializer_list<_CharT> __l)
-      {
-	_GLIBCXX_DEBUG_PEDASSERT(__p >= _M_ibegin() && __p <= _M_iend());
-	this->insert(__p - _M_ibegin(), __l.begin(), __l.size());
-      }
-#endif // C++11
-
-      /**
-       *  @brief  Insert value of a string.
-       *  @param __pos1  Position in string to insert at.
-       *  @param __str  The string to insert.
-       *  @return  Reference to this string.
-       *  @throw  std::length_error  If new length exceeds @c max_size().
-       *
-       *  Inserts value of @a __str starting at @a __pos1.  If adding
-       *  characters causes the length to exceed max_size(),
-       *  length_error is thrown.  The value of the string doesn't
-       *  change if an error is thrown.
-      */
-      basic_string&
-      insert(size_type __pos1, const basic_string& __str)
-      { return this->insert(__pos1, __str, size_type(0), __str.size()); }
-
-      /**
-       *  @brief  Insert a substring.
-       *  @param __pos1  Position in string to insert at.
-       *  @param __str  The string to insert.
-       *  @param __pos2  Start of characters in str to insert.
-       *  @param __n  Number of characters to insert.
-       *  @return  Reference to this string.
-       *  @throw  std::length_error  If new length exceeds @c max_size().
-       *  @throw  std::out_of_range  If @a pos1 > size() or
-       *  @a __pos2 > @a str.size().
-       *
-       *  Starting at @a pos1, insert @a __n character of @a __str
-       *  beginning with @a __pos2.  If adding characters causes the
-       *  length to exceed max_size(), length_error is thrown.  If @a
-       *  __pos1 is beyond the end of this string or @a __pos2 is
-       *  beyond the end of @a __str, out_of_range is thrown.  The
-       *  value of the string doesn't change if an error is thrown.
-      */
-      basic_string&
-      insert(size_type __pos1, const basic_string& __str,
-	     size_type __pos2, size_type __n = npos)
-      { return this->insert(__pos1, __str._M_data()
-			    + __str._M_check(__pos2, "basic_string::insert"),
-			    __str._M_limit(__pos2, __n)); }
-
-      /**
-       *  @brief  Insert a C substring.
-       *  @param __pos  Position in string to insert at.
-       *  @param __s  The C string to insert.
-       *  @param __n  The number of characters to insert.
-       *  @return  Reference to this string.
-       *  @throw  std::length_error  If new length exceeds @c max_size().
-       *  @throw  std::out_of_range  If @a __pos is beyond the end of this
-       *  string.
-       *
-       *  Inserts the first @a __n characters of @a __s starting at @a
-       *  __pos.  If adding characters causes the length to exceed
-       *  max_size(), length_error is thrown.  If @a __pos is beyond
-       *  end(), out_of_range is thrown.  The value of the string
-       *  doesn't change if an error is thrown.
-      */
-      basic_string&
-      insert(size_type __pos, const _CharT* __s, size_type __n);
-
-      /**
-       *  @brief  Insert a C string.
-       *  @param __pos  Position in string to insert at.
-       *  @param __s  The C string to insert.
-       *  @return  Reference to this string.
-       *  @throw  std::length_error  If new length exceeds @c max_size().
-       *  @throw  std::out_of_range  If @a pos is beyond the end of this
-       *  string.
-       *
-       *  Inserts the first @a n characters of @a __s starting at @a __pos.  If
-       *  adding characters causes the length to exceed max_size(),
-       *  length_error is thrown.  If @a __pos is beyond end(), out_of_range is
-       *  thrown.  The value of the string doesn't change if an error is
-       *  thrown.
-      */
-      basic_string&
-      insert(size_type __pos, const _CharT* __s)
-      {
-	__glibcxx_requires_string(__s);
-	return this->insert(__pos, __s, traits_type::length(__s));
-      }
-
-      /**
-       *  @brief  Insert multiple characters.
-       *  @param __pos  Index in string to insert at.
-       *  @param __n  Number of characters to insert
-       *  @param __c  The character to insert.
-       *  @return  Reference to this string.
-       *  @throw  std::length_error  If new length exceeds @c max_size().
-       *  @throw  std::out_of_range  If @a __pos is beyond the end of this
-       *  string.
-       *
-       *  Inserts @a __n copies of character @a __c starting at index
-       *  @a __pos.  If adding characters causes the length to exceed
-       *  max_size(), length_error is thrown.  If @a __pos > length(),
-       *  out_of_range is thrown.  The value of the string doesn't
-       *  change if an error is thrown.
-      */
-      basic_string&
-      insert(size_type __pos, size_type __n, _CharT __c)
-      { return _M_replace_aux(_M_check(__pos, "basic_string::insert"),
-			      size_type(0), __n, __c); }
-
-      /**
-       *  @brief  Insert one character.
-       *  @param __p  Iterator referencing position in string to insert at.
-       *  @param __c  The character to insert.
-       *  @return  Iterator referencing newly inserted char.
-       *  @throw  std::length_error  If new length exceeds @c max_size().
-       *
-       *  Inserts character @a __c at position referenced by @a __p.
-       *  If adding character causes the length to exceed max_size(),
-       *  length_error is thrown.  If @a __p is beyond end of string,
-       *  out_of_range is thrown.  The value of the string doesn't
-       *  change if an error is thrown.
-      */
-      iterator
-      insert(iterator __p, _CharT __c)
-      {
-	_GLIBCXX_DEBUG_PEDASSERT(__p >= _M_ibegin() && __p <= _M_iend());
-	const size_type __pos = __p - _M_ibegin();
-	_M_replace_aux(__pos, size_type(0), size_type(1), __c);
-	_M_rep()->_M_set_leaked();
-	return iterator(_M_data() + __pos);
-      }
-
-#if __cplusplus >= 201703L
-      /**
-       *  @brief  Insert a string_view.
-       *  @param __pos  Position in string to insert at.
-       *  @param __svt  The object convertible to string_view to insert.
-       *  @return  Reference to this string.
-      */
-      template<typename _Tp>
-	_If_sv<_Tp, basic_string&>
-	insert(size_type __pos, const _Tp& __svt)
-	{
-	  __sv_type __sv = __svt;
-	  return this->insert(__pos, __sv.data(), __sv.size());
-	}
-
-      /**
-       *  @brief  Insert a string_view.
-       *  @param __pos1  Position in string to insert at.
-       *  @param __svt   The object convertible to string_view to insert from.
-       *  @param __pos2  Position in string_view to insert from.
-       *  @param __n    The number of characters to insert.
-       *  @return  Reference to this string.
-      */
-      template<typename _Tp>
-        _If_sv<_Tp, basic_string&>
-        insert(size_type __pos1, const _Tp& __svt,
-	       size_type __pos2, size_type __n = npos)
-	{
-	  __sv_type __sv = __svt;
-	  return this->replace(__pos1, size_type(0), __sv.data()
-	      + std::__sv_check(__sv.size(), __pos2, "basic_string::insert"),
-	      std::__sv_limit(__sv.size(), __pos2, __n));
-	}
-#endif // C++17
-
-      /**
-       *  @brief  Remove characters.
-       *  @param __pos  Index of first character to remove (default 0).
-       *  @param __n  Number of characters to remove (default remainder).
-       *  @return  Reference to this string.
-       *  @throw  std::out_of_range  If @a pos is beyond the end of this
-       *  string.
-       *
-       *  Removes @a __n characters from this string starting at @a
-       *  __pos.  The length of the string is reduced by @a __n.  If
-       *  there are < @a __n characters to remove, the remainder of
-       *  the string is truncated.  If @a __p is beyond end of string,
-       *  out_of_range is thrown.  The value of the string doesn't
-       *  change if an error is thrown.
-      */
-      basic_string&
-      erase(size_type __pos = 0, size_type __n = npos)
-      { 
-	_M_mutate(_M_check(__pos, "basic_string::erase"),
-		  _M_limit(__pos, __n), size_type(0));
-	return *this;
-      }
-
-      /**
-       *  @brief  Remove one character.
-       *  @param __position  Iterator referencing the character to remove.
-       *  @return  iterator referencing same location after removal.
-       *
-       *  Removes the character at @a __position from this string. The value
-       *  of the string doesn't change if an error is thrown.
-      */
-      iterator
-      erase(iterator __position)
-      {
-	_GLIBCXX_DEBUG_PEDASSERT(__position >= _M_ibegin()
-				 && __position < _M_iend());
-	const size_type __pos = __position - _M_ibegin();
-	_M_mutate(__pos, size_type(1), size_type(0));
-	_M_rep()->_M_set_leaked();
-	return iterator(_M_data() + __pos);
-      }
-
-      /**
-       *  @brief  Remove a range of characters.
-       *  @param __first  Iterator referencing the first character to remove.
-       *  @param __last  Iterator referencing the end of the range.
-       *  @return  Iterator referencing location of first after removal.
-       *
-       *  Removes the characters in the range [first,last) from this string.
-       *  The value of the string doesn't change if an error is thrown.
-      */
-      iterator
-      erase(iterator __first, iterator __last);
- 
-#if __cplusplus >= 201103L
-      /**
-       *  @brief  Remove the last character.
-       *
-       *  The string must be non-empty.
-       */
-      void
-      pop_back() // FIXME C++11: should be noexcept.
-      {
-	__glibcxx_assert(!empty());
-	erase(size() - 1, 1);
-      }
-#endif // C++11
-
-      /**
-       *  @brief  Replace characters with value from another string.
-       *  @param __pos  Index of first character to replace.
-       *  @param __n  Number of characters to be replaced.
-       *  @param __str  String to insert.
-       *  @return  Reference to this string.
-       *  @throw  std::out_of_range  If @a pos is beyond the end of this
-       *  string.
-       *  @throw  std::length_error  If new length exceeds @c max_size().
-       *
-       *  Removes the characters in the range [__pos,__pos+__n) from
-       *  this string.  In place, the value of @a __str is inserted.
-       *  If @a __pos is beyond end of string, out_of_range is thrown.
-       *  If the length of the result exceeds max_size(), length_error
-       *  is thrown.  The value of the string doesn't change if an
-       *  error is thrown.
-      */
-      basic_string&
-      replace(size_type __pos, size_type __n, const basic_string& __str)
-      { return this->replace(__pos, __n, __str._M_data(), __str.size()); }
-
-      /**
-       *  @brief  Replace characters with value from another string.
-       *  @param __pos1  Index of first character to replace.
-       *  @param __n1  Number of characters to be replaced.
-       *  @param __str  String to insert.
-       *  @param __pos2  Index of first character of str to use.
-       *  @param __n2  Number of characters from str to use.
-       *  @return  Reference to this string.
-       *  @throw  std::out_of_range  If @a __pos1 > size() or @a __pos2 >
-       *  __str.size().
-       *  @throw  std::length_error  If new length exceeds @c max_size().
-       *
-       *  Removes the characters in the range [__pos1,__pos1 + n) from this
-       *  string.  In place, the value of @a __str is inserted.  If @a __pos is
-       *  beyond end of string, out_of_range is thrown.  If the length of the
-       *  result exceeds max_size(), length_error is thrown.  The value of the
-       *  string doesn't change if an error is thrown.
-      */
-      basic_string&
-      replace(size_type __pos1, size_type __n1, const basic_string& __str,
-	      size_type __pos2, size_type __n2 = npos)
-      { return this->replace(__pos1, __n1, __str._M_data()
-			     + __str._M_check(__pos2, "basic_string::replace"),
-			     __str._M_limit(__pos2, __n2)); }
-
-      /**
-       *  @brief  Replace characters with value of a C substring.
-       *  @param __pos  Index of first character to replace.
-       *  @param __n1  Number of characters to be replaced.
-       *  @param __s  C string to insert.
-       *  @param __n2  Number of characters from @a s to use.
-       *  @return  Reference to this string.
-       *  @throw  std::out_of_range  If @a pos1 > size().
-       *  @throw  std::length_error  If new length exceeds @c max_size().
-       *
-       *  Removes the characters in the range [__pos,__pos + __n1)
-       *  from this string.  In place, the first @a __n2 characters of
-       *  @a __s are inserted, or all of @a __s if @a __n2 is too large.  If
-       *  @a __pos is beyond end of string, out_of_range is thrown.  If
-       *  the length of result exceeds max_size(), length_error is
-       *  thrown.  The value of the string doesn't change if an error
-       *  is thrown.
-      */
-      basic_string&
-      replace(size_type __pos, size_type __n1, const _CharT* __s,
-	      size_type __n2);
-
-      /**
-       *  @brief  Replace characters with value of a C string.
-       *  @param __pos  Index of first character to replace.
-       *  @param __n1  Number of characters to be replaced.
-       *  @param __s  C string to insert.
-       *  @return  Reference to this string.
-       *  @throw  std::out_of_range  If @a pos > size().
-       *  @throw  std::length_error  If new length exceeds @c max_size().
-       *
-       *  Removes the characters in the range [__pos,__pos + __n1)
-       *  from this string.  In place, the characters of @a __s are
-       *  inserted.  If @a __pos is beyond end of string, out_of_range
-       *  is thrown.  If the length of result exceeds max_size(),
-       *  length_error is thrown.  The value of the string doesn't
-       *  change if an error is thrown.
-      */
-      basic_string&
-      replace(size_type __pos, size_type __n1, const _CharT* __s)
-      {
-	__glibcxx_requires_string(__s);
-	return this->replace(__pos, __n1, __s, traits_type::length(__s));
-      }
-
-      /**
-       *  @brief  Replace characters with multiple characters.
-       *  @param __pos  Index of first character to replace.
-       *  @param __n1  Number of characters to be replaced.
-       *  @param __n2  Number of characters to insert.
-       *  @param __c  Character to insert.
-       *  @return  Reference to this string.
-       *  @throw  std::out_of_range  If @a __pos > size().
-       *  @throw  std::length_error  If new length exceeds @c max_size().
-       *
-       *  Removes the characters in the range [pos,pos + n1) from this
-       *  string.  In place, @a __n2 copies of @a __c are inserted.
-       *  If @a __pos is beyond end of string, out_of_range is thrown.
-       *  If the length of result exceeds max_size(), length_error is
-       *  thrown.  The value of the string doesn't change if an error
-       *  is thrown.
-      */
-      basic_string&
-      replace(size_type __pos, size_type __n1, size_type __n2, _CharT __c)
-      { return _M_replace_aux(_M_check(__pos, "basic_string::replace"),
-			      _M_limit(__pos, __n1), __n2, __c); }
-
-      /**
-       *  @brief  Replace range of characters with string.
-       *  @param __i1  Iterator referencing start of range to replace.
-       *  @param __i2  Iterator referencing end of range to replace.
-       *  @param __str  String value to insert.
-       *  @return  Reference to this string.
-       *  @throw  std::length_error  If new length exceeds @c max_size().
-       *
-       *  Removes the characters in the range [__i1,__i2).  In place,
-       *  the value of @a __str is inserted.  If the length of result
-       *  exceeds max_size(), length_error is thrown.  The value of
-       *  the string doesn't change if an error is thrown.
-      */
-      basic_string&
-      replace(iterator __i1, iterator __i2, const basic_string& __str)
-      { return this->replace(__i1, __i2, __str._M_data(), __str.size()); }
-
-      /**
-       *  @brief  Replace range of characters with C substring.
-       *  @param __i1  Iterator referencing start of range to replace.
-       *  @param __i2  Iterator referencing end of range to replace.
-       *  @param __s  C string value to insert.
-       *  @param __n  Number of characters from s to insert.
-       *  @return  Reference to this string.
-       *  @throw  std::length_error  If new length exceeds @c max_size().
-       *
-       *  Removes the characters in the range [__i1,__i2).  In place,
-       *  the first @a __n characters of @a __s are inserted.  If the
-       *  length of result exceeds max_size(), length_error is thrown.
-       *  The value of the string doesn't change if an error is
-       *  thrown.
-      */
-      basic_string&
-      replace(iterator __i1, iterator __i2, const _CharT* __s, size_type __n)
-      {
-	_GLIBCXX_DEBUG_PEDASSERT(_M_ibegin() <= __i1 && __i1 <= __i2
-				 && __i2 <= _M_iend());
-	return this->replace(__i1 - _M_ibegin(), __i2 - __i1, __s, __n);
-      }
-
-      /**
-       *  @brief  Replace range of characters with C string.
-       *  @param __i1  Iterator referencing start of range to replace.
-       *  @param __i2  Iterator referencing end of range to replace.
-       *  @param __s  C string value to insert.
-       *  @return  Reference to this string.
-       *  @throw  std::length_error  If new length exceeds @c max_size().
-       *
-       *  Removes the characters in the range [__i1,__i2).  In place,
-       *  the characters of @a __s are inserted.  If the length of
-       *  result exceeds max_size(), length_error is thrown.  The
-       *  value of the string doesn't change if an error is thrown.
-      */
-      basic_string&
-      replace(iterator __i1, iterator __i2, const _CharT* __s)
-      {
-	__glibcxx_requires_string(__s);
-	return this->replace(__i1, __i2, __s, traits_type::length(__s));
-      }
-
-      /**
-       *  @brief  Replace range of characters with multiple characters
-       *  @param __i1  Iterator referencing start of range to replace.
-       *  @param __i2  Iterator referencing end of range to replace.
-       *  @param __n  Number of characters to insert.
-       *  @param __c  Character to insert.
-       *  @return  Reference to this string.
-       *  @throw  std::length_error  If new length exceeds @c max_size().
-       *
-       *  Removes the characters in the range [__i1,__i2).  In place,
-       *  @a __n copies of @a __c are inserted.  If the length of
-       *  result exceeds max_size(), length_error is thrown.  The
-       *  value of the string doesn't change if an error is thrown.
-      */
-      basic_string&
-      replace(iterator __i1, iterator __i2, size_type __n, _CharT __c)
-      {
-	_GLIBCXX_DEBUG_PEDASSERT(_M_ibegin() <= __i1 && __i1 <= __i2
-				 && __i2 <= _M_iend());
-	return _M_replace_aux(__i1 - _M_ibegin(), __i2 - __i1, __n, __c);
-      }
-
-      /**
-       *  @brief  Replace range of characters with range.
-       *  @param __i1  Iterator referencing start of range to replace.
-       *  @param __i2  Iterator referencing end of range to replace.
-       *  @param __k1  Iterator referencing start of range to insert.
-       *  @param __k2  Iterator referencing end of range to insert.
-       *  @return  Reference to this string.
-       *  @throw  std::length_error  If new length exceeds @c max_size().
-       *
-       *  Removes the characters in the range [__i1,__i2).  In place,
-       *  characters in the range [__k1,__k2) are inserted.  If the
-       *  length of result exceeds max_size(), length_error is thrown.
-       *  The value of the string doesn't change if an error is
-       *  thrown.
-      */
-      template<class _InputIterator>
-        basic_string&
-        replace(iterator __i1, iterator __i2,
-		_InputIterator __k1, _InputIterator __k2)
-        {
-	  _GLIBCXX_DEBUG_PEDASSERT(_M_ibegin() <= __i1 && __i1 <= __i2
-				   && __i2 <= _M_iend());
-	  __glibcxx_requires_valid_range(__k1, __k2);
-	  typedef typename std::__is_integer<_InputIterator>::__type _Integral;
-	  return _M_replace_dispatch(__i1, __i2, __k1, __k2, _Integral());
-	}
-
-      // Specializations for the common case of pointer and iterator:
-      // useful to avoid the overhead of temporary buffering in _M_replace.
-      basic_string&
-      replace(iterator __i1, iterator __i2, _CharT* __k1, _CharT* __k2)
-      {
-	_GLIBCXX_DEBUG_PEDASSERT(_M_ibegin() <= __i1 && __i1 <= __i2
-				 && __i2 <= _M_iend());
-	__glibcxx_requires_valid_range(__k1, __k2);
-	return this->replace(__i1 - _M_ibegin(), __i2 - __i1,
-			     __k1, __k2 - __k1);
-      }
-
-      basic_string&
-      replace(iterator __i1, iterator __i2,
-	      const _CharT* __k1, const _CharT* __k2)
-      {
-	_GLIBCXX_DEBUG_PEDASSERT(_M_ibegin() <= __i1 && __i1 <= __i2
-				 && __i2 <= _M_iend());
-	__glibcxx_requires_valid_range(__k1, __k2);
-	return this->replace(__i1 - _M_ibegin(), __i2 - __i1,
-			     __k1, __k2 - __k1);
-      }
-
-      basic_string&
-      replace(iterator __i1, iterator __i2, iterator __k1, iterator __k2)
-      {
-	_GLIBCXX_DEBUG_PEDASSERT(_M_ibegin() <= __i1 && __i1 <= __i2
-				 && __i2 <= _M_iend());
-	__glibcxx_requires_valid_range(__k1, __k2);
-	return this->replace(__i1 - _M_ibegin(), __i2 - __i1,
-			     __k1.base(), __k2 - __k1);
-      }
-
-      basic_string&
-      replace(iterator __i1, iterator __i2,
-	      const_iterator __k1, const_iterator __k2)
-      {
-	_GLIBCXX_DEBUG_PEDASSERT(_M_ibegin() <= __i1 && __i1 <= __i2
-				 && __i2 <= _M_iend());
-	__glibcxx_requires_valid_range(__k1, __k2);
-	return this->replace(__i1 - _M_ibegin(), __i2 - __i1,
-			     __k1.base(), __k2 - __k1);
-      }
-
-#if __cplusplus >= 201103L
-      /**
-       *  @brief  Replace range of characters with initializer_list.
-       *  @param __i1  Iterator referencing start of range to replace.
-       *  @param __i2  Iterator referencing end of range to replace.
-       *  @param __l  The initializer_list of characters to insert.
-       *  @return  Reference to this string.
-       *  @throw  std::length_error  If new length exceeds @c max_size().
-       *
-       *  Removes the characters in the range [__i1,__i2).  In place,
-       *  characters in the range [__k1,__k2) are inserted.  If the
-       *  length of result exceeds max_size(), length_error is thrown.
-       *  The value of the string doesn't change if an error is
-       *  thrown.
-      */
-      basic_string& replace(iterator __i1, iterator __i2,
-			    initializer_list<_CharT> __l)
-      { return this->replace(__i1, __i2, __l.begin(), __l.end()); }
-#endif // C++11
-
-#if __cplusplus >= 201703L
-      /**
-       *  @brief  Replace range of characters with string_view.
-       *  @param __pos  The position to replace at.
-       *  @param __n    The number of characters to replace.
-       *  @param __svt  The object convertible to string_view to insert.
-       *  @return  Reference to this string.
-      */
-      template<typename _Tp>
-	_If_sv<_Tp, basic_string&>
-	replace(size_type __pos, size_type __n, const _Tp& __svt)
-	{
-	  __sv_type __sv = __svt;
-	  return this->replace(__pos, __n, __sv.data(), __sv.size());
-	}
-
-      /**
-       *  @brief  Replace range of characters with string_view.
-       *  @param __pos1  The position to replace at.
-       *  @param __n1    The number of characters to replace.
-       *  @param __svt   The object convertible to string_view to insert from.
-       *  @param __pos2  The position in the string_view to insert from.
-       *  @param __n2    The number of characters to insert.
-       *  @return  Reference to this string.
-      */
-      template<typename _Tp>
-        _If_sv<_Tp, basic_string&>
-        replace(size_type __pos1, size_type __n1, const _Tp& __svt,
-		size_type __pos2, size_type __n2 = npos)
-	{
-	  __sv_type __sv = __svt;
-	  return this->replace(__pos1, __n1,
-	      __sv.data()
-	      + std::__sv_check(__sv.size(), __pos2, "basic_string::replace"),
-	      std::__sv_limit(__sv.size(), __pos2, __n2));
-	}
-
-      /**
-       *  @brief  Replace range of characters with string_view.
-       *  @param __i1    An iterator referencing the start position
-          to replace at.
-       *  @param __i2    An iterator referencing the end position
-          for the replace.
-       *  @param __svt   The object convertible to string_view to insert from.
-       *  @return  Reference to this string.
-      */
-      template<typename _Tp>
-	_If_sv<_Tp, basic_string&>
-	replace(const_iterator __i1, const_iterator __i2, const _Tp& __svt)
-	{
-	  __sv_type __sv = __svt;
-	  return this->replace(__i1 - begin(), __i2 - __i1, __sv);
-	}
-#endif // C++17
-
-    private:
-      template<class _Integer>
-	basic_string&
-	_M_replace_dispatch(iterator __i1, iterator __i2, _Integer __n,
-			    _Integer __val, __true_type)
-        { return _M_replace_aux(__i1 - _M_ibegin(), __i2 - __i1, __n, __val); }
-
-      template<class _InputIterator>
-	basic_string&
-	_M_replace_dispatch(iterator __i1, iterator __i2, _InputIterator __k1,
-			    _InputIterator __k2, __false_type);
-
-      basic_string&
-      _M_replace_aux(size_type __pos1, size_type __n1, size_type __n2,
-		     _CharT __c);
-
-      basic_string&
-      _M_replace_safe(size_type __pos1, size_type __n1, const _CharT* __s,
-		      size_type __n2);
-
-      // _S_construct_aux is used to implement the 21.3.1 para 15 which
-      // requires special behaviour if _InIter is an integral type
-      template<class _InIterator>
-        static _CharT*
-        _S_construct_aux(_InIterator __beg, _InIterator __end,
-			 const _Alloc& __a, __false_type)
-	{
-          typedef typename iterator_traits<_InIterator>::iterator_category _Tag;
-          return _S_construct(__beg, __end, __a, _Tag());
-	}
-
-      // _GLIBCXX_RESOLVE_LIB_DEFECTS
-      // 438. Ambiguity in the "do the right thing" clause
-      template<class _Integer>
-        static _CharT*
-        _S_construct_aux(_Integer __beg, _Integer __end,
-			 const _Alloc& __a, __true_type)
-        { return _S_construct_aux_2(static_cast<size_type>(__beg),
-				    __end, __a); }
-
-      static _CharT*
-      _S_construct_aux_2(size_type __req, _CharT __c, const _Alloc& __a)
-      { return _S_construct(__req, __c, __a); }
-
-      template<class _InIterator>
-        static _CharT*
-        _S_construct(_InIterator __beg, _InIterator __end, const _Alloc& __a)
-	{
-	  typedef typename std::__is_integer<_InIterator>::__type _Integral;
-	  return _S_construct_aux(__beg, __end, __a, _Integral());
-        }
-
-      // For Input Iterators, used in istreambuf_iterators, etc.
-      template<class _InIterator>
-        static _CharT*
-         _S_construct(_InIterator __beg, _InIterator __end, const _Alloc& __a,
-		      input_iterator_tag);
-
-      // For forward_iterators up to random_access_iterators, used for
-      // string::iterator, _CharT*, etc.
-      template<class _FwdIterator>
-        static _CharT*
-        _S_construct(_FwdIterator __beg, _FwdIterator __end, const _Alloc& __a,
-		     forward_iterator_tag);
-
-      static _CharT*
-      _S_construct(size_type __req, _CharT __c, const _Alloc& __a);
-
-    public:
-
-      /**
-       *  @brief  Copy substring into C string.
-       *  @param __s  C string to copy value into.
-       *  @param __n  Number of characters to copy.
-       *  @param __pos  Index of first character to copy.
-       *  @return  Number of characters actually copied
-       *  @throw  std::out_of_range  If __pos > size().
-       *
-       *  Copies up to @a __n characters starting at @a __pos into the
-       *  C string @a __s.  If @a __pos is %greater than size(),
-       *  out_of_range is thrown.
-      */
-      size_type
-      copy(_CharT* __s, size_type __n, size_type __pos = 0) const;
-
-      /**
-       *  @brief  Swap contents with another string.
-       *  @param __s  String to swap with.
-       *
-       *  Exchanges the contents of this string with that of @a __s in constant
-       *  time.
-      */
-      void
-      swap(basic_string& __s)
-      _GLIBCXX_NOEXCEPT_IF(allocator_traits<_Alloc>::is_always_equal::value);
-
-      // String operations:
-      /**
-       *  @brief  Return const pointer to null-terminated contents.
-       *
-       *  This is a handle to internal data.  Do not modify or dire things may
-       *  happen.
-      */
-      const _CharT*
-      c_str() const _GLIBCXX_NOEXCEPT
-      { return _M_data(); }
-
-      /**
-       *  @brief  Return const pointer to contents.
-       *
-       *  This is a pointer to internal data.  It is undefined to modify
-       *  the contents through the returned pointer. To get a pointer that
-       *  allows modifying the contents use @c &str[0] instead,
-       *  (or in C++17 the non-const @c str.data() overload).
-      */
-      const _CharT*
-      data() const _GLIBCXX_NOEXCEPT
-      { return _M_data(); }
-
-#if __cplusplus >= 201703L
-      /**
-       *  @brief  Return non-const pointer to contents.
-       *
-       *  This is a pointer to the character sequence held by the string.
-       *  Modifying the characters in the sequence is allowed.
-      */
-      _CharT*
-      data() noexcept
-      {
-	_M_leak();
-	return _M_data();
-      }
-#endif
-
-      /**
-       *  @brief  Return copy of allocator used to construct this string.
-      */
-      allocator_type
-      get_allocator() const _GLIBCXX_NOEXCEPT
-      { return _M_dataplus; }
-
-      /**
-       *  @brief  Find position of a C substring.
-       *  @param __s  C string to locate.
-       *  @param __pos  Index of character to search from.
-       *  @param __n  Number of characters from @a s to search for.
-       *  @return  Index of start of first occurrence.
-       *
-       *  Starting from @a __pos, searches forward for the first @a
-       *  __n characters in @a __s within this string.  If found,
-       *  returns the index where it begins.  If not found, returns
-       *  npos.
-      */
-      size_type
-      find(const _CharT* __s, size_type __pos, size_type __n) const
-      _GLIBCXX_NOEXCEPT;
-
-      /**
-       *  @brief  Find position of a string.
-       *  @param __str  String to locate.
-       *  @param __pos  Index of character to search from (default 0).
-       *  @return  Index of start of first occurrence.
-       *
-       *  Starting from @a __pos, searches forward for value of @a __str within
-       *  this string.  If found, returns the index where it begins.  If not
-       *  found, returns npos.
-      */
-      size_type
-      find(const basic_string& __str, size_type __pos = 0) const
-      _GLIBCXX_NOEXCEPT
-      { return this->find(__str.data(), __pos, __str.size()); }
-
-      /**
-       *  @brief  Find position of a C string.
-       *  @param __s  C string to locate.
-       *  @param __pos  Index of character to search from (default 0).
-       *  @return  Index of start of first occurrence.
-       *
-       *  Starting from @a __pos, searches forward for the value of @a
-       *  __s within this string.  If found, returns the index where
-       *  it begins.  If not found, returns npos.
-      */
-      size_type
-      find(const _CharT* __s, size_type __pos = 0) const _GLIBCXX_NOEXCEPT
-      {
-	__glibcxx_requires_string(__s);
-	return this->find(__s, __pos, traits_type::length(__s));
-      }
-
-      /**
-       *  @brief  Find position of a character.
-       *  @param __c  Character to locate.
-       *  @param __pos  Index of character to search from (default 0).
-       *  @return  Index of first occurrence.
-       *
-       *  Starting from @a __pos, searches forward for @a __c within
-       *  this string.  If found, returns the index where it was
-       *  found.  If not found, returns npos.
-      */
-      size_type
-      find(_CharT __c, size_type __pos = 0) const _GLIBCXX_NOEXCEPT;
-
-#if __cplusplus >= 201703L
-      /**
-       *  @brief  Find position of a string_view.
-       *  @param __svt  The object convertible to string_view to locate.
-       *  @param __pos  Index of character to search from (default 0).
-       *  @return  Index of start of first occurrence.
-      */
-      template<typename _Tp>
-	_If_sv<_Tp, size_type>
-	find(const _Tp& __svt, size_type __pos = 0) const
-	noexcept(is_same<_Tp, __sv_type>::value)
-	{
-	  __sv_type __sv = __svt;
-	  return this->find(__sv.data(), __pos, __sv.size());
-	}
-#endif // C++17
-
-      /**
-       *  @brief  Find last position of a string.
-       *  @param __str  String to locate.
-       *  @param __pos  Index of character to search back from (default end).
-       *  @return  Index of start of last occurrence.
-       *
-       *  Starting from @a __pos, searches backward for value of @a
-       *  __str within this string.  If found, returns the index where
-       *  it begins.  If not found, returns npos.
-      */
-      size_type
-      rfind(const basic_string& __str, size_type __pos = npos) const
-      _GLIBCXX_NOEXCEPT
-      { return this->rfind(__str.data(), __pos, __str.size()); }
-
-      /**
-       *  @brief  Find last position of a C substring.
-       *  @param __s  C string to locate.
-       *  @param __pos  Index of character to search back from.
-       *  @param __n  Number of characters from s to search for.
-       *  @return  Index of start of last occurrence.
-       *
-       *  Starting from @a __pos, searches backward for the first @a
-       *  __n characters in @a __s within this string.  If found,
-       *  returns the index where it begins.  If not found, returns
-       *  npos.
-      */
-      size_type
-      rfind(const _CharT* __s, size_type __pos, size_type __n) const
-      _GLIBCXX_NOEXCEPT;
-
-      /**
-       *  @brief  Find last position of a C string.
-       *  @param __s  C string to locate.
-       *  @param __pos  Index of character to start search at (default end).
-       *  @return  Index of start of  last occurrence.
-       *
-       *  Starting from @a __pos, searches backward for the value of
-       *  @a __s within this string.  If found, returns the index
-       *  where it begins.  If not found, returns npos.
-      */
-      size_type
-      rfind(const _CharT* __s, size_type __pos = npos) const _GLIBCXX_NOEXCEPT
-      {
-	__glibcxx_requires_string(__s);
-	return this->rfind(__s, __pos, traits_type::length(__s));
-      }
-
-      /**
-       *  @brief  Find last position of a character.
-       *  @param __c  Character to locate.
-       *  @param __pos  Index of character to search back from (default end).
-       *  @return  Index of last occurrence.
-       *
-       *  Starting from @a __pos, searches backward for @a __c within
-       *  this string.  If found, returns the index where it was
-       *  found.  If not found, returns npos.
-      */
-      size_type
-      rfind(_CharT __c, size_type __pos = npos) const _GLIBCXX_NOEXCEPT;
-
-#if __cplusplus >= 201703L
-      /**
-       *  @brief  Find last position of a string_view.
-       *  @param __svt  The object convertible to string_view to locate.
-       *  @param __pos  Index of character to search back from (default end).
-       *  @return  Index of start of last occurrence.
-      */
-      template<typename _Tp>
-	_If_sv<_Tp, size_type>
-	rfind(const _Tp& __svt, size_type __pos = npos) const
-	noexcept(is_same<_Tp, __sv_type>::value)
-	{
-	  __sv_type __sv = __svt;
-	  return this->rfind(__sv.data(), __pos, __sv.size());
-	}
-#endif // C++17
-
-      /**
-       *  @brief  Find position of a character of string.
-       *  @param __str  String containing characters to locate.
-       *  @param __pos  Index of character to search from (default 0).
-       *  @return  Index of first occurrence.
-       *
-       *  Starting from @a __pos, searches forward for one of the
-       *  characters of @a __str within this string.  If found,
-       *  returns the index where it was found.  If not found, returns
-       *  npos.
-      */
-      size_type
-      find_first_of(const basic_string& __str, size_type __pos = 0) const
-      _GLIBCXX_NOEXCEPT
-      { return this->find_first_of(__str.data(), __pos, __str.size()); }
-
-      /**
-       *  @brief  Find position of a character of C substring.
-       *  @param __s  String containing characters to locate.
-       *  @param __pos  Index of character to search from.
-       *  @param __n  Number of characters from s to search for.
-       *  @return  Index of first occurrence.
-       *
-       *  Starting from @a __pos, searches forward for one of the
-       *  first @a __n characters of @a __s within this string.  If
-       *  found, returns the index where it was found.  If not found,
-       *  returns npos.
-      */
-      size_type
-      find_first_of(const _CharT* __s, size_type __pos, size_type __n) const
-      _GLIBCXX_NOEXCEPT;
-
-      /**
-       *  @brief  Find position of a character of C string.
-       *  @param __s  String containing characters to locate.
-       *  @param __pos  Index of character to search from (default 0).
-       *  @return  Index of first occurrence.
-       *
-       *  Starting from @a __pos, searches forward for one of the
-       *  characters of @a __s within this string.  If found, returns
-       *  the index where it was found.  If not found, returns npos.
-      */
-      size_type
-      find_first_of(const _CharT* __s, size_type __pos = 0) const
-      _GLIBCXX_NOEXCEPT
-      {
-	__glibcxx_requires_string(__s);
-	return this->find_first_of(__s, __pos, traits_type::length(__s));
-      }
-
-      /**
-       *  @brief  Find position of a character.
-       *  @param __c  Character to locate.
-       *  @param __pos  Index of character to search from (default 0).
-       *  @return  Index of first occurrence.
-       *
-       *  Starting from @a __pos, searches forward for the character
-       *  @a __c within this string.  If found, returns the index
-       *  where it was found.  If not found, returns npos.
-       *
-       *  Note: equivalent to find(__c, __pos).
-      */
-      size_type
-      find_first_of(_CharT __c, size_type __pos = 0) const _GLIBCXX_NOEXCEPT
-      { return this->find(__c, __pos); }
-
-#if __cplusplus >= 201703L
-      /**
-       *  @brief  Find position of a character of a string_view.
-       *  @param __svt  An object convertible to string_view containing
-       *                characters to locate.
-       *  @param __pos  Index of character to search from (default 0).
-       *  @return  Index of first occurrence.
-      */
-      template<typename _Tp>
-	_If_sv<_Tp, size_type>
-	find_first_of(const _Tp& __svt, size_type __pos = 0) const
-	noexcept(is_same<_Tp, __sv_type>::value)
-	{
-	  __sv_type __sv = __svt;
-	  return this->find_first_of(__sv.data(), __pos, __sv.size());
-	}
-#endif // C++17
-
-      /**
-       *  @brief  Find last position of a character of string.
-       *  @param __str  String containing characters to locate.
-       *  @param __pos  Index of character to search back from (default end).
-       *  @return  Index of last occurrence.
-       *
-       *  Starting from @a __pos, searches backward for one of the
-       *  characters of @a __str within this string.  If found,
-       *  returns the index where it was found.  If not found, returns
-       *  npos.
-      */
-      size_type
-      find_last_of(const basic_string& __str, size_type __pos = npos) const
-      _GLIBCXX_NOEXCEPT
-      { return this->find_last_of(__str.data(), __pos, __str.size()); }
-
-      /**
-       *  @brief  Find last position of a character of C substring.
-       *  @param __s  C string containing characters to locate.
-       *  @param __pos  Index of character to search back from.
-       *  @param __n  Number of characters from s to search for.
-       *  @return  Index of last occurrence.
-       *
-       *  Starting from @a __pos, searches backward for one of the
-       *  first @a __n characters of @a __s within this string.  If
-       *  found, returns the index where it was found.  If not found,
-       *  returns npos.
-      */
-      size_type
-      find_last_of(const _CharT* __s, size_type __pos, size_type __n) const
-      _GLIBCXX_NOEXCEPT;
-
-      /**
-       *  @brief  Find last position of a character of C string.
-       *  @param __s  C string containing characters to locate.
-       *  @param __pos  Index of character to search back from (default end).
-       *  @return  Index of last occurrence.
-       *
-       *  Starting from @a __pos, searches backward for one of the
-       *  characters of @a __s within this string.  If found, returns
-       *  the index where it was found.  If not found, returns npos.
-      */
-      size_type
-      find_last_of(const _CharT* __s, size_type __pos = npos) const
-      _GLIBCXX_NOEXCEPT
-      {
-	__glibcxx_requires_string(__s);
-	return this->find_last_of(__s, __pos, traits_type::length(__s));
-      }
-
-      /**
-       *  @brief  Find last position of a character.
-       *  @param __c  Character to locate.
-       *  @param __pos  Index of character to search back from (default end).
-       *  @return  Index of last occurrence.
-       *
-       *  Starting from @a __pos, searches backward for @a __c within
-       *  this string.  If found, returns the index where it was
-       *  found.  If not found, returns npos.
-       *
-       *  Note: equivalent to rfind(__c, __pos).
-      */
-      size_type
-      find_last_of(_CharT __c, size_type __pos = npos) const _GLIBCXX_NOEXCEPT
-      { return this->rfind(__c, __pos); }
-
-#if __cplusplus >= 201703L
-      /**
-       *  @brief  Find last position of a character of string.
-       *  @param __svt  An object convertible to string_view containing
-       *                characters to locate.
-       *  @param __pos  Index of character to search back from (default end).
-       *  @return  Index of last occurrence.
-      */
-      template<typename _Tp>
-	_If_sv<_Tp, size_type>
-	find_last_of(const _Tp& __svt, size_type __pos = npos) const
-	noexcept(is_same<_Tp, __sv_type>::value)
-	{
-	  __sv_type __sv = __svt;
-	  return this->find_last_of(__sv.data(), __pos, __sv.size());
-	}
-#endif // C++17
-
-      /**
-       *  @brief  Find position of a character not in string.
-       *  @param __str  String containing characters to avoid.
-       *  @param __pos  Index of character to search from (default 0).
-       *  @return  Index of first occurrence.
-       *
-       *  Starting from @a __pos, searches forward for a character not contained
-       *  in @a __str within this string.  If found, returns the index where it
-       *  was found.  If not found, returns npos.
-      */
-      size_type
-      find_first_not_of(const basic_string& __str, size_type __pos = 0) const
-      _GLIBCXX_NOEXCEPT
-      { return this->find_first_not_of(__str.data(), __pos, __str.size()); }
-
-      /**
-       *  @brief  Find position of a character not in C substring.
-       *  @param __s  C string containing characters to avoid.
-       *  @param __pos  Index of character to search from.
-       *  @param __n  Number of characters from __s to consider.
-       *  @return  Index of first occurrence.
-       *
-       *  Starting from @a __pos, searches forward for a character not
-       *  contained in the first @a __n characters of @a __s within
-       *  this string.  If found, returns the index where it was
-       *  found.  If not found, returns npos.
-      */
-      size_type
-      find_first_not_of(const _CharT* __s, size_type __pos,
-			size_type __n) const _GLIBCXX_NOEXCEPT;
-
-      /**
-       *  @brief  Find position of a character not in C string.
-       *  @param __s  C string containing characters to avoid.
-       *  @param __pos  Index of character to search from (default 0).
-       *  @return  Index of first occurrence.
-       *
-       *  Starting from @a __pos, searches forward for a character not
-       *  contained in @a __s within this string.  If found, returns
-       *  the index where it was found.  If not found, returns npos.
-      */
-      size_type
-      find_first_not_of(const _CharT* __s, size_type __pos = 0) const
-      _GLIBCXX_NOEXCEPT
-      {
-	__glibcxx_requires_string(__s);
-	return this->find_first_not_of(__s, __pos, traits_type::length(__s));
-      }
-
-      /**
-       *  @brief  Find position of a different character.
-       *  @param __c  Character to avoid.
-       *  @param __pos  Index of character to search from (default 0).
-       *  @return  Index of first occurrence.
-       *
-       *  Starting from @a __pos, searches forward for a character
-       *  other than @a __c within this string.  If found, returns the
-       *  index where it was found.  If not found, returns npos.
-      */
-      size_type
-      find_first_not_of(_CharT __c, size_type __pos = 0) const
-      _GLIBCXX_NOEXCEPT;
-
-#if __cplusplus >= 201703L
-      /**
-       *  @brief  Find position of a character not in a string_view.
-       *  @param __svt  An object convertible to string_view containing
-       *                characters to avoid.
-       *  @param __pos  Index of character to search from (default 0).
-       *  @return  Index of first occurrence.
-       */
-      template<typename _Tp>
-	_If_sv<_Tp, size_type>
-	find_first_not_of(const _Tp& __svt, size_type __pos = 0) const
-	noexcept(is_same<_Tp, __sv_type>::value)
-	{
-	  __sv_type __sv = __svt;
-	  return this->find_first_not_of(__sv.data(), __pos, __sv.size());
-	}
-#endif // C++17
-
-      /**
-       *  @brief  Find last position of a character not in string.
-       *  @param __str  String containing characters to avoid.
-       *  @param __pos  Index of character to search back from (default end).
-       *  @return  Index of last occurrence.
-       *
-       *  Starting from @a __pos, searches backward for a character
-       *  not contained in @a __str within this string.  If found,
-       *  returns the index where it was found.  If not found, returns
-       *  npos.
-      */
-      size_type
-      find_last_not_of(const basic_string& __str, size_type __pos = npos) const
-      _GLIBCXX_NOEXCEPT
-      { return this->find_last_not_of(__str.data(), __pos, __str.size()); }
-
-      /**
-       *  @brief  Find last position of a character not in C substring.
-       *  @param __s  C string containing characters to avoid.
-       *  @param __pos  Index of character to search back from.
-       *  @param __n  Number of characters from s to consider.
-       *  @return  Index of last occurrence.
-       *
-       *  Starting from @a __pos, searches backward for a character not
-       *  contained in the first @a __n characters of @a __s within this string.
-       *  If found, returns the index where it was found.  If not found,
-       *  returns npos.
-      */
-      size_type
-      find_last_not_of(const _CharT* __s, size_type __pos,
-		       size_type __n) const _GLIBCXX_NOEXCEPT;
-      /**
-       *  @brief  Find last position of a character not in C string.
-       *  @param __s  C string containing characters to avoid.
-       *  @param __pos  Index of character to search back from (default end).
-       *  @return  Index of last occurrence.
-       *
-       *  Starting from @a __pos, searches backward for a character
-       *  not contained in @a __s within this string.  If found,
-       *  returns the index where it was found.  If not found, returns
-       *  npos.
-      */
-      size_type
-      find_last_not_of(const _CharT* __s, size_type __pos = npos) const
-      _GLIBCXX_NOEXCEPT
-      {
-	__glibcxx_requires_string(__s);
-	return this->find_last_not_of(__s, __pos, traits_type::length(__s));
-      }
-
-      /**
-       *  @brief  Find last position of a different character.
-       *  @param __c  Character to avoid.
-       *  @param __pos  Index of character to search back from (default end).
-       *  @return  Index of last occurrence.
-       *
-       *  Starting from @a __pos, searches backward for a character other than
-       *  @a __c within this string.  If found, returns the index where it was
-       *  found.  If not found, returns npos.
-      */
-      size_type
-      find_last_not_of(_CharT __c, size_type __pos = npos) const
-      _GLIBCXX_NOEXCEPT;
-
-#if __cplusplus >= 201703L
-      /**
-       *  @brief  Find last position of a character not in a string_view.
-       *  @param __svt  An object convertible to string_view containing
-       *                characters to avoid.
-       *  @param __pos  Index of character to search back from (default end).
-       *  @return  Index of last occurrence.
-       */
-      template<typename _Tp>
-	_If_sv<_Tp, size_type>
-	find_last_not_of(const _Tp& __svt, size_type __pos = npos) const
-	noexcept(is_same<_Tp, __sv_type>::value)
-	{
-	  __sv_type __sv = __svt;
-	  return this->find_last_not_of(__sv.data(), __pos, __sv.size());
-	}
-#endif // C++17
-
-      /**
-       *  @brief  Get a substring.
-       *  @param __pos  Index of first character (default 0).
-       *  @param __n  Number of characters in substring (default remainder).
-       *  @return  The new string.
-       *  @throw  std::out_of_range  If __pos > size().
-       *
-       *  Construct and return a new string using the @a __n
-       *  characters starting at @a __pos.  If the string is too
-       *  short, use the remainder of the characters.  If @a __pos is
-       *  beyond the end of the string, out_of_range is thrown.
-      */
-      basic_string
-      substr(size_type __pos = 0, size_type __n = npos) const
-      { return basic_string(*this,
-			    _M_check(__pos, "basic_string::substr"), __n); }
-
-      /**
-       *  @brief  Compare to a string.
-       *  @param __str  String to compare against.
-       *  @return  Integer < 0, 0, or > 0.
-       *
-       *  Returns an integer < 0 if this string is ordered before @a
-       *  __str, 0 if their values are equivalent, or > 0 if this
-       *  string is ordered after @a __str.  Determines the effective
-       *  length rlen of the strings to compare as the smallest of
-       *  size() and str.size().  The function then compares the two
-       *  strings by calling traits::compare(data(), str.data(),rlen).
-       *  If the result of the comparison is nonzero returns it,
-       *  otherwise the shorter one is ordered first.
-      */
-      int
-      compare(const basic_string& __str) const
-      {
-	const size_type __size = this->size();
-	const size_type __osize = __str.size();
-	const size_type __len = std::min(__size, __osize);
-
-	int __r = traits_type::compare(_M_data(), __str.data(), __len);
-	if (!__r)
-	  __r = _S_compare(__size, __osize);
-	return __r;
-      }
-
-#if __cplusplus >= 201703L
-      /**
-       *  @brief  Compare to a string_view.
-       *  @param __svt An object convertible to string_view to compare against.
-       *  @return  Integer < 0, 0, or > 0.
-       */
-      template<typename _Tp>
-	_If_sv<_Tp, int>
-	compare(const _Tp& __svt) const
-	noexcept(is_same<_Tp, __sv_type>::value)
-	{
-	   __sv_type __sv = __svt;
-	  const size_type __size = this->size();
-	  const size_type __osize = __sv.size();
-	  const size_type __len = std::min(__size, __osize);
-
-	  int __r = traits_type::compare(_M_data(), __sv.data(), __len);
-	  if (!__r)
-	    __r = _S_compare(__size, __osize);
-	  return __r;
-	}
-
-      /**
-       *  @brief  Compare to a string_view.
-       *  @param __pos  A position in the string to start comparing from.
-       *  @param __n  The number of characters to compare.
-       *  @param __svt  An object convertible to string_view to compare
-       *                against.
-       *  @return  Integer < 0, 0, or > 0.
-       */
-      template<typename _Tp>
-	_If_sv<_Tp, int>
-	compare(size_type __pos, size_type __n, const _Tp& __svt) const
-	noexcept(is_same<_Tp, __sv_type>::value)
-	{
-	  __sv_type __sv = __svt;
-	  return __sv_type(*this).substr(__pos, __n).compare(__sv);
-	}
-
-      /**
-       *  @brief  Compare to a string_view.
-       *  @param __pos1  A position in the string to start comparing from.
-       *  @param __n1  The number of characters to compare.
-       *  @param __svt   An object convertible to string_view to compare
-       *                 against.
-       *  @param __pos2  A position in the string_view to start comparing from.
-       *  @param __n2  The number of characters to compare.
-       *  @return  Integer < 0, 0, or > 0.
-       */
-      template<typename _Tp>
-	_If_sv<_Tp, int>
-	compare(size_type __pos1, size_type __n1, const _Tp& __svt,
-		size_type __pos2, size_type __n2 = npos) const
-	noexcept(is_same<_Tp, __sv_type>::value)
-	{
-	  __sv_type __sv = __svt;
-	  return __sv_type(*this)
-	    .substr(__pos1, __n1).compare(__sv.substr(__pos2, __n2));
-	}
-#endif // C++17
-
-      /**
-       *  @brief  Compare substring to a string.
-       *  @param __pos  Index of first character of substring.
-       *  @param __n  Number of characters in substring.
-       *  @param __str  String to compare against.
-       *  @return  Integer < 0, 0, or > 0.
-       *
-       *  Form the substring of this string from the @a __n characters
-       *  starting at @a __pos.  Returns an integer < 0 if the
-       *  substring is ordered before @a __str, 0 if their values are
-       *  equivalent, or > 0 if the substring is ordered after @a
-       *  __str.  Determines the effective length rlen of the strings
-       *  to compare as the smallest of the length of the substring
-       *  and @a __str.size().  The function then compares the two
-       *  strings by calling
-       *  traits::compare(substring.data(),str.data(),rlen).  If the
-       *  result of the comparison is nonzero returns it, otherwise
-       *  the shorter one is ordered first.
-      */
-      int
-      compare(size_type __pos, size_type __n, const basic_string& __str) const;
-
-      /**
-       *  @brief  Compare substring to a substring.
-       *  @param __pos1  Index of first character of substring.
-       *  @param __n1  Number of characters in substring.
-       *  @param __str  String to compare against.
-       *  @param __pos2  Index of first character of substring of str.
-       *  @param __n2  Number of characters in substring of str.
-       *  @return  Integer < 0, 0, or > 0.
-       *
-       *  Form the substring of this string from the @a __n1
-       *  characters starting at @a __pos1.  Form the substring of @a
-       *  __str from the @a __n2 characters starting at @a __pos2.
-       *  Returns an integer < 0 if this substring is ordered before
-       *  the substring of @a __str, 0 if their values are equivalent,
-       *  or > 0 if this substring is ordered after the substring of
-       *  @a __str.  Determines the effective length rlen of the
-       *  strings to compare as the smallest of the lengths of the
-       *  substrings.  The function then compares the two strings by
-       *  calling
-       *  traits::compare(substring.data(),str.substr(pos2,n2).data(),rlen).
-       *  If the result of the comparison is nonzero returns it,
-       *  otherwise the shorter one is ordered first.
-      */
-      int
-      compare(size_type __pos1, size_type __n1, const basic_string& __str,
-	      size_type __pos2, size_type __n2 = npos) const;
-
-      /**
-       *  @brief  Compare to a C string.
-       *  @param __s  C string to compare against.
-       *  @return  Integer < 0, 0, or > 0.
-       *
-       *  Returns an integer < 0 if this string is ordered before @a __s, 0 if
-       *  their values are equivalent, or > 0 if this string is ordered after
-       *  @a __s.  Determines the effective length rlen of the strings to
-       *  compare as the smallest of size() and the length of a string
-       *  constructed from @a __s.  The function then compares the two strings
-       *  by calling traits::compare(data(),s,rlen).  If the result of the
-       *  comparison is nonzero returns it, otherwise the shorter one is
-       *  ordered first.
-      */
-      int
-      compare(const _CharT* __s) const _GLIBCXX_NOEXCEPT;
-
-      // _GLIBCXX_RESOLVE_LIB_DEFECTS
-      // 5 String::compare specification questionable
-      /**
-       *  @brief  Compare substring to a C string.
-       *  @param __pos  Index of first character of substring.
-       *  @param __n1  Number of characters in substring.
-       *  @param __s  C string to compare against.
-       *  @return  Integer < 0, 0, or > 0.
-       *
-       *  Form the substring of this string from the @a __n1
-       *  characters starting at @a pos.  Returns an integer < 0 if
-       *  the substring is ordered before @a __s, 0 if their values
-       *  are equivalent, or > 0 if the substring is ordered after @a
-       *  __s.  Determines the effective length rlen of the strings to
-       *  compare as the smallest of the length of the substring and
-       *  the length of a string constructed from @a __s.  The
-       *  function then compares the two string by calling
-       *  traits::compare(substring.data(),__s,rlen).  If the result of
-       *  the comparison is nonzero returns it, otherwise the shorter
-       *  one is ordered first.
-      */
-      int
-      compare(size_type __pos, size_type __n1, const _CharT* __s) const;
-
-      /**
-       *  @brief  Compare substring against a character %array.
-       *  @param __pos  Index of first character of substring.
-       *  @param __n1  Number of characters in substring.
-       *  @param __s  character %array to compare against.
-       *  @param __n2  Number of characters of s.
-       *  @return  Integer < 0, 0, or > 0.
-       *
-       *  Form the substring of this string from the @a __n1
-       *  characters starting at @a __pos.  Form a string from the
-       *  first @a __n2 characters of @a __s.  Returns an integer < 0
-       *  if this substring is ordered before the string from @a __s,
-       *  0 if their values are equivalent, or > 0 if this substring
-       *  is ordered after the string from @a __s.  Determines the
-       *  effective length rlen of the strings to compare as the
-       *  smallest of the length of the substring and @a __n2.  The
-       *  function then compares the two strings by calling
-       *  traits::compare(substring.data(),s,rlen).  If the result of
-       *  the comparison is nonzero returns it, otherwise the shorter
-       *  one is ordered first.
-       *
-       *  NB: s must have at least n2 characters, &apos;\\0&apos; has
-       *  no special meaning.
-      */
-      int
-      compare(size_type __pos, size_type __n1, const _CharT* __s,
-	      size_type __n2) const;
-
-#if __cplusplus > 201703L
-      bool
-      starts_with(basic_string_view<_CharT, _Traits> __x) const noexcept
-      { return __sv_type(this->data(), this->size()).starts_with(__x); }
-
-      bool
-      starts_with(_CharT __x) const noexcept
-      { return __sv_type(this->data(), this->size()).starts_with(__x); }
-
-      bool
-      starts_with(const _CharT* __x) const noexcept
-      { return __sv_type(this->data(), this->size()).starts_with(__x); }
-
-      bool
-      ends_with(basic_string_view<_CharT, _Traits> __x) const noexcept
-      { return __sv_type(this->data(), this->size()).ends_with(__x); }
-
-      bool
-      ends_with(_CharT __x) const noexcept
-      { return __sv_type(this->data(), this->size()).ends_with(__x); }
-
-      bool
-      ends_with(const _CharT* __x) const noexcept
-      { return __sv_type(this->data(), this->size()).ends_with(__x); }
-#endif // C++20
-
-#if __cplusplus >= 202011L \
-  || (__cplusplus == 202002L && !defined __STRICT_ANSI__)
-      bool
-      contains(basic_string_view<_CharT, _Traits> __x) const noexcept
-      { return __sv_type(this->data(), this->size()).contains(__x); }
-
-      bool
-      contains(_CharT __x) const noexcept
-      { return __sv_type(this->data(), this->size()).contains(__x); }
-
-      bool
-      contains(const _CharT* __x) const noexcept
-      { return __sv_type(this->data(), this->size()).contains(__x); }
-#endif // C++23
-
-# ifdef _GLIBCXX_TM_TS_INTERNAL
-      friend void
-      ::_txnal_cow_string_C1_for_exceptions(void* that, const char* s,
-					    void* exc);
-      friend const char*
-      ::_txnal_cow_string_c_str(const void *that);
-      friend void
-      ::_txnal_cow_string_D1(void *that);
-      friend void
-      ::_txnal_cow_string_D1_commit(void *that);
-# endif
-  };
-#endif  // !_GLIBCXX_USE_CXX11_ABI
+namespace std _GLIBCXX_VISIBILITY(default)
+{
+_GLIBCXX_BEGIN_NAMESPACE_VERSION
 
 #if __cpp_deduction_guides >= 201606
 _GLIBCXX_BEGIN_NAMESPACE_CXX11
diff --git a/libstdc++-v3/include/bits/basic_string.tcc b/libstdc++-v3/include/bits/basic_string.tcc
index 0c13e744747..371f1c3ccee 100644
--- a/libstdc++-v3/include/bits/basic_string.tcc
+++ b/libstdc++-v3/include/bits/basic_string.tcc
@@ -515,654 +515,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       return __n;
     }
 
-#else  // !_GLIBCXX_USE_CXX11_ABI
-
-  template<typename _CharT, typename _Traits, typename _Alloc>
-    const typename basic_string<_CharT, _Traits, _Alloc>::size_type
-    basic_string<_CharT, _Traits, _Alloc>::
-    _Rep::_S_max_size = (((npos - sizeof(_Rep_base))/sizeof(_CharT)) - 1) / 4;
-
-  template<typename _CharT, typename _Traits, typename _Alloc>
-    const _CharT
-    basic_string<_CharT, _Traits, _Alloc>::
-    _Rep::_S_terminal = _CharT();
-
-  template<typename _CharT, typename _Traits, typename _Alloc>
-    const typename basic_string<_CharT, _Traits, _Alloc>::size_type
-    basic_string<_CharT, _Traits, _Alloc>::npos;
-
-  // Linker sets _S_empty_rep_storage to all 0s (one reference, empty string)
-  // at static init time (before static ctors are run).
-  template<typename _CharT, typename _Traits, typename _Alloc>
-    typename basic_string<_CharT, _Traits, _Alloc>::size_type
-    basic_string<_CharT, _Traits, _Alloc>::_Rep::_S_empty_rep_storage[
-    (sizeof(_Rep_base) + sizeof(_CharT) + sizeof(size_type) - 1) /
-      sizeof(size_type)];
-
-  // NB: This is the special case for Input Iterators, used in
-  // istreambuf_iterators, etc.
-  // Input Iterators have a cost structure very different from
-  // pointers, calling for a different coding style.
-  template<typename _CharT, typename _Traits, typename _Alloc>
-    template<typename _InIterator>
-      _CharT*
-      basic_string<_CharT, _Traits, _Alloc>::
-      _S_construct(_InIterator __beg, _InIterator __end, const _Alloc& __a,
-		   input_iterator_tag)
-      {
-#if _GLIBCXX_FULLY_DYNAMIC_STRING == 0
-	if (__beg == __end && __a == _Alloc())
-	  return _S_empty_rep()._M_refdata();
-#endif
-	// Avoid reallocation for common case.
-	_CharT __buf[128];
-	size_type __len = 0;
-	while (__beg != __end && __len < sizeof(__buf) / sizeof(_CharT))
-	  {
-	    __buf[__len++] = *__beg;
-	    ++__beg;
-	  }
-	_Rep* __r = _Rep::_S_create(__len, size_type(0), __a);
-	_M_copy(__r->_M_refdata(), __buf, __len);
-	__try
-	  {
-	    while (__beg != __end)
-	      {
-		if (__len == __r->_M_capacity)
-		  {
-		    // Allocate more space.
-		    _Rep* __another = _Rep::_S_create(__len + 1, __len, __a);
-		    _M_copy(__another->_M_refdata(), __r->_M_refdata(), __len);
-		    __r->_M_destroy(__a);
-		    __r = __another;
-		  }
-		__r->_M_refdata()[__len++] = *__beg;
-		++__beg;
-	      }
-	  }
-	__catch(...)
-	  {
-	    __r->_M_destroy(__a);
-	    __throw_exception_again;
-	  }
-	__r->_M_set_length_and_sharable(__len);
-	return __r->_M_refdata();
-      }
-
-  template<typename _CharT, typename _Traits, typename _Alloc>
-    template <typename _InIterator>
-      _CharT*
-      basic_string<_CharT, _Traits, _Alloc>::
-      _S_construct(_InIterator __beg, _InIterator __end, const _Alloc& __a,
-		   forward_iterator_tag)
-      {
-#if _GLIBCXX_FULLY_DYNAMIC_STRING == 0
-	if (__beg == __end && __a == _Alloc())
-	  return _S_empty_rep()._M_refdata();
-#endif
-	// NB: Not required, but considered best practice.
-	if (__gnu_cxx::__is_null_pointer(__beg) && __beg != __end)
-	  __throw_logic_error(__N("basic_string::_S_construct null not valid"));
-
-	const size_type __dnew = static_cast<size_type>(std::distance(__beg,
-								      __end));
-	// Check for out_of_range and length_error exceptions.
-	_Rep* __r = _Rep::_S_create(__dnew, size_type(0), __a);
-	__try
-	  { _S_copy_chars(__r->_M_refdata(), __beg, __end); }
-	__catch(...)
-	  {
-	    __r->_M_destroy(__a);
-	    __throw_exception_again;
-	  }
-	__r->_M_set_length_and_sharable(__dnew);
-	return __r->_M_refdata();
-      }
-
-  template<typename _CharT, typename _Traits, typename _Alloc>
-    _CharT*
-    basic_string<_CharT, _Traits, _Alloc>::
-    _S_construct(size_type __n, _CharT __c, const _Alloc& __a)
-    {
-#if _GLIBCXX_FULLY_DYNAMIC_STRING == 0
-      if (__n == 0 && __a == _Alloc())
-	return _S_empty_rep()._M_refdata();
-#endif
-      // Check for out_of_range and length_error exceptions.
-      _Rep* __r = _Rep::_S_create(__n, size_type(0), __a);
-      if (__n)
-	_M_assign(__r->_M_refdata(), __n, __c);
-
-      __r->_M_set_length_and_sharable(__n);
-      return __r->_M_refdata();
-    }
-
-  template<typename _CharT, typename _Traits, typename _Alloc>
-    basic_string<_CharT, _Traits, _Alloc>::
-    basic_string(const basic_string& __str, size_type __pos, const _Alloc& __a)
-    : _M_dataplus(_S_construct(__str._M_data()
-			       + __str._M_check(__pos,
-						"basic_string::basic_string"),
-			       __str._M_data() + __str._M_limit(__pos, npos)
-			       + __pos, __a), __a)
-    { }
-
-  template<typename _CharT, typename _Traits, typename _Alloc>
-    basic_string<_CharT, _Traits, _Alloc>::
-    basic_string(const basic_string& __str, size_type __pos, size_type __n)
-    : _M_dataplus(_S_construct(__str._M_data()
-			       + __str._M_check(__pos,
-						"basic_string::basic_string"),
-			       __str._M_data() + __str._M_limit(__pos, __n)
-			       + __pos, _Alloc()), _Alloc())
-    { }
-
-  template<typename _CharT, typename _Traits, typename _Alloc>
-    basic_string<_CharT, _Traits, _Alloc>::
-    basic_string(const basic_string& __str, size_type __pos,
-		 size_type __n, const _Alloc& __a)
-    : _M_dataplus(_S_construct(__str._M_data()
-			       + __str._M_check(__pos,
-						"basic_string::basic_string"),
-			       __str._M_data() + __str._M_limit(__pos, __n)
-			       + __pos, __a), __a)
-    { }
-
-  template<typename _CharT, typename _Traits, typename _Alloc>
-    basic_string<_CharT, _Traits, _Alloc>&
-    basic_string<_CharT, _Traits, _Alloc>::
-    assign(const basic_string& __str)
-    {
-      if (_M_rep() != __str._M_rep())
-	{
-	  // XXX MT
-	  const allocator_type __a = this->get_allocator();
-	  _CharT* __tmp = __str._M_rep()->_M_grab(__a, __str.get_allocator());
-	  _M_rep()->_M_dispose(__a);
-	  _M_data(__tmp);
-	}
-      return *this;
-    }
-
-  template<typename _CharT, typename _Traits, typename _Alloc>
-    basic_string<_CharT, _Traits, _Alloc>&
-    basic_string<_CharT, _Traits, _Alloc>::
-    assign(const _CharT* __s, size_type __n)
-    {
-      __glibcxx_requires_string_len(__s, __n);
-      _M_check_length(this->size(), __n, "basic_string::assign");
-      if (_M_disjunct(__s) || _M_rep()->_M_is_shared())
-	return _M_replace_safe(size_type(0), this->size(), __s, __n);
-      else
-	{
-	  // Work in-place.
-	  const size_type __pos = __s - _M_data();
-	  if (__pos >= __n)
-	    _M_copy(_M_data(), __s, __n);
-	  else if (__pos)
-	    _M_move(_M_data(), __s, __n);
-	  _M_rep()->_M_set_length_and_sharable(__n);
-	  return *this;
-	}
-     }
-
-  template<typename _CharT, typename _Traits, typename _Alloc>
-    basic_string<_CharT, _Traits, _Alloc>&
-    basic_string<_CharT, _Traits, _Alloc>::
-    append(size_type __n, _CharT __c)
-    {
-      if (__n)
-	{
-	  _M_check_length(size_type(0), __n, "basic_string::append");	  
-	  const size_type __len = __n + this->size();
-	  if (__len > this->capacity() || _M_rep()->_M_is_shared())
-	    this->reserve(__len);
-	  _M_assign(_M_data() + this->size(), __n, __c);
-	  _M_rep()->_M_set_length_and_sharable(__len);
-	}
-      return *this;
-    }
-
-  template<typename _CharT, typename _Traits, typename _Alloc>
-    basic_string<_CharT, _Traits, _Alloc>&
-    basic_string<_CharT, _Traits, _Alloc>::
-    append(const _CharT* __s, size_type __n)
-    {
-      __glibcxx_requires_string_len(__s, __n);
-      if (__n)
-	{
-	  _M_check_length(size_type(0), __n, "basic_string::append");
-	  const size_type __len = __n + this->size();
-	  if (__len > this->capacity() || _M_rep()->_M_is_shared())
-	    {
-	      if (_M_disjunct(__s))
-		this->reserve(__len);
-	      else
-		{
-		  const size_type __off = __s - _M_data();
-		  this->reserve(__len);
-		  __s = _M_data() + __off;
-		}
-	    }
-	  _M_copy(_M_data() + this->size(), __s, __n);
-	  _M_rep()->_M_set_length_and_sharable(__len);
-	}
-      return *this;
-    }
-
-  template<typename _CharT, typename _Traits, typename _Alloc>
-    basic_string<_CharT, _Traits, _Alloc>&
-    basic_string<_CharT, _Traits, _Alloc>::
-    append(const basic_string& __str)
-    {
-      const size_type __size = __str.size();
-      if (__size)
-	{
-	  const size_type __len = __size + this->size();
-	  if (__len > this->capacity() || _M_rep()->_M_is_shared())
-	    this->reserve(__len);
-	  _M_copy(_M_data() + this->size(), __str._M_data(), __size);
-	  _M_rep()->_M_set_length_and_sharable(__len);
-	}
-      return *this;
-    }    
-
-  template<typename _CharT, typename _Traits, typename _Alloc>
-    basic_string<_CharT, _Traits, _Alloc>&
-    basic_string<_CharT, _Traits, _Alloc>::
-    append(const basic_string& __str, size_type __pos, size_type __n)
-    {
-      __str._M_check(__pos, "basic_string::append");
-      __n = __str._M_limit(__pos, __n);
-      if (__n)
-	{
-	  const size_type __len = __n + this->size();
-	  if (__len > this->capacity() || _M_rep()->_M_is_shared())
-	    this->reserve(__len);
-	  _M_copy(_M_data() + this->size(), __str._M_data() + __pos, __n);
-	  _M_rep()->_M_set_length_and_sharable(__len);	  
-	}
-      return *this;
-    }
-
-   template<typename _CharT, typename _Traits, typename _Alloc>
-     basic_string<_CharT, _Traits, _Alloc>&
-     basic_string<_CharT, _Traits, _Alloc>::
-     insert(size_type __pos, const _CharT* __s, size_type __n)
-     {
-       __glibcxx_requires_string_len(__s, __n);
-       _M_check(__pos, "basic_string::insert");
-       _M_check_length(size_type(0), __n, "basic_string::insert");
-       if (_M_disjunct(__s) || _M_rep()->_M_is_shared())
-         return _M_replace_safe(__pos, size_type(0), __s, __n);
-       else
-         {
-           // Work in-place.
-           const size_type __off = __s - _M_data();
-           _M_mutate(__pos, 0, __n);
-           __s = _M_data() + __off;
-           _CharT* __p = _M_data() + __pos;
-           if (__s  + __n <= __p)
-             _M_copy(__p, __s, __n);
-           else if (__s >= __p)
-             _M_copy(__p, __s + __n, __n);
-           else
-             {
-	       const size_type __nleft = __p - __s;
-               _M_copy(__p, __s, __nleft);
-               _M_copy(__p + __nleft, __p + __n, __n - __nleft);
-             }
-           return *this;
-         }
-     }
-
-   template<typename _CharT, typename _Traits, typename _Alloc>
-     typename basic_string<_CharT, _Traits, _Alloc>::iterator
-     basic_string<_CharT, _Traits, _Alloc>::
-     erase(iterator __first, iterator __last)
-     {
-       _GLIBCXX_DEBUG_PEDASSERT(__first >= _M_ibegin() && __first <= __last
-				&& __last <= _M_iend());
-
-       // NB: This isn't just an optimization (bail out early when
-       // there is nothing to do, really), it's also a correctness
-       // issue vs MT, see libstdc++/40518.
-       const size_type __size = __last - __first;
-       if (__size)
-	 {
-	   const size_type __pos = __first - _M_ibegin();
-	   _M_mutate(__pos, __size, size_type(0));
-	   _M_rep()->_M_set_leaked();
-	   return iterator(_M_data() + __pos);
-	 }
-       else
-	 return __first;
-     }
-
-   template<typename _CharT, typename _Traits, typename _Alloc>
-     basic_string<_CharT, _Traits, _Alloc>&
-     basic_string<_CharT, _Traits, _Alloc>::
-     replace(size_type __pos, size_type __n1, const _CharT* __s,
-	     size_type __n2)
-     {
-       __glibcxx_requires_string_len(__s, __n2);
-       _M_check(__pos, "basic_string::replace");
-       __n1 = _M_limit(__pos, __n1);
-       _M_check_length(__n1, __n2, "basic_string::replace");
-       bool __left;
-       if (_M_disjunct(__s) || _M_rep()->_M_is_shared())
-         return _M_replace_safe(__pos, __n1, __s, __n2);
-       else if ((__left = __s + __n2 <= _M_data() + __pos)
-		|| _M_data() + __pos + __n1 <= __s)
-	 {
-	   // Work in-place: non-overlapping case.
-	   size_type __off = __s - _M_data();
-	   __left ? __off : (__off += __n2 - __n1);
-	   _M_mutate(__pos, __n1, __n2);
-	   _M_copy(_M_data() + __pos, _M_data() + __off, __n2);
-	   return *this;
-	 }
-       else
-	 {
-	   // Todo: overlapping case.
-	   const basic_string __tmp(__s, __n2);
-	   return _M_replace_safe(__pos, __n1, __tmp._M_data(), __n2);
-	 }
-     }
-
-  template<typename _CharT, typename _Traits, typename _Alloc>
-    void
-    basic_string<_CharT, _Traits, _Alloc>::_Rep::
-    _M_destroy(const _Alloc& __a) throw ()
-    {
-      const size_type __size = sizeof(_Rep_base) +
-	                       (this->_M_capacity + 1) * sizeof(_CharT);
-      _Raw_bytes_alloc(__a).deallocate(reinterpret_cast<char*>(this), __size);
-    }
-
-  template<typename _CharT, typename _Traits, typename _Alloc>
-    void
-    basic_string<_CharT, _Traits, _Alloc>::
-    _M_leak_hard()
-    {
-#if _GLIBCXX_FULLY_DYNAMIC_STRING == 0
-      if (_M_rep() == &_S_empty_rep())
-	return;
-#endif
-      if (_M_rep()->_M_is_shared())
-	_M_mutate(0, 0, 0);
-      _M_rep()->_M_set_leaked();
-    }
-
-  template<typename _CharT, typename _Traits, typename _Alloc>
-    void
-    basic_string<_CharT, _Traits, _Alloc>::
-    _M_mutate(size_type __pos, size_type __len1, size_type __len2)
-    {
-      const size_type __old_size = this->size();
-      const size_type __new_size = __old_size + __len2 - __len1;
-      const size_type __how_much = __old_size - __pos - __len1;
-
-      if (__new_size > this->capacity() || _M_rep()->_M_is_shared())
-	{
-	  // Must reallocate.
-	  const allocator_type __a = get_allocator();
-	  _Rep* __r = _Rep::_S_create(__new_size, this->capacity(), __a);
-
-	  if (__pos)
-	    _M_copy(__r->_M_refdata(), _M_data(), __pos);
-	  if (__how_much)
-	    _M_copy(__r->_M_refdata() + __pos + __len2,
-		    _M_data() + __pos + __len1, __how_much);
-
-	  _M_rep()->_M_dispose(__a);
-	  _M_data(__r->_M_refdata());
-	}
-      else if (__how_much && __len1 != __len2)
-	{
-	  // Work in-place.
-	  _M_move(_M_data() + __pos + __len2,
-		  _M_data() + __pos + __len1, __how_much);
-	}
-      _M_rep()->_M_set_length_and_sharable(__new_size);
-    }
-
-  template<typename _CharT, typename _Traits, typename _Alloc>
-    void
-    basic_string<_CharT, _Traits, _Alloc>::
-    reserve(size_type __res)
-    {
-      const size_type __capacity = capacity();
-
-      // _GLIBCXX_RESOLVE_LIB_DEFECTS
-      // 2968. Inconsistencies between basic_string reserve and
-      // vector/unordered_map/unordered_set reserve functions
-      // P0966 reserve should not shrink
-      if (__res <= __capacity)
-	{
-	  if (!_M_rep()->_M_is_shared())
-	    return;
-
-	  // unshare, but keep same capacity
-	  __res = __capacity;
-	}
-
-      const allocator_type __a = get_allocator();
-      _CharT* __tmp = _M_rep()->_M_clone(__a, __res - this->size());
-      _M_rep()->_M_dispose(__a);
-      _M_data(__tmp);
-    }
-
-  template<typename _CharT, typename _Traits, typename _Alloc>
-    void
-    basic_string<_CharT, _Traits, _Alloc>::
-    swap(basic_string& __s)
-    _GLIBCXX_NOEXCEPT_IF(allocator_traits<_Alloc>::is_always_equal::value)
-    {
-      if (_M_rep()->_M_is_leaked())
-	_M_rep()->_M_set_sharable();
-      if (__s._M_rep()->_M_is_leaked())
-	__s._M_rep()->_M_set_sharable();
-      if (this->get_allocator() == __s.get_allocator())
-	{
-	  _CharT* __tmp = _M_data();
-	  _M_data(__s._M_data());
-	  __s._M_data(__tmp);
-	}
-      // The code below can usually be optimized away.
-      else
-	{
-	  const basic_string __tmp1(_M_ibegin(), _M_iend(),
-				    __s.get_allocator());
-	  const basic_string __tmp2(__s._M_ibegin(), __s._M_iend(),
-				    this->get_allocator());
-	  *this = __tmp2;
-	  __s = __tmp1;
-	}
-    }
-
-  template<typename _CharT, typename _Traits, typename _Alloc>
-    typename basic_string<_CharT, _Traits, _Alloc>::_Rep*
-    basic_string<_CharT, _Traits, _Alloc>::_Rep::
-    _S_create(size_type __capacity, size_type __old_capacity,
-	      const _Alloc& __alloc)
-    {
-      // _GLIBCXX_RESOLVE_LIB_DEFECTS
-      // 83.  String::npos vs. string::max_size()
-      if (__capacity > _S_max_size)
-	__throw_length_error(__N("basic_string::_S_create"));
-
-      // The standard places no restriction on allocating more memory
-      // than is strictly needed within this layer at the moment or as
-      // requested by an explicit application call to reserve(n).
-
-      // Many malloc implementations perform quite poorly when an
-      // application attempts to allocate memory in a stepwise fashion
-      // growing each allocation size by only 1 char.  Additionally,
-      // it makes little sense to allocate less linear memory than the
-      // natural blocking size of the malloc implementation.
-      // Unfortunately, we would need a somewhat low-level calculation
-      // with tuned parameters to get this perfect for any particular
-      // malloc implementation.  Fortunately, generalizations about
-      // common features seen among implementations seems to suffice.
-
-      // __pagesize need not match the actual VM page size for good
-      // results in practice, thus we pick a common value on the low
-      // side.  __malloc_header_size is an estimate of the amount of
-      // overhead per memory allocation (in practice seen N * sizeof
-      // (void*) where N is 0, 2 or 4).  According to folklore,
-      // picking this value on the high side is better than
-      // low-balling it (especially when this algorithm is used with
-      // malloc implementations that allocate memory blocks rounded up
-      // to a size which is a power of 2).
-      const size_type __pagesize = 4096;
-      const size_type __malloc_header_size = 4 * sizeof(void*);
-
-      // The below implements an exponential growth policy, necessary to
-      // meet amortized linear time requirements of the library: see
-      // http://gcc.gnu.org/ml/libstdc++/2001-07/msg00085.html.
-      // It's active for allocations requiring an amount of memory above
-      // system pagesize. This is consistent with the requirements of the
-      // standard: http://gcc.gnu.org/ml/libstdc++/2001-07/msg00130.html
-      if (__capacity > __old_capacity && __capacity < 2 * __old_capacity)
-	__capacity = 2 * __old_capacity;
-
-      // NB: Need an array of char_type[__capacity], plus a terminating
-      // null char_type() element, plus enough for the _Rep data structure.
-      // Whew. Seemingly so needy, yet so elemental.
-      size_type __size = (__capacity + 1) * sizeof(_CharT) + sizeof(_Rep);
-
-      const size_type __adj_size = __size + __malloc_header_size;
-      if (__adj_size > __pagesize && __capacity > __old_capacity)
-	{
-	  const size_type __extra = __pagesize - __adj_size % __pagesize;
-	  __capacity += __extra / sizeof(_CharT);
-	  // Never allocate a string bigger than _S_max_size.
-	  if (__capacity > _S_max_size)
-	    __capacity = _S_max_size;
-	  __size = (__capacity + 1) * sizeof(_CharT) + sizeof(_Rep);
-	}
-
-      // NB: Might throw, but no worries about a leak, mate: _Rep()
-      // does not throw.
-      void* __place = _Raw_bytes_alloc(__alloc).allocate(__size);
-      _Rep *__p = new (__place) _Rep;
-      __p->_M_capacity = __capacity;
-      // ABI compatibility - 3.4.x set in _S_create both
-      // _M_refcount and _M_length.  All callers of _S_create
-      // in basic_string.tcc then set just _M_length.
-      // In 4.0.x and later both _M_refcount and _M_length
-      // are initialized in the callers, unfortunately we can
-      // have 3.4.x compiled code with _S_create callers inlined
-      // calling 4.0.x+ _S_create.
-      __p->_M_set_sharable();
-      return __p;
-    }
-
-  template<typename _CharT, typename _Traits, typename _Alloc>
-    _CharT*
-    basic_string<_CharT, _Traits, _Alloc>::_Rep::
-    _M_clone(const _Alloc& __alloc, size_type __res)
-    {
-      // Requested capacity of the clone.
-      const size_type __requested_cap = this->_M_length + __res;
-      _Rep* __r = _Rep::_S_create(__requested_cap, this->_M_capacity,
-				  __alloc);
-      if (this->_M_length)
-	_M_copy(__r->_M_refdata(), _M_refdata(), this->_M_length);
-
-      __r->_M_set_length_and_sharable(this->_M_length);
-      return __r->_M_refdata();
-    }
-
-  template<typename _CharT, typename _Traits, typename _Alloc>
-    void
-    basic_string<_CharT, _Traits, _Alloc>::
-    resize(size_type __n, _CharT __c)
-    {
-      const size_type __size = this->size();
-      _M_check_length(__size, __n, "basic_string::resize");
-      if (__size < __n)
-	this->append(__n - __size, __c);
-      else if (__n < __size)
-	this->erase(__n);
-      // else nothing (in particular, avoid calling _M_mutate() unnecessarily.)
-    }
-
-  template<typename _CharT, typename _Traits, typename _Alloc>
-    template<typename _InputIterator>
-      basic_string<_CharT, _Traits, _Alloc>&
-      basic_string<_CharT, _Traits, _Alloc>::
-      _M_replace_dispatch(iterator __i1, iterator __i2, _InputIterator __k1,
-			  _InputIterator __k2, __false_type)
-      {
-	const basic_string __s(__k1, __k2);
-	const size_type __n1 = __i2 - __i1;
-	_M_check_length(__n1, __s.size(), "basic_string::_M_replace_dispatch");
-	return _M_replace_safe(__i1 - _M_ibegin(), __n1, __s._M_data(),
-			       __s.size());
-      }
-
-  template<typename _CharT, typename _Traits, typename _Alloc>
-    basic_string<_CharT, _Traits, _Alloc>&
-    basic_string<_CharT, _Traits, _Alloc>::
-    _M_replace_aux(size_type __pos1, size_type __n1, size_type __n2,
-		   _CharT __c)
-    {
-      _M_check_length(__n1, __n2, "basic_string::_M_replace_aux");
-      _M_mutate(__pos1, __n1, __n2);
-      if (__n2)
-	_M_assign(_M_data() + __pos1, __n2, __c);
-      return *this;
-    }
-
-  template<typename _CharT, typename _Traits, typename _Alloc>
-    basic_string<_CharT, _Traits, _Alloc>&
-    basic_string<_CharT, _Traits, _Alloc>::
-    _M_replace_safe(size_type __pos1, size_type __n1, const _CharT* __s,
-		    size_type __n2)
-    {
-      _M_mutate(__pos1, __n1, __n2);
-      if (__n2)
-	_M_copy(_M_data() + __pos1, __s, __n2);
-      return *this;
-    }
-
-  template<typename _CharT, typename _Traits, typename _Alloc>
-    void
-    basic_string<_CharT, _Traits, _Alloc>::
-    reserve()
-    {
-#if __cpp_exceptions
-      if (length() < capacity() || _M_rep()->_M_is_shared())
-	try
-	  {
-	    const allocator_type __a = get_allocator();
-	    _CharT* __tmp = _M_rep()->_M_clone(__a);
-	    _M_rep()->_M_dispose(__a);
-	    _M_data(__tmp);
-	  }
-	catch (const __cxxabiv1::__forced_unwind&)
-	  { throw; }
-	catch (...)
-	  { /* swallow the exception */ }
-#endif
-    }
-
-    template<typename _CharT, typename _Traits, typename _Alloc>
-    typename basic_string<_CharT, _Traits, _Alloc>::size_type
-    basic_string<_CharT, _Traits, _Alloc>::
-    copy(_CharT* __s, size_type __n, size_type __pos) const
-    {
-      _M_check(__pos, "basic_string::copy");
-      __n = _M_limit(__pos, __n);
-      __glibcxx_requires_string_len(__s, __n);
-      if (__n)
-	_M_copy(__s, _M_data() + __pos, __n);
-      // 21.3.5.7 par 3: do not append null.  (good.)
-      return __n;
-    }
-#endif  // !_GLIBCXX_USE_CXX11_ABI
+#endif  // _GLIBCXX_USE_CXX11_ABI
    
   template<typename _CharT, typename _Traits, typename _Alloc>
     basic_string<_CharT, _Traits, _Alloc>
diff --git a/libstdc++-v3/include/bits/cow_string.h b/libstdc++-v3/include/bits/cow_string.h
new file mode 100644
index 00000000000..53532657492
--- /dev/null
+++ b/libstdc++-v3/include/bits/cow_string.h
@@ -0,0 +1,3642 @@
+// Definition of gcc4-compatible Copy-on-Write basic_string -*- C++ -*-
+
+// Copyright (C) 1997-2021 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library.  This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3, or (at your option)
+// any later version.
+
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// Under Section 7 of GPL version 3, you are granted additional
+// permissions described in the GCC Runtime Library Exception, version
+// 3.1, as published by the Free Software Foundation.
+
+// You should have received a copy of the GNU General Public License and
+// a copy of the GCC Runtime Library Exception along with this program;
+// see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
+// <http://www.gnu.org/licenses/>.
+
+/** @file bits/cow_string.h
+ *  This is an internal header file, included by other library headers.
+ *  Do not attempt to use it directly. @headername{string}
+ *
+ *  Defines the reference-counted COW string implentation.
+ */
+
+#ifndef _COW_STRING_H
+#define _COW_STRING_H 1
+
+#if ! _GLIBCXX_USE_CXX11_ABI
+
+#ifdef __cpp_lib_is_constant_evaluated
+// Support P1032R1 in C++20 (but not P0980R1 yet).
+# define __cpp_lib_constexpr_string 201811L
+#elif __cplusplus >= 201703L && _GLIBCXX_HAVE_BUILTIN_IS_CONSTANT_EVALUATED
+// Support P0426R1 changes to char_traits in C++17.
+# define __cpp_lib_constexpr_string 201611L
+#endif
+
+namespace std _GLIBCXX_VISIBILITY(default)
+{
+_GLIBCXX_BEGIN_NAMESPACE_VERSION
+
+  /**
+   *  @class basic_string basic_string.h <string>
+   *  @brief  Managing sequences of characters and character-like objects.
+   *
+   *  @ingroup strings
+   *  @ingroup sequences
+   *
+   *  @tparam _CharT  Type of character
+   *  @tparam _Traits  Traits for character type, defaults to
+   *                   char_traits<_CharT>.
+   *  @tparam _Alloc  Allocator type, defaults to allocator<_CharT>.
+   *
+   *  Meets the requirements of a <a href="tables.html#65">container</a>, a
+   *  <a href="tables.html#66">reversible container</a>, and a
+   *  <a href="tables.html#67">sequence</a>.  Of the
+   *  <a href="tables.html#68">optional sequence requirements</a>, only
+   *  @c push_back, @c at, and @c %array access are supported.
+   *
+   *  @doctodo
+   *
+   *
+   *  Documentation?  What's that?
+   *  Nathan Myers <ncm@cantrip.org>.
+   *
+   *  A string looks like this:
+   *
+   *  @code
+   *                                        [_Rep]
+   *                                        _M_length
+   *   [basic_string<char_type>]            _M_capacity
+   *   _M_dataplus                          _M_refcount
+   *   _M_p ---------------->               unnamed array of char_type
+   *  @endcode
+   *
+   *  Where the _M_p points to the first character in the string, and
+   *  you cast it to a pointer-to-_Rep and subtract 1 to get a
+   *  pointer to the header.
+   *
+   *  This approach has the enormous advantage that a string object
+   *  requires only one allocation.  All the ugliness is confined
+   *  within a single %pair of inline functions, which each compile to
+   *  a single @a add instruction: _Rep::_M_data(), and
+   *  string::_M_rep(); and the allocation function which gets a
+   *  block of raw bytes and with room enough and constructs a _Rep
+   *  object at the front.
+   *
+   *  The reason you want _M_data pointing to the character %array and
+   *  not the _Rep is so that the debugger can see the string
+   *  contents. (Probably we should add a non-inline member to get
+   *  the _Rep for the debugger to use, so users can check the actual
+   *  string length.)
+   *
+   *  Note that the _Rep object is a POD so that you can have a
+   *  static <em>empty string</em> _Rep object already @a constructed before
+   *  static constructors have run.  The reference-count encoding is
+   *  chosen so that a 0 indicates one reference, so you never try to
+   *  destroy the empty-string _Rep object.
+   *
+   *  All but the last paragraph is considered pretty conventional
+   *  for a C++ string implementation.
+  */
+  // 21.3  Template class basic_string
+  template<typename _CharT, typename _Traits, typename _Alloc>
+    class basic_string
+    {
+      typedef typename __gnu_cxx::__alloc_traits<_Alloc>::template
+	rebind<_CharT>::other _CharT_alloc_type;
+      typedef __gnu_cxx::__alloc_traits<_CharT_alloc_type> _CharT_alloc_traits;
+
+      // Types:
+    public:
+      typedef _Traits					    traits_type;
+      typedef typename _Traits::char_type		    value_type;
+      typedef _Alloc					    allocator_type;
+      typedef typename _CharT_alloc_traits::size_type	    size_type;
+      typedef typename _CharT_alloc_traits::difference_type difference_type;
+#if __cplusplus < 201103L
+      typedef typename _CharT_alloc_type::reference	    reference;
+      typedef typename _CharT_alloc_type::const_reference   const_reference;
+#else
+      typedef value_type&				    reference;
+      typedef const value_type&				    const_reference;
+#endif
+      typedef typename _CharT_alloc_traits::pointer	    pointer;
+      typedef typename _CharT_alloc_traits::const_pointer   const_pointer;
+      typedef __gnu_cxx::__normal_iterator<pointer, basic_string>  iterator;
+      typedef __gnu_cxx::__normal_iterator<const_pointer, basic_string>
+                                                            const_iterator;
+      typedef std::reverse_iterator<const_iterator>	const_reverse_iterator;
+      typedef std::reverse_iterator<iterator>		    reverse_iterator;
+
+    protected:
+      // type used for positions in insert, erase etc.
+      typedef iterator __const_iterator;
+
+    private:
+      // _Rep: string representation
+      //   Invariants:
+      //   1. String really contains _M_length + 1 characters: due to 21.3.4
+      //      must be kept null-terminated.
+      //   2. _M_capacity >= _M_length
+      //      Allocated memory is always (_M_capacity + 1) * sizeof(_CharT).
+      //   3. _M_refcount has three states:
+      //      -1: leaked, one reference, no ref-copies allowed, non-const.
+      //       0: one reference, non-const.
+      //     n>0: n + 1 references, operations require a lock, const.
+      //   4. All fields==0 is an empty string, given the extra storage
+      //      beyond-the-end for a null terminator; thus, the shared
+      //      empty string representation needs no constructor.
+
+      struct _Rep_base
+      {
+	size_type		_M_length;
+	size_type		_M_capacity;
+	_Atomic_word		_M_refcount;
+      };
+
+      struct _Rep : _Rep_base
+      {
+	// Types:
+	typedef typename __gnu_cxx::__alloc_traits<_Alloc>::template
+	  rebind<char>::other _Raw_bytes_alloc;
+
+	// (Public) Data members:
+
+	// The maximum number of individual char_type elements of an
+	// individual string is determined by _S_max_size. This is the
+	// value that will be returned by max_size().  (Whereas npos
+	// is the maximum number of bytes the allocator can allocate.)
+	// If one was to divvy up the theoretical largest size string,
+	// with a terminating character and m _CharT elements, it'd
+	// look like this:
+	// npos = sizeof(_Rep) + (m * sizeof(_CharT)) + sizeof(_CharT)
+	// Solving for m:
+	// m = ((npos - sizeof(_Rep))/sizeof(CharT)) - 1
+	// In addition, this implementation quarters this amount.
+	static const size_type	_S_max_size;
+	static const _CharT	_S_terminal;
+
+	// The following storage is init'd to 0 by the linker, resulting
+        // (carefully) in an empty string with one reference.
+        static size_type _S_empty_rep_storage[];
+
+        static _Rep&
+        _S_empty_rep() _GLIBCXX_NOEXCEPT
+        {
+	  // NB: Mild hack to avoid strict-aliasing warnings.  Note that
+	  // _S_empty_rep_storage is never modified and the punning should
+	  // be reasonably safe in this case.
+	  void* __p = reinterpret_cast<void*>(&_S_empty_rep_storage);
+	  return *reinterpret_cast<_Rep*>(__p);
+	}
+
+        bool
+	_M_is_leaked() const _GLIBCXX_NOEXCEPT
+        {
+#if defined(__GTHREADS)
+          // _M_refcount is mutated concurrently by _M_refcopy/_M_dispose,
+          // so we need to use an atomic load. However, _M_is_leaked
+          // predicate does not change concurrently (i.e. the string is either
+          // leaked or not), so a relaxed load is enough.
+          return __atomic_load_n(&this->_M_refcount, __ATOMIC_RELAXED) < 0;
+#else
+          return this->_M_refcount < 0;
+#endif
+        }
+
+        bool
+	_M_is_shared() const _GLIBCXX_NOEXCEPT
+	{
+#if defined(__GTHREADS)
+          // _M_refcount is mutated concurrently by _M_refcopy/_M_dispose,
+          // so we need to use an atomic load. Another thread can drop last
+          // but one reference concurrently with this check, so we need this
+          // load to be acquire to synchronize with release fetch_and_add in
+          // _M_dispose.
+          return __atomic_load_n(&this->_M_refcount, __ATOMIC_ACQUIRE) > 0;
+#else
+          return this->_M_refcount > 0;
+#endif
+        }
+
+        void
+	_M_set_leaked() _GLIBCXX_NOEXCEPT
+        { this->_M_refcount = -1; }
+
+        void
+	_M_set_sharable() _GLIBCXX_NOEXCEPT
+        { this->_M_refcount = 0; }
+
+	void
+	_M_set_length_and_sharable(size_type __n) _GLIBCXX_NOEXCEPT
+	{
+#if _GLIBCXX_FULLY_DYNAMIC_STRING == 0
+	  if (__builtin_expect(this != &_S_empty_rep(), false))
+#endif
+	    {
+	      this->_M_set_sharable();  // One reference.
+	      this->_M_length = __n;
+	      traits_type::assign(this->_M_refdata()[__n], _S_terminal);
+	      // grrr. (per 21.3.4)
+	      // You cannot leave those LWG people alone for a second.
+	    }
+	}
+
+	_CharT*
+	_M_refdata() throw()
+	{ return reinterpret_cast<_CharT*>(this + 1); }
+
+	_CharT*
+	_M_grab(const _Alloc& __alloc1, const _Alloc& __alloc2)
+	{
+	  return (!_M_is_leaked() && __alloc1 == __alloc2)
+	          ? _M_refcopy() : _M_clone(__alloc1);
+	}
+
+	// Create & Destroy
+	static _Rep*
+	_S_create(size_type, size_type, const _Alloc&);
+
+	void
+	_M_dispose(const _Alloc& __a) _GLIBCXX_NOEXCEPT
+	{
+#if _GLIBCXX_FULLY_DYNAMIC_STRING == 0
+	  if (__builtin_expect(this != &_S_empty_rep(), false))
+#endif
+	    {
+	      // Be race-detector-friendly.  For more info see bits/c++config.
+	      _GLIBCXX_SYNCHRONIZATION_HAPPENS_BEFORE(&this->_M_refcount);
+              // Decrement of _M_refcount is acq_rel, because:
+              // - all but last decrements need to release to synchronize with
+              //   the last decrement that will delete the object.
+              // - the last decrement needs to acquire to synchronize with
+              //   all the previous decrements.
+              // - last but one decrement needs to release to synchronize with
+              //   the acquire load in _M_is_shared that will conclude that
+              //   the object is not shared anymore.
+	      if (__gnu_cxx::__exchange_and_add_dispatch(&this->_M_refcount,
+							 -1) <= 0)
+		{
+		  _GLIBCXX_SYNCHRONIZATION_HAPPENS_AFTER(&this->_M_refcount);
+		  _M_destroy(__a);
+		}
+	    }
+	}  // XXX MT
+
+	void
+	_M_destroy(const _Alloc&) throw();
+
+	_CharT*
+	_M_refcopy() throw()
+	{
+#if _GLIBCXX_FULLY_DYNAMIC_STRING == 0
+	  if (__builtin_expect(this != &_S_empty_rep(), false))
+#endif
+            __gnu_cxx::__atomic_add_dispatch(&this->_M_refcount, 1);
+	  return _M_refdata();
+	}  // XXX MT
+
+	_CharT*
+	_M_clone(const _Alloc&, size_type __res = 0);
+      };
+
+      // Use empty-base optimization: http://www.cantrip.org/emptyopt.html
+      struct _Alloc_hider : _Alloc
+      {
+	_Alloc_hider(_CharT* __dat, const _Alloc& __a) _GLIBCXX_NOEXCEPT
+	: _Alloc(__a), _M_p(__dat) { }
+
+	_CharT* _M_p; // The actual data.
+      };
+
+    public:
+      // Data Members (public):
+      // NB: This is an unsigned type, and thus represents the maximum
+      // size that the allocator can hold.
+      ///  Value returned by various member functions when they fail.
+      static const size_type	npos = static_cast<size_type>(-1);
+
+    private:
+      // Data Members (private):
+      mutable _Alloc_hider	_M_dataplus;
+
+      _CharT*
+      _M_data() const _GLIBCXX_NOEXCEPT
+      { return  _M_dataplus._M_p; }
+
+      _CharT*
+      _M_data(_CharT* __p) _GLIBCXX_NOEXCEPT
+      { return (_M_dataplus._M_p = __p); }
+
+      _Rep*
+      _M_rep() const _GLIBCXX_NOEXCEPT
+      { return &((reinterpret_cast<_Rep*> (_M_data()))[-1]); }
+
+      // For the internal use we have functions similar to `begin'/`end'
+      // but they do not call _M_leak.
+      iterator
+      _M_ibegin() const _GLIBCXX_NOEXCEPT
+      { return iterator(_M_data()); }
+
+      iterator
+      _M_iend() const _GLIBCXX_NOEXCEPT
+      { return iterator(_M_data() + this->size()); }
+
+      void
+      _M_leak()    // for use in begin() & non-const op[]
+      {
+	if (!_M_rep()->_M_is_leaked())
+	  _M_leak_hard();
+      }
+
+      size_type
+      _M_check(size_type __pos, const char* __s) const
+      {
+	if (__pos > this->size())
+	  __throw_out_of_range_fmt(__N("%s: __pos (which is %zu) > "
+				       "this->size() (which is %zu)"),
+				   __s, __pos, this->size());
+	return __pos;
+      }
+
+      void
+      _M_check_length(size_type __n1, size_type __n2, const char* __s) const
+      {
+	if (this->max_size() - (this->size() - __n1) < __n2)
+	  __throw_length_error(__N(__s));
+      }
+
+      // NB: _M_limit doesn't check for a bad __pos value.
+      size_type
+      _M_limit(size_type __pos, size_type __off) const _GLIBCXX_NOEXCEPT
+      {
+	const bool __testoff =  __off < this->size() - __pos;
+	return __testoff ? __off : this->size() - __pos;
+      }
+
+      // True if _Rep and source do not overlap.
+      bool
+      _M_disjunct(const _CharT* __s) const _GLIBCXX_NOEXCEPT
+      {
+	return (less<const _CharT*>()(__s, _M_data())
+		|| less<const _CharT*>()(_M_data() + this->size(), __s));
+      }
+
+      // When __n = 1 way faster than the general multichar
+      // traits_type::copy/move/assign.
+      static void
+      _M_copy(_CharT* __d, const _CharT* __s, size_type __n) _GLIBCXX_NOEXCEPT
+      {
+	if (__n == 1)
+	  traits_type::assign(*__d, *__s);
+	else
+	  traits_type::copy(__d, __s, __n);
+      }
+
+      static void
+      _M_move(_CharT* __d, const _CharT* __s, size_type __n) _GLIBCXX_NOEXCEPT
+      {
+	if (__n == 1)
+	  traits_type::assign(*__d, *__s);
+	else
+	  traits_type::move(__d, __s, __n);
+      }
+
+      static void
+      _M_assign(_CharT* __d, size_type __n, _CharT __c) _GLIBCXX_NOEXCEPT
+      {
+	if (__n == 1)
+	  traits_type::assign(*__d, __c);
+	else
+	  traits_type::assign(__d, __n, __c);
+      }
+
+      // _S_copy_chars is a separate template to permit specialization
+      // to optimize for the common case of pointers as iterators.
+      template<class _Iterator>
+        static void
+        _S_copy_chars(_CharT* __p, _Iterator __k1, _Iterator __k2)
+        {
+	  for (; __k1 != __k2; ++__k1, (void)++__p)
+	    traits_type::assign(*__p, *__k1); // These types are off.
+	}
+
+      static void
+      _S_copy_chars(_CharT* __p, iterator __k1, iterator __k2) _GLIBCXX_NOEXCEPT
+      { _S_copy_chars(__p, __k1.base(), __k2.base()); }
+
+      static void
+      _S_copy_chars(_CharT* __p, const_iterator __k1, const_iterator __k2)
+      _GLIBCXX_NOEXCEPT
+      { _S_copy_chars(__p, __k1.base(), __k2.base()); }
+
+      static void
+      _S_copy_chars(_CharT* __p, _CharT* __k1, _CharT* __k2) _GLIBCXX_NOEXCEPT
+      { _M_copy(__p, __k1, __k2 - __k1); }
+
+      static void
+      _S_copy_chars(_CharT* __p, const _CharT* __k1, const _CharT* __k2)
+      _GLIBCXX_NOEXCEPT
+      { _M_copy(__p, __k1, __k2 - __k1); }
+
+      static int
+      _S_compare(size_type __n1, size_type __n2) _GLIBCXX_NOEXCEPT
+      {
+	const difference_type __d = difference_type(__n1 - __n2);
+
+	if (__d > __gnu_cxx::__numeric_traits<int>::__max)
+	  return __gnu_cxx::__numeric_traits<int>::__max;
+	else if (__d < __gnu_cxx::__numeric_traits<int>::__min)
+	  return __gnu_cxx::__numeric_traits<int>::__min;
+	else
+	  return int(__d);
+      }
+
+      void
+      _M_mutate(size_type __pos, size_type __len1, size_type __len2);
+
+      void
+      _M_leak_hard();
+
+      static _Rep&
+      _S_empty_rep() _GLIBCXX_NOEXCEPT
+      { return _Rep::_S_empty_rep(); }
+
+#if __cplusplus >= 201703L
+      // A helper type for avoiding boiler-plate.
+      typedef basic_string_view<_CharT, _Traits> __sv_type;
+
+      template<typename _Tp, typename _Res>
+	using _If_sv = enable_if_t<
+	  __and_<is_convertible<const _Tp&, __sv_type>,
+		 __not_<is_convertible<const _Tp*, const basic_string*>>,
+		 __not_<is_convertible<const _Tp&, const _CharT*>>>::value,
+	  _Res>;
+
+      // Allows an implicit conversion to __sv_type.
+      static __sv_type
+      _S_to_string_view(__sv_type __svt) noexcept
+      { return __svt; }
+
+      // Wraps a string_view by explicit conversion and thus
+      // allows to add an internal constructor that does not
+      // participate in overload resolution when a string_view
+      // is provided.
+      struct __sv_wrapper
+      {
+	explicit __sv_wrapper(__sv_type __sv) noexcept : _M_sv(__sv) { }
+	__sv_type _M_sv;
+      };
+
+      /**
+       *  @brief  Only internally used: Construct string from a string view
+       *          wrapper.
+       *  @param  __svw  string view wrapper.
+       *  @param  __a  Allocator to use.
+       */
+      explicit
+      basic_string(__sv_wrapper __svw, const _Alloc& __a)
+      : basic_string(__svw._M_sv.data(), __svw._M_sv.size(), __a) { }
+#endif
+
+    public:
+      // Construct/copy/destroy:
+      // NB: We overload ctors in some cases instead of using default
+      // arguments, per 17.4.4.4 para. 2 item 2.
+
+      /**
+       *  @brief  Default constructor creates an empty string.
+       */
+      basic_string()
+#if _GLIBCXX_FULLY_DYNAMIC_STRING == 0
+      _GLIBCXX_NOEXCEPT
+      : _M_dataplus(_S_empty_rep()._M_refdata(), _Alloc())
+#else
+      : _M_dataplus(_S_construct(size_type(), _CharT(), _Alloc()), _Alloc())
+#endif
+      { }
+
+      /**
+       *  @brief  Construct an empty string using allocator @a a.
+       */
+      explicit
+      basic_string(const _Alloc& __a)
+      : _M_dataplus(_S_construct(size_type(), _CharT(), __a), __a)
+      { }
+
+      // NB: per LWG issue 42, semantics different from IS:
+      /**
+       *  @brief  Construct string with copy of value of @a str.
+       *  @param  __str  Source string.
+       */
+      basic_string(const basic_string& __str)
+      : _M_dataplus(__str._M_rep()->_M_grab(_Alloc(__str.get_allocator()),
+					    __str.get_allocator()),
+		    __str.get_allocator())
+      { }
+
+      // _GLIBCXX_RESOLVE_LIB_DEFECTS
+      // 2583. no way to supply an allocator for basic_string(str, pos)
+      /**
+       *  @brief  Construct string as copy of a substring.
+       *  @param  __str  Source string.
+       *  @param  __pos  Index of first character to copy from.
+       *  @param  __a  Allocator to use.
+       */
+      basic_string(const basic_string& __str, size_type __pos,
+		   const _Alloc& __a = _Alloc());
+
+      /**
+       *  @brief  Construct string as copy of a substring.
+       *  @param  __str  Source string.
+       *  @param  __pos  Index of first character to copy from.
+       *  @param  __n  Number of characters to copy.
+       */
+      basic_string(const basic_string& __str, size_type __pos,
+		   size_type __n);
+      /**
+       *  @brief  Construct string as copy of a substring.
+       *  @param  __str  Source string.
+       *  @param  __pos  Index of first character to copy from.
+       *  @param  __n  Number of characters to copy.
+       *  @param  __a  Allocator to use.
+       */
+      basic_string(const basic_string& __str, size_type __pos,
+		   size_type __n, const _Alloc& __a);
+
+      /**
+       *  @brief  Construct string initialized by a character %array.
+       *  @param  __s  Source character %array.
+       *  @param  __n  Number of characters to copy.
+       *  @param  __a  Allocator to use (default is default allocator).
+       *
+       *  NB: @a __s must have at least @a __n characters, &apos;\\0&apos;
+       *  has no special meaning.
+       */
+      basic_string(const _CharT* __s, size_type __n,
+		   const _Alloc& __a = _Alloc())
+      : _M_dataplus(_S_construct(__s, __s + __n, __a), __a)
+      { }
+
+      /**
+       *  @brief  Construct string as copy of a C string.
+       *  @param  __s  Source C string.
+       *  @param  __a  Allocator to use (default is default allocator).
+       */
+#if __cpp_deduction_guides && ! defined _GLIBCXX_DEFINING_STRING_INSTANTIATIONS
+      // _GLIBCXX_RESOLVE_LIB_DEFECTS
+      // 3076. basic_string CTAD ambiguity
+      template<typename = _RequireAllocator<_Alloc>>
+#endif
+      basic_string(const _CharT* __s, const _Alloc& __a = _Alloc())
+      : _M_dataplus(_S_construct(__s, __s ? __s + traits_type::length(__s) :
+				 __s + npos, __a), __a)
+      { }
+
+      /**
+       *  @brief  Construct string as multiple characters.
+       *  @param  __n  Number of characters.
+       *  @param  __c  Character to use.
+       *  @param  __a  Allocator to use (default is default allocator).
+       */
+      basic_string(size_type __n, _CharT __c, const _Alloc& __a = _Alloc())
+      : _M_dataplus(_S_construct(__n, __c, __a), __a)
+      { }
+
+#if __cplusplus >= 201103L
+      /**
+       *  @brief  Move construct string.
+       *  @param  __str  Source string.
+       *
+       *  The newly-created string contains the exact contents of @a __str.
+       *  @a __str is a valid, but unspecified string.
+       */
+      basic_string(basic_string&& __str)
+#if _GLIBCXX_FULLY_DYNAMIC_STRING == 0
+      noexcept // FIXME C++11: should always be noexcept.
+#endif
+      : _M_dataplus(std::move(__str._M_dataplus))
+      {
+#if _GLIBCXX_FULLY_DYNAMIC_STRING == 0
+	__str._M_data(_S_empty_rep()._M_refdata());
+#else
+	__str._M_data(_S_construct(size_type(), _CharT(), get_allocator()));
+#endif
+      }
+
+      /**
+       *  @brief  Construct string from an initializer %list.
+       *  @param  __l  std::initializer_list of characters.
+       *  @param  __a  Allocator to use (default is default allocator).
+       */
+      basic_string(initializer_list<_CharT> __l, const _Alloc& __a = _Alloc())
+      : _M_dataplus(_S_construct(__l.begin(), __l.end(), __a), __a)
+      { }
+
+      basic_string(const basic_string& __str, const _Alloc& __a)
+      : _M_dataplus(__str._M_rep()->_M_grab(__a, __str.get_allocator()), __a)
+      { }
+
+      basic_string(basic_string&& __str, const _Alloc& __a)
+      : _M_dataplus(__str._M_data(), __a)
+      {
+	if (__a == __str.get_allocator())
+	  {
+#if _GLIBCXX_FULLY_DYNAMIC_STRING == 0
+	    __str._M_data(_S_empty_rep()._M_refdata());
+#else
+	    __str._M_data(_S_construct(size_type(), _CharT(), __a));
+#endif
+	  }
+	else
+	  _M_dataplus._M_p = _S_construct(__str.begin(), __str.end(), __a);
+      }
+#endif // C++11
+
+      /**
+       *  @brief  Construct string as copy of a range.
+       *  @param  __beg  Start of range.
+       *  @param  __end  End of range.
+       *  @param  __a  Allocator to use (default is default allocator).
+       */
+      template<class _InputIterator>
+        basic_string(_InputIterator __beg, _InputIterator __end,
+		     const _Alloc& __a = _Alloc())
+	: _M_dataplus(_S_construct(__beg, __end, __a), __a)
+	{ }
+
+#if __cplusplus >= 201703L
+      /**
+       *  @brief  Construct string from a substring of a string_view.
+       *  @param  __t   Source object convertible to string view.
+       *  @param  __pos The index of the first character to copy from __t.
+       *  @param  __n   The number of characters to copy from __t.
+       *  @param  __a   Allocator to use.
+       */
+      template<typename _Tp, typename = _If_sv<_Tp, void>>
+	basic_string(const _Tp& __t, size_type __pos, size_type __n,
+		     const _Alloc& __a = _Alloc())
+	: basic_string(_S_to_string_view(__t).substr(__pos, __n), __a) { }
+
+      /**
+       *  @brief  Construct string from a string_view.
+       *  @param  __t  Source object convertible to string view.
+       *  @param  __a  Allocator to use (default is default allocator).
+       */
+      template<typename _Tp, typename = _If_sv<_Tp, void>>
+	explicit
+	basic_string(const _Tp& __t, const _Alloc& __a = _Alloc())
+	: basic_string(__sv_wrapper(_S_to_string_view(__t)), __a) { }
+#endif // C++17
+
+      /**
+       *  @brief  Destroy the string instance.
+       */
+      ~basic_string() _GLIBCXX_NOEXCEPT
+      { _M_rep()->_M_dispose(this->get_allocator()); }
+
+      /**
+       *  @brief  Assign the value of @a str to this string.
+       *  @param  __str  Source string.
+       */
+      basic_string&
+      operator=(const basic_string& __str)
+      { return this->assign(__str); }
+
+      /**
+       *  @brief  Copy contents of @a s into this string.
+       *  @param  __s  Source null-terminated string.
+       */
+      basic_string&
+      operator=(const _CharT* __s)
+      { return this->assign(__s); }
+
+      /**
+       *  @brief  Set value to string of length 1.
+       *  @param  __c  Source character.
+       *
+       *  Assigning to a character makes this string length 1 and
+       *  (*this)[0] == @a c.
+       */
+      basic_string&
+      operator=(_CharT __c)
+      {
+	this->assign(1, __c);
+	return *this;
+      }
+
+#if __cplusplus >= 201103L
+      /**
+       *  @brief  Move assign the value of @a str to this string.
+       *  @param  __str  Source string.
+       *
+       *  The contents of @a str are moved into this string (without copying).
+       *  @a str is a valid, but unspecified string.
+       */
+      basic_string&
+      operator=(basic_string&& __str)
+      _GLIBCXX_NOEXCEPT_IF(allocator_traits<_Alloc>::is_always_equal::value)
+      {
+	// NB: DR 1204.
+	this->swap(__str);
+	return *this;
+      }
+
+      /**
+       *  @brief  Set value to string constructed from initializer %list.
+       *  @param  __l  std::initializer_list.
+       */
+      basic_string&
+      operator=(initializer_list<_CharT> __l)
+      {
+	this->assign(__l.begin(), __l.size());
+	return *this;
+      }
+#endif // C++11
+
+#if __cplusplus >= 201703L
+      /**
+       *  @brief  Set value to string constructed from a string_view.
+       *  @param  __svt An object convertible to  string_view.
+       */
+      template<typename _Tp>
+	_If_sv<_Tp, basic_string&>
+	operator=(const _Tp& __svt)
+	{ return this->assign(__svt); }
+
+      /**
+       *  @brief  Convert to a string_view.
+       *  @return A string_view.
+       */
+      operator __sv_type() const noexcept
+      { return __sv_type(data(), size()); }
+#endif // C++17
+
+      // Iterators:
+      /**
+       *  Returns a read/write iterator that points to the first character in
+       *  the %string.  Unshares the string.
+       */
+      iterator
+      begin() // FIXME C++11: should be noexcept.
+      {
+	_M_leak();
+	return iterator(_M_data());
+      }
+
+      /**
+       *  Returns a read-only (constant) iterator that points to the first
+       *  character in the %string.
+       */
+      const_iterator
+      begin() const _GLIBCXX_NOEXCEPT
+      { return const_iterator(_M_data()); }
+
+      /**
+       *  Returns a read/write iterator that points one past the last
+       *  character in the %string.  Unshares the string.
+       */
+      iterator
+      end() // FIXME C++11: should be noexcept.
+      {
+	_M_leak();
+	return iterator(_M_data() + this->size());
+      }
+
+      /**
+       *  Returns a read-only (constant) iterator that points one past the
+       *  last character in the %string.
+       */
+      const_iterator
+      end() const _GLIBCXX_NOEXCEPT
+      { return const_iterator(_M_data() + this->size()); }
+
+      /**
+       *  Returns a read/write reverse iterator that points to the last
+       *  character in the %string.  Iteration is done in reverse element
+       *  order.  Unshares the string.
+       */
+      reverse_iterator
+      rbegin() // FIXME C++11: should be noexcept.
+      { return reverse_iterator(this->end()); }
+
+      /**
+       *  Returns a read-only (constant) reverse iterator that points
+       *  to the last character in the %string.  Iteration is done in
+       *  reverse element order.
+       */
+      const_reverse_iterator
+      rbegin() const _GLIBCXX_NOEXCEPT
+      { return const_reverse_iterator(this->end()); }
+
+      /**
+       *  Returns a read/write reverse iterator that points to one before the
+       *  first character in the %string.  Iteration is done in reverse
+       *  element order.  Unshares the string.
+       */
+      reverse_iterator
+      rend() // FIXME C++11: should be noexcept.
+      { return reverse_iterator(this->begin()); }
+
+      /**
+       *  Returns a read-only (constant) reverse iterator that points
+       *  to one before the first character in the %string.  Iteration
+       *  is done in reverse element order.
+       */
+      const_reverse_iterator
+      rend() const _GLIBCXX_NOEXCEPT
+      { return const_reverse_iterator(this->begin()); }
+
+#if __cplusplus >= 201103L
+      /**
+       *  Returns a read-only (constant) iterator that points to the first
+       *  character in the %string.
+       */
+      const_iterator
+      cbegin() const noexcept
+      { return const_iterator(this->_M_data()); }
+
+      /**
+       *  Returns a read-only (constant) iterator that points one past the
+       *  last character in the %string.
+       */
+      const_iterator
+      cend() const noexcept
+      { return const_iterator(this->_M_data() + this->size()); }
+
+      /**
+       *  Returns a read-only (constant) reverse iterator that points
+       *  to the last character in the %string.  Iteration is done in
+       *  reverse element order.
+       */
+      const_reverse_iterator
+      crbegin() const noexcept
+      { return const_reverse_iterator(this->end()); }
+
+      /**
+       *  Returns a read-only (constant) reverse iterator that points
+       *  to one before the first character in the %string.  Iteration
+       *  is done in reverse element order.
+       */
+      const_reverse_iterator
+      crend() const noexcept
+      { return const_reverse_iterator(this->begin()); }
+#endif
+
+    public:
+      // Capacity:
+      ///  Returns the number of characters in the string, not including any
+      ///  null-termination.
+      size_type
+      size() const _GLIBCXX_NOEXCEPT
+      { return _M_rep()->_M_length; }
+
+      ///  Returns the number of characters in the string, not including any
+      ///  null-termination.
+      size_type
+      length() const _GLIBCXX_NOEXCEPT
+      { return _M_rep()->_M_length; }
+
+      ///  Returns the size() of the largest possible %string.
+      size_type
+      max_size() const _GLIBCXX_NOEXCEPT
+      { return _Rep::_S_max_size; }
+
+      /**
+       *  @brief  Resizes the %string to the specified number of characters.
+       *  @param  __n  Number of characters the %string should contain.
+       *  @param  __c  Character to fill any new elements.
+       *
+       *  This function will %resize the %string to the specified
+       *  number of characters.  If the number is smaller than the
+       *  %string's current size the %string is truncated, otherwise
+       *  the %string is extended and new elements are %set to @a __c.
+       */
+      void
+      resize(size_type __n, _CharT __c);
+
+      /**
+       *  @brief  Resizes the %string to the specified number of characters.
+       *  @param  __n  Number of characters the %string should contain.
+       *
+       *  This function will resize the %string to the specified length.  If
+       *  the new size is smaller than the %string's current size the %string
+       *  is truncated, otherwise the %string is extended and new characters
+       *  are default-constructed.  For basic types such as char, this means
+       *  setting them to 0.
+       */
+      void
+      resize(size_type __n)
+      { this->resize(__n, _CharT()); }
+
+#if __cplusplus >= 201103L
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wdeprecated-declarations"
+      ///  A non-binding request to reduce capacity() to size().
+      void
+      shrink_to_fit() noexcept
+      { reserve(); }
+#pragma GCC diagnostic pop
+#endif
+
+      /**
+       *  Returns the total number of characters that the %string can hold
+       *  before needing to allocate more memory.
+       */
+      size_type
+      capacity() const _GLIBCXX_NOEXCEPT
+      { return _M_rep()->_M_capacity; }
+
+      /**
+       *  @brief  Attempt to preallocate enough memory for specified number of
+       *          characters.
+       *  @param  __res_arg  Number of characters required.
+       *  @throw  std::length_error  If @a __res_arg exceeds @c max_size().
+       *
+       *  This function attempts to reserve enough memory for the
+       *  %string to hold the specified number of characters.  If the
+       *  number requested is more than max_size(), length_error is
+       *  thrown.
+       *
+       *  The advantage of this function is that if optimal code is a
+       *  necessity and the user can determine the string length that will be
+       *  required, the user can reserve the memory in %advance, and thus
+       *  prevent a possible reallocation of memory and copying of %string
+       *  data.
+       */
+      void
+      reserve(size_type __res_arg);
+
+      /// Equivalent to shrink_to_fit().
+#if __cplusplus > 201703L
+      [[deprecated("use shrink_to_fit() instead")]]
+#endif
+      void
+      reserve();
+
+      /**
+       *  Erases the string, making it empty.
+       */
+#if _GLIBCXX_FULLY_DYNAMIC_STRING == 0
+      void
+      clear() _GLIBCXX_NOEXCEPT
+      {
+	if (_M_rep()->_M_is_shared())
+	  {
+	    _M_rep()->_M_dispose(this->get_allocator());
+	    _M_data(_S_empty_rep()._M_refdata());
+	  }
+	else
+	  _M_rep()->_M_set_length_and_sharable(0);
+      }
+#else
+      // PR 56166: this should not throw.
+      void
+      clear()
+      { _M_mutate(0, this->size(), 0); }
+#endif
+
+      /**
+       *  Returns true if the %string is empty.  Equivalent to
+       *  <code>*this == ""</code>.
+       */
+      _GLIBCXX_NODISCARD bool
+      empty() const _GLIBCXX_NOEXCEPT
+      { return this->size() == 0; }
+
+      // Element access:
+      /**
+       *  @brief  Subscript access to the data contained in the %string.
+       *  @param  __pos  The index of the character to access.
+       *  @return  Read-only (constant) reference to the character.
+       *
+       *  This operator allows for easy, array-style, data access.
+       *  Note that data access with this operator is unchecked and
+       *  out_of_range lookups are not defined. (For checked lookups
+       *  see at().)
+       */
+      const_reference
+      operator[] (size_type __pos) const _GLIBCXX_NOEXCEPT
+      {
+	__glibcxx_assert(__pos <= size());
+	return _M_data()[__pos];
+      }
+
+      /**
+       *  @brief  Subscript access to the data contained in the %string.
+       *  @param  __pos  The index of the character to access.
+       *  @return  Read/write reference to the character.
+       *
+       *  This operator allows for easy, array-style, data access.
+       *  Note that data access with this operator is unchecked and
+       *  out_of_range lookups are not defined. (For checked lookups
+       *  see at().)  Unshares the string.
+       */
+      reference
+      operator[](size_type __pos)
+      {
+        // Allow pos == size() both in C++98 mode, as v3 extension,
+	// and in C++11 mode.
+	__glibcxx_assert(__pos <= size());
+        // In pedantic mode be strict in C++98 mode.
+	_GLIBCXX_DEBUG_PEDASSERT(__cplusplus >= 201103L || __pos < size());
+	_M_leak();
+	return _M_data()[__pos];
+      }
+
+      /**
+       *  @brief  Provides access to the data contained in the %string.
+       *  @param __n The index of the character to access.
+       *  @return  Read-only (const) reference to the character.
+       *  @throw  std::out_of_range  If @a n is an invalid index.
+       *
+       *  This function provides for safer data access.  The parameter is
+       *  first checked that it is in the range of the string.  The function
+       *  throws out_of_range if the check fails.
+       */
+      const_reference
+      at(size_type __n) const
+      {
+	if (__n >= this->size())
+	  __throw_out_of_range_fmt(__N("basic_string::at: __n "
+				       "(which is %zu) >= this->size() "
+				       "(which is %zu)"),
+				   __n, this->size());
+	return _M_data()[__n];
+      }
+
+      /**
+       *  @brief  Provides access to the data contained in the %string.
+       *  @param __n The index of the character to access.
+       *  @return  Read/write reference to the character.
+       *  @throw  std::out_of_range  If @a n is an invalid index.
+       *
+       *  This function provides for safer data access.  The parameter is
+       *  first checked that it is in the range of the string.  The function
+       *  throws out_of_range if the check fails.  Success results in
+       *  unsharing the string.
+       */
+      reference
+      at(size_type __n)
+      {
+	if (__n >= size())
+	  __throw_out_of_range_fmt(__N("basic_string::at: __n "
+				       "(which is %zu) >= this->size() "
+				       "(which is %zu)"),
+				   __n, this->size());
+	_M_leak();
+	return _M_data()[__n];
+      }
+
+#if __cplusplus >= 201103L
+      /**
+       *  Returns a read/write reference to the data at the first
+       *  element of the %string.
+       */
+      reference
+      front()
+      {
+	__glibcxx_assert(!empty());
+	return operator[](0);
+      }
+
+      /**
+       *  Returns a read-only (constant) reference to the data at the first
+       *  element of the %string.
+       */
+      const_reference
+      front() const noexcept
+      {
+	__glibcxx_assert(!empty());
+	return operator[](0);
+      }
+
+      /**
+       *  Returns a read/write reference to the data at the last
+       *  element of the %string.
+       */
+      reference
+      back()
+      {
+	__glibcxx_assert(!empty());
+	return operator[](this->size() - 1);
+      }
+
+      /**
+       *  Returns a read-only (constant) reference to the data at the
+       *  last element of the %string.
+       */
+      const_reference
+      back() const noexcept
+      {
+	__glibcxx_assert(!empty());
+	return operator[](this->size() - 1);
+      }
+#endif
+
+      // Modifiers:
+      /**
+       *  @brief  Append a string to this string.
+       *  @param __str  The string to append.
+       *  @return  Reference to this string.
+       */
+      basic_string&
+      operator+=(const basic_string& __str)
+      { return this->append(__str); }
+
+      /**
+       *  @brief  Append a C string.
+       *  @param __s  The C string to append.
+       *  @return  Reference to this string.
+       */
+      basic_string&
+      operator+=(const _CharT* __s)
+      { return this->append(__s); }
+
+      /**
+       *  @brief  Append a character.
+       *  @param __c  The character to append.
+       *  @return  Reference to this string.
+       */
+      basic_string&
+      operator+=(_CharT __c)
+      {
+	this->push_back(__c);
+	return *this;
+      }
+
+#if __cplusplus >= 201103L
+      /**
+       *  @brief  Append an initializer_list of characters.
+       *  @param __l  The initializer_list of characters to be appended.
+       *  @return  Reference to this string.
+       */
+      basic_string&
+      operator+=(initializer_list<_CharT> __l)
+      { return this->append(__l.begin(), __l.size()); }
+#endif // C++11
+
+#if __cplusplus >= 201703L
+      /**
+       *  @brief  Append a string_view.
+       *  @param __svt The object convertible to string_view to be appended.
+       *  @return  Reference to this string.
+       */
+      template<typename _Tp>
+	_If_sv<_Tp, basic_string&>
+	operator+=(const _Tp& __svt)
+	{ return this->append(__svt); }
+#endif // C++17
+
+      /**
+       *  @brief  Append a string to this string.
+       *  @param __str  The string to append.
+       *  @return  Reference to this string.
+       */
+      basic_string&
+      append(const basic_string& __str);
+
+      /**
+       *  @brief  Append a substring.
+       *  @param __str  The string to append.
+       *  @param __pos  Index of the first character of str to append.
+       *  @param __n  The number of characters to append.
+       *  @return  Reference to this string.
+       *  @throw  std::out_of_range if @a __pos is not a valid index.
+       *
+       *  This function appends @a __n characters from @a __str
+       *  starting at @a __pos to this string.  If @a __n is is larger
+       *  than the number of available characters in @a __str, the
+       *  remainder of @a __str is appended.
+       */
+      basic_string&
+      append(const basic_string& __str, size_type __pos, size_type __n = npos);
+
+      /**
+       *  @brief  Append a C substring.
+       *  @param __s  The C string to append.
+       *  @param __n  The number of characters to append.
+       *  @return  Reference to this string.
+       */
+      basic_string&
+      append(const _CharT* __s, size_type __n);
+
+      /**
+       *  @brief  Append a C string.
+       *  @param __s  The C string to append.
+       *  @return  Reference to this string.
+       */
+      basic_string&
+      append(const _CharT* __s)
+      {
+	__glibcxx_requires_string(__s);
+	return this->append(__s, traits_type::length(__s));
+      }
+
+      /**
+       *  @brief  Append multiple characters.
+       *  @param __n  The number of characters to append.
+       *  @param __c  The character to use.
+       *  @return  Reference to this string.
+       *
+       *  Appends __n copies of __c to this string.
+       */
+      basic_string&
+      append(size_type __n, _CharT __c);
+
+#if __cplusplus >= 201103L
+      /**
+       *  @brief  Append an initializer_list of characters.
+       *  @param __l  The initializer_list of characters to append.
+       *  @return  Reference to this string.
+       */
+      basic_string&
+      append(initializer_list<_CharT> __l)
+      { return this->append(__l.begin(), __l.size()); }
+#endif // C++11
+
+      /**
+       *  @brief  Append a range of characters.
+       *  @param __first  Iterator referencing the first character to append.
+       *  @param __last  Iterator marking the end of the range.
+       *  @return  Reference to this string.
+       *
+       *  Appends characters in the range [__first,__last) to this string.
+       */
+      template<class _InputIterator>
+        basic_string&
+        append(_InputIterator __first, _InputIterator __last)
+        { return this->replace(_M_iend(), _M_iend(), __first, __last); }
+
+#if __cplusplus >= 201703L
+      /**
+       *  @brief  Append a string_view.
+       *  @param __svt The object convertible to string_view to be appended.
+       *  @return  Reference to this string.
+       */
+      template<typename _Tp>
+	_If_sv<_Tp, basic_string&>
+	append(const _Tp& __svt)
+	{
+	  __sv_type __sv = __svt;
+	  return this->append(__sv.data(), __sv.size());
+	}
+
+      /**
+       *  @brief  Append a range of characters from a string_view.
+       *  @param __svt The object convertible to string_view to be appended
+       *               from.
+       *  @param __pos The position in the string_view to append from.
+       *  @param __n   The number of characters to append from the string_view.
+       *  @return  Reference to this string.
+       */
+      template<typename _Tp>
+        _If_sv<_Tp, basic_string&>
+	append(const _Tp& __svt, size_type __pos, size_type __n = npos)
+	{
+	  __sv_type __sv = __svt;
+	  return append(__sv.data()
+	      + std::__sv_check(__sv.size(), __pos, "basic_string::append"),
+	      std::__sv_limit(__sv.size(), __pos, __n));
+	}
+#endif // C++17
+
+      /**
+       *  @brief  Append a single character.
+       *  @param __c  Character to append.
+       */
+      void
+      push_back(_CharT __c)
+      {
+	const size_type __len = 1 + this->size();
+	if (__len > this->capacity() || _M_rep()->_M_is_shared())
+	  this->reserve(__len);
+	traits_type::assign(_M_data()[this->size()], __c);
+	_M_rep()->_M_set_length_and_sharable(__len);
+      }
+
+      /**
+       *  @brief  Set value to contents of another string.
+       *  @param  __str  Source string to use.
+       *  @return  Reference to this string.
+       */
+      basic_string&
+      assign(const basic_string& __str);
+
+#if __cplusplus >= 201103L
+      /**
+       *  @brief  Set value to contents of another string.
+       *  @param  __str  Source string to use.
+       *  @return  Reference to this string.
+       *
+       *  This function sets this string to the exact contents of @a __str.
+       *  @a __str is a valid, but unspecified string.
+       */
+      basic_string&
+      assign(basic_string&& __str)
+      noexcept(allocator_traits<_Alloc>::is_always_equal::value)
+      {
+	this->swap(__str);
+	return *this;
+      }
+#endif // C++11
+
+      /**
+       *  @brief  Set value to a substring of a string.
+       *  @param __str  The string to use.
+       *  @param __pos  Index of the first character of str.
+       *  @param __n  Number of characters to use.
+       *  @return  Reference to this string.
+       *  @throw  std::out_of_range if @a pos is not a valid index.
+       *
+       *  This function sets this string to the substring of @a __str
+       *  consisting of @a __n characters at @a __pos.  If @a __n is
+       *  is larger than the number of available characters in @a
+       *  __str, the remainder of @a __str is used.
+       */
+      basic_string&
+      assign(const basic_string& __str, size_type __pos, size_type __n = npos)
+      { return this->assign(__str._M_data()
+			    + __str._M_check(__pos, "basic_string::assign"),
+			    __str._M_limit(__pos, __n)); }
+
+      /**
+       *  @brief  Set value to a C substring.
+       *  @param __s  The C string to use.
+       *  @param __n  Number of characters to use.
+       *  @return  Reference to this string.
+       *
+       *  This function sets the value of this string to the first @a __n
+       *  characters of @a __s.  If @a __n is is larger than the number of
+       *  available characters in @a __s, the remainder of @a __s is used.
+       */
+      basic_string&
+      assign(const _CharT* __s, size_type __n);
+
+      /**
+       *  @brief  Set value to contents of a C string.
+       *  @param __s  The C string to use.
+       *  @return  Reference to this string.
+       *
+       *  This function sets the value of this string to the value of @a __s.
+       *  The data is copied, so there is no dependence on @a __s once the
+       *  function returns.
+       */
+      basic_string&
+      assign(const _CharT* __s)
+      {
+	__glibcxx_requires_string(__s);
+	return this->assign(__s, traits_type::length(__s));
+      }
+
+      /**
+       *  @brief  Set value to multiple characters.
+       *  @param __n  Length of the resulting string.
+       *  @param __c  The character to use.
+       *  @return  Reference to this string.
+       *
+       *  This function sets the value of this string to @a __n copies of
+       *  character @a __c.
+       */
+      basic_string&
+      assign(size_type __n, _CharT __c)
+      { return _M_replace_aux(size_type(0), this->size(), __n, __c); }
+
+      /**
+       *  @brief  Set value to a range of characters.
+       *  @param __first  Iterator referencing the first character to append.
+       *  @param __last  Iterator marking the end of the range.
+       *  @return  Reference to this string.
+       *
+       *  Sets value of string to characters in the range [__first,__last).
+      */
+      template<class _InputIterator>
+        basic_string&
+        assign(_InputIterator __first, _InputIterator __last)
+        { return this->replace(_M_ibegin(), _M_iend(), __first, __last); }
+
+#if __cplusplus >= 201103L
+      /**
+       *  @brief  Set value to an initializer_list of characters.
+       *  @param __l  The initializer_list of characters to assign.
+       *  @return  Reference to this string.
+       */
+      basic_string&
+      assign(initializer_list<_CharT> __l)
+      { return this->assign(__l.begin(), __l.size()); }
+#endif // C++11
+
+#if __cplusplus >= 201703L
+      /**
+       *  @brief  Set value from a string_view.
+       *  @param __svt The source object convertible to string_view.
+       *  @return  Reference to this string.
+       */
+      template<typename _Tp>
+	_If_sv<_Tp, basic_string&>
+	assign(const _Tp& __svt)
+	{
+	  __sv_type __sv = __svt;
+	  return this->assign(__sv.data(), __sv.size());
+	}
+
+      /**
+       *  @brief  Set value from a range of characters in a string_view.
+       *  @param __svt  The source object convertible to string_view.
+       *  @param __pos  The position in the string_view to assign from.
+       *  @param __n  The number of characters to assign.
+       *  @return  Reference to this string.
+       */
+      template<typename _Tp>
+        _If_sv<_Tp, basic_string&>
+        assign(const _Tp& __svt, size_type __pos, size_type __n = npos)
+	{
+	  __sv_type __sv = __svt;
+	  return assign(__sv.data()
+	      + std::__sv_check(__sv.size(), __pos, "basic_string::assign"),
+	      std::__sv_limit(__sv.size(), __pos, __n));
+	}
+#endif // C++17
+
+      /**
+       *  @brief  Insert multiple characters.
+       *  @param __p  Iterator referencing location in string to insert at.
+       *  @param __n  Number of characters to insert
+       *  @param __c  The character to insert.
+       *  @throw  std::length_error  If new length exceeds @c max_size().
+       *
+       *  Inserts @a __n copies of character @a __c starting at the
+       *  position referenced by iterator @a __p.  If adding
+       *  characters causes the length to exceed max_size(),
+       *  length_error is thrown.  The value of the string doesn't
+       *  change if an error is thrown.
+      */
+      void
+      insert(iterator __p, size_type __n, _CharT __c)
+      {	this->replace(__p, __p, __n, __c);  }
+
+      /**
+       *  @brief  Insert a range of characters.
+       *  @param __p  Iterator referencing location in string to insert at.
+       *  @param __beg  Start of range.
+       *  @param __end  End of range.
+       *  @throw  std::length_error  If new length exceeds @c max_size().
+       *
+       *  Inserts characters in range [__beg,__end).  If adding
+       *  characters causes the length to exceed max_size(),
+       *  length_error is thrown.  The value of the string doesn't
+       *  change if an error is thrown.
+      */
+      template<class _InputIterator>
+        void
+        insert(iterator __p, _InputIterator __beg, _InputIterator __end)
+        { this->replace(__p, __p, __beg, __end); }
+
+#if __cplusplus >= 201103L
+      /**
+       *  @brief  Insert an initializer_list of characters.
+       *  @param __p  Iterator referencing location in string to insert at.
+       *  @param __l  The initializer_list of characters to insert.
+       *  @throw  std::length_error  If new length exceeds @c max_size().
+       */
+      void
+      insert(iterator __p, initializer_list<_CharT> __l)
+      {
+	_GLIBCXX_DEBUG_PEDASSERT(__p >= _M_ibegin() && __p <= _M_iend());
+	this->insert(__p - _M_ibegin(), __l.begin(), __l.size());
+      }
+#endif // C++11
+
+      /**
+       *  @brief  Insert value of a string.
+       *  @param __pos1  Position in string to insert at.
+       *  @param __str  The string to insert.
+       *  @return  Reference to this string.
+       *  @throw  std::length_error  If new length exceeds @c max_size().
+       *
+       *  Inserts value of @a __str starting at @a __pos1.  If adding
+       *  characters causes the length to exceed max_size(),
+       *  length_error is thrown.  The value of the string doesn't
+       *  change if an error is thrown.
+      */
+      basic_string&
+      insert(size_type __pos1, const basic_string& __str)
+      { return this->insert(__pos1, __str, size_type(0), __str.size()); }
+
+      /**
+       *  @brief  Insert a substring.
+       *  @param __pos1  Position in string to insert at.
+       *  @param __str  The string to insert.
+       *  @param __pos2  Start of characters in str to insert.
+       *  @param __n  Number of characters to insert.
+       *  @return  Reference to this string.
+       *  @throw  std::length_error  If new length exceeds @c max_size().
+       *  @throw  std::out_of_range  If @a pos1 > size() or
+       *  @a __pos2 > @a str.size().
+       *
+       *  Starting at @a pos1, insert @a __n character of @a __str
+       *  beginning with @a __pos2.  If adding characters causes the
+       *  length to exceed max_size(), length_error is thrown.  If @a
+       *  __pos1 is beyond the end of this string or @a __pos2 is
+       *  beyond the end of @a __str, out_of_range is thrown.  The
+       *  value of the string doesn't change if an error is thrown.
+      */
+      basic_string&
+      insert(size_type __pos1, const basic_string& __str,
+	     size_type __pos2, size_type __n = npos)
+      { return this->insert(__pos1, __str._M_data()
+			    + __str._M_check(__pos2, "basic_string::insert"),
+			    __str._M_limit(__pos2, __n)); }
+
+      /**
+       *  @brief  Insert a C substring.
+       *  @param __pos  Position in string to insert at.
+       *  @param __s  The C string to insert.
+       *  @param __n  The number of characters to insert.
+       *  @return  Reference to this string.
+       *  @throw  std::length_error  If new length exceeds @c max_size().
+       *  @throw  std::out_of_range  If @a __pos is beyond the end of this
+       *  string.
+       *
+       *  Inserts the first @a __n characters of @a __s starting at @a
+       *  __pos.  If adding characters causes the length to exceed
+       *  max_size(), length_error is thrown.  If @a __pos is beyond
+       *  end(), out_of_range is thrown.  The value of the string
+       *  doesn't change if an error is thrown.
+      */
+      basic_string&
+      insert(size_type __pos, const _CharT* __s, size_type __n);
+
+      /**
+       *  @brief  Insert a C string.
+       *  @param __pos  Position in string to insert at.
+       *  @param __s  The C string to insert.
+       *  @return  Reference to this string.
+       *  @throw  std::length_error  If new length exceeds @c max_size().
+       *  @throw  std::out_of_range  If @a pos is beyond the end of this
+       *  string.
+       *
+       *  Inserts the first @a n characters of @a __s starting at @a __pos.  If
+       *  adding characters causes the length to exceed max_size(),
+       *  length_error is thrown.  If @a __pos is beyond end(), out_of_range is
+       *  thrown.  The value of the string doesn't change if an error is
+       *  thrown.
+      */
+      basic_string&
+      insert(size_type __pos, const _CharT* __s)
+      {
+	__glibcxx_requires_string(__s);
+	return this->insert(__pos, __s, traits_type::length(__s));
+      }
+
+      /**
+       *  @brief  Insert multiple characters.
+       *  @param __pos  Index in string to insert at.
+       *  @param __n  Number of characters to insert
+       *  @param __c  The character to insert.
+       *  @return  Reference to this string.
+       *  @throw  std::length_error  If new length exceeds @c max_size().
+       *  @throw  std::out_of_range  If @a __pos is beyond the end of this
+       *  string.
+       *
+       *  Inserts @a __n copies of character @a __c starting at index
+       *  @a __pos.  If adding characters causes the length to exceed
+       *  max_size(), length_error is thrown.  If @a __pos > length(),
+       *  out_of_range is thrown.  The value of the string doesn't
+       *  change if an error is thrown.
+      */
+      basic_string&
+      insert(size_type __pos, size_type __n, _CharT __c)
+      { return _M_replace_aux(_M_check(__pos, "basic_string::insert"),
+			      size_type(0), __n, __c); }
+
+      /**
+       *  @brief  Insert one character.
+       *  @param __p  Iterator referencing position in string to insert at.
+       *  @param __c  The character to insert.
+       *  @return  Iterator referencing newly inserted char.
+       *  @throw  std::length_error  If new length exceeds @c max_size().
+       *
+       *  Inserts character @a __c at position referenced by @a __p.
+       *  If adding character causes the length to exceed max_size(),
+       *  length_error is thrown.  If @a __p is beyond end of string,
+       *  out_of_range is thrown.  The value of the string doesn't
+       *  change if an error is thrown.
+      */
+      iterator
+      insert(iterator __p, _CharT __c)
+      {
+	_GLIBCXX_DEBUG_PEDASSERT(__p >= _M_ibegin() && __p <= _M_iend());
+	const size_type __pos = __p - _M_ibegin();
+	_M_replace_aux(__pos, size_type(0), size_type(1), __c);
+	_M_rep()->_M_set_leaked();
+	return iterator(_M_data() + __pos);
+      }
+
+#if __cplusplus >= 201703L
+      /**
+       *  @brief  Insert a string_view.
+       *  @param __pos  Position in string to insert at.
+       *  @param __svt  The object convertible to string_view to insert.
+       *  @return  Reference to this string.
+      */
+      template<typename _Tp>
+	_If_sv<_Tp, basic_string&>
+	insert(size_type __pos, const _Tp& __svt)
+	{
+	  __sv_type __sv = __svt;
+	  return this->insert(__pos, __sv.data(), __sv.size());
+	}
+
+      /**
+       *  @brief  Insert a string_view.
+       *  @param __pos1  Position in string to insert at.
+       *  @param __svt   The object convertible to string_view to insert from.
+       *  @param __pos2  Position in string_view to insert from.
+       *  @param __n    The number of characters to insert.
+       *  @return  Reference to this string.
+      */
+      template<typename _Tp>
+        _If_sv<_Tp, basic_string&>
+        insert(size_type __pos1, const _Tp& __svt,
+	       size_type __pos2, size_type __n = npos)
+	{
+	  __sv_type __sv = __svt;
+	  return this->replace(__pos1, size_type(0), __sv.data()
+	      + std::__sv_check(__sv.size(), __pos2, "basic_string::insert"),
+	      std::__sv_limit(__sv.size(), __pos2, __n));
+	}
+#endif // C++17
+
+      /**
+       *  @brief  Remove characters.
+       *  @param __pos  Index of first character to remove (default 0).
+       *  @param __n  Number of characters to remove (default remainder).
+       *  @return  Reference to this string.
+       *  @throw  std::out_of_range  If @a pos is beyond the end of this
+       *  string.
+       *
+       *  Removes @a __n characters from this string starting at @a
+       *  __pos.  The length of the string is reduced by @a __n.  If
+       *  there are < @a __n characters to remove, the remainder of
+       *  the string is truncated.  If @a __p is beyond end of string,
+       *  out_of_range is thrown.  The value of the string doesn't
+       *  change if an error is thrown.
+      */
+      basic_string&
+      erase(size_type __pos = 0, size_type __n = npos)
+      {
+	_M_mutate(_M_check(__pos, "basic_string::erase"),
+		  _M_limit(__pos, __n), size_type(0));
+	return *this;
+      }
+
+      /**
+       *  @brief  Remove one character.
+       *  @param __position  Iterator referencing the character to remove.
+       *  @return  iterator referencing same location after removal.
+       *
+       *  Removes the character at @a __position from this string. The value
+       *  of the string doesn't change if an error is thrown.
+      */
+      iterator
+      erase(iterator __position)
+      {
+	_GLIBCXX_DEBUG_PEDASSERT(__position >= _M_ibegin()
+				 && __position < _M_iend());
+	const size_type __pos = __position - _M_ibegin();
+	_M_mutate(__pos, size_type(1), size_type(0));
+	_M_rep()->_M_set_leaked();
+	return iterator(_M_data() + __pos);
+      }
+
+      /**
+       *  @brief  Remove a range of characters.
+       *  @param __first  Iterator referencing the first character to remove.
+       *  @param __last  Iterator referencing the end of the range.
+       *  @return  Iterator referencing location of first after removal.
+       *
+       *  Removes the characters in the range [first,last) from this string.
+       *  The value of the string doesn't change if an error is thrown.
+      */
+      iterator
+      erase(iterator __first, iterator __last);
+
+#if __cplusplus >= 201103L
+      /**
+       *  @brief  Remove the last character.
+       *
+       *  The string must be non-empty.
+       */
+      void
+      pop_back() // FIXME C++11: should be noexcept.
+      {
+	__glibcxx_assert(!empty());
+	erase(size() - 1, 1);
+      }
+#endif // C++11
+
+      /**
+       *  @brief  Replace characters with value from another string.
+       *  @param __pos  Index of first character to replace.
+       *  @param __n  Number of characters to be replaced.
+       *  @param __str  String to insert.
+       *  @return  Reference to this string.
+       *  @throw  std::out_of_range  If @a pos is beyond the end of this
+       *  string.
+       *  @throw  std::length_error  If new length exceeds @c max_size().
+       *
+       *  Removes the characters in the range [__pos,__pos+__n) from
+       *  this string.  In place, the value of @a __str is inserted.
+       *  If @a __pos is beyond end of string, out_of_range is thrown.
+       *  If the length of the result exceeds max_size(), length_error
+       *  is thrown.  The value of the string doesn't change if an
+       *  error is thrown.
+      */
+      basic_string&
+      replace(size_type __pos, size_type __n, const basic_string& __str)
+      { return this->replace(__pos, __n, __str._M_data(), __str.size()); }
+
+      /**
+       *  @brief  Replace characters with value from another string.
+       *  @param __pos1  Index of first character to replace.
+       *  @param __n1  Number of characters to be replaced.
+       *  @param __str  String to insert.
+       *  @param __pos2  Index of first character of str to use.
+       *  @param __n2  Number of characters from str to use.
+       *  @return  Reference to this string.
+       *  @throw  std::out_of_range  If @a __pos1 > size() or @a __pos2 >
+       *  __str.size().
+       *  @throw  std::length_error  If new length exceeds @c max_size().
+       *
+       *  Removes the characters in the range [__pos1,__pos1 + n) from this
+       *  string.  In place, the value of @a __str is inserted.  If @a __pos is
+       *  beyond end of string, out_of_range is thrown.  If the length of the
+       *  result exceeds max_size(), length_error is thrown.  The value of the
+       *  string doesn't change if an error is thrown.
+      */
+      basic_string&
+      replace(size_type __pos1, size_type __n1, const basic_string& __str,
+	      size_type __pos2, size_type __n2 = npos)
+      { return this->replace(__pos1, __n1, __str._M_data()
+			     + __str._M_check(__pos2, "basic_string::replace"),
+			     __str._M_limit(__pos2, __n2)); }
+
+      /**
+       *  @brief  Replace characters with value of a C substring.
+       *  @param __pos  Index of first character to replace.
+       *  @param __n1  Number of characters to be replaced.
+       *  @param __s  C string to insert.
+       *  @param __n2  Number of characters from @a s to use.
+       *  @return  Reference to this string.
+       *  @throw  std::out_of_range  If @a pos1 > size().
+       *  @throw  std::length_error  If new length exceeds @c max_size().
+       *
+       *  Removes the characters in the range [__pos,__pos + __n1)
+       *  from this string.  In place, the first @a __n2 characters of
+       *  @a __s are inserted, or all of @a __s if @a __n2 is too large.  If
+       *  @a __pos is beyond end of string, out_of_range is thrown.  If
+       *  the length of result exceeds max_size(), length_error is
+       *  thrown.  The value of the string doesn't change if an error
+       *  is thrown.
+      */
+      basic_string&
+      replace(size_type __pos, size_type __n1, const _CharT* __s,
+	      size_type __n2);
+
+      /**
+       *  @brief  Replace characters with value of a C string.
+       *  @param __pos  Index of first character to replace.
+       *  @param __n1  Number of characters to be replaced.
+       *  @param __s  C string to insert.
+       *  @return  Reference to this string.
+       *  @throw  std::out_of_range  If @a pos > size().
+       *  @throw  std::length_error  If new length exceeds @c max_size().
+       *
+       *  Removes the characters in the range [__pos,__pos + __n1)
+       *  from this string.  In place, the characters of @a __s are
+       *  inserted.  If @a __pos is beyond end of string, out_of_range
+       *  is thrown.  If the length of result exceeds max_size(),
+       *  length_error is thrown.  The value of the string doesn't
+       *  change if an error is thrown.
+      */
+      basic_string&
+      replace(size_type __pos, size_type __n1, const _CharT* __s)
+      {
+	__glibcxx_requires_string(__s);
+	return this->replace(__pos, __n1, __s, traits_type::length(__s));
+      }
+
+      /**
+       *  @brief  Replace characters with multiple characters.
+       *  @param __pos  Index of first character to replace.
+       *  @param __n1  Number of characters to be replaced.
+       *  @param __n2  Number of characters to insert.
+       *  @param __c  Character to insert.
+       *  @return  Reference to this string.
+       *  @throw  std::out_of_range  If @a __pos > size().
+       *  @throw  std::length_error  If new length exceeds @c max_size().
+       *
+       *  Removes the characters in the range [pos,pos + n1) from this
+       *  string.  In place, @a __n2 copies of @a __c are inserted.
+       *  If @a __pos is beyond end of string, out_of_range is thrown.
+       *  If the length of result exceeds max_size(), length_error is
+       *  thrown.  The value of the string doesn't change if an error
+       *  is thrown.
+      */
+      basic_string&
+      replace(size_type __pos, size_type __n1, size_type __n2, _CharT __c)
+      { return _M_replace_aux(_M_check(__pos, "basic_string::replace"),
+			      _M_limit(__pos, __n1), __n2, __c); }
+
+      /**
+       *  @brief  Replace range of characters with string.
+       *  @param __i1  Iterator referencing start of range to replace.
+       *  @param __i2  Iterator referencing end of range to replace.
+       *  @param __str  String value to insert.
+       *  @return  Reference to this string.
+       *  @throw  std::length_error  If new length exceeds @c max_size().
+       *
+       *  Removes the characters in the range [__i1,__i2).  In place,
+       *  the value of @a __str is inserted.  If the length of result
+       *  exceeds max_size(), length_error is thrown.  The value of
+       *  the string doesn't change if an error is thrown.
+      */
+      basic_string&
+      replace(iterator __i1, iterator __i2, const basic_string& __str)
+      { return this->replace(__i1, __i2, __str._M_data(), __str.size()); }
+
+      /**
+       *  @brief  Replace range of characters with C substring.
+       *  @param __i1  Iterator referencing start of range to replace.
+       *  @param __i2  Iterator referencing end of range to replace.
+       *  @param __s  C string value to insert.
+       *  @param __n  Number of characters from s to insert.
+       *  @return  Reference to this string.
+       *  @throw  std::length_error  If new length exceeds @c max_size().
+       *
+       *  Removes the characters in the range [__i1,__i2).  In place,
+       *  the first @a __n characters of @a __s are inserted.  If the
+       *  length of result exceeds max_size(), length_error is thrown.
+       *  The value of the string doesn't change if an error is
+       *  thrown.
+      */
+      basic_string&
+      replace(iterator __i1, iterator __i2, const _CharT* __s, size_type __n)
+      {
+	_GLIBCXX_DEBUG_PEDASSERT(_M_ibegin() <= __i1 && __i1 <= __i2
+				 && __i2 <= _M_iend());
+	return this->replace(__i1 - _M_ibegin(), __i2 - __i1, __s, __n);
+      }
+
+      /**
+       *  @brief  Replace range of characters with C string.
+       *  @param __i1  Iterator referencing start of range to replace.
+       *  @param __i2  Iterator referencing end of range to replace.
+       *  @param __s  C string value to insert.
+       *  @return  Reference to this string.
+       *  @throw  std::length_error  If new length exceeds @c max_size().
+       *
+       *  Removes the characters in the range [__i1,__i2).  In place,
+       *  the characters of @a __s are inserted.  If the length of
+       *  result exceeds max_size(), length_error is thrown.  The
+       *  value of the string doesn't change if an error is thrown.
+      */
+      basic_string&
+      replace(iterator __i1, iterator __i2, const _CharT* __s)
+      {
+	__glibcxx_requires_string(__s);
+	return this->replace(__i1, __i2, __s, traits_type::length(__s));
+      }
+
+      /**
+       *  @brief  Replace range of characters with multiple characters
+       *  @param __i1  Iterator referencing start of range to replace.
+       *  @param __i2  Iterator referencing end of range to replace.
+       *  @param __n  Number of characters to insert.
+       *  @param __c  Character to insert.
+       *  @return  Reference to this string.
+       *  @throw  std::length_error  If new length exceeds @c max_size().
+       *
+       *  Removes the characters in the range [__i1,__i2).  In place,
+       *  @a __n copies of @a __c are inserted.  If the length of
+       *  result exceeds max_size(), length_error is thrown.  The
+       *  value of the string doesn't change if an error is thrown.
+      */
+      basic_string&
+      replace(iterator __i1, iterator __i2, size_type __n, _CharT __c)
+      {
+	_GLIBCXX_DEBUG_PEDASSERT(_M_ibegin() <= __i1 && __i1 <= __i2
+				 && __i2 <= _M_iend());
+	return _M_replace_aux(__i1 - _M_ibegin(), __i2 - __i1, __n, __c);
+      }
+
+      /**
+       *  @brief  Replace range of characters with range.
+       *  @param __i1  Iterator referencing start of range to replace.
+       *  @param __i2  Iterator referencing end of range to replace.
+       *  @param __k1  Iterator referencing start of range to insert.
+       *  @param __k2  Iterator referencing end of range to insert.
+       *  @return  Reference to this string.
+       *  @throw  std::length_error  If new length exceeds @c max_size().
+       *
+       *  Removes the characters in the range [__i1,__i2).  In place,
+       *  characters in the range [__k1,__k2) are inserted.  If the
+       *  length of result exceeds max_size(), length_error is thrown.
+       *  The value of the string doesn't change if an error is
+       *  thrown.
+      */
+      template<class _InputIterator>
+        basic_string&
+        replace(iterator __i1, iterator __i2,
+		_InputIterator __k1, _InputIterator __k2)
+        {
+	  _GLIBCXX_DEBUG_PEDASSERT(_M_ibegin() <= __i1 && __i1 <= __i2
+				   && __i2 <= _M_iend());
+	  __glibcxx_requires_valid_range(__k1, __k2);
+	  typedef typename std::__is_integer<_InputIterator>::__type _Integral;
+	  return _M_replace_dispatch(__i1, __i2, __k1, __k2, _Integral());
+	}
+
+      // Specializations for the common case of pointer and iterator:
+      // useful to avoid the overhead of temporary buffering in _M_replace.
+      basic_string&
+      replace(iterator __i1, iterator __i2, _CharT* __k1, _CharT* __k2)
+      {
+	_GLIBCXX_DEBUG_PEDASSERT(_M_ibegin() <= __i1 && __i1 <= __i2
+				 && __i2 <= _M_iend());
+	__glibcxx_requires_valid_range(__k1, __k2);
+	return this->replace(__i1 - _M_ibegin(), __i2 - __i1,
+			     __k1, __k2 - __k1);
+      }
+
+      basic_string&
+      replace(iterator __i1, iterator __i2,
+	      const _CharT* __k1, const _CharT* __k2)
+      {
+	_GLIBCXX_DEBUG_PEDASSERT(_M_ibegin() <= __i1 && __i1 <= __i2
+				 && __i2 <= _M_iend());
+	__glibcxx_requires_valid_range(__k1, __k2);
+	return this->replace(__i1 - _M_ibegin(), __i2 - __i1,
+			     __k1, __k2 - __k1);
+      }
+
+      basic_string&
+      replace(iterator __i1, iterator __i2, iterator __k1, iterator __k2)
+      {
+	_GLIBCXX_DEBUG_PEDASSERT(_M_ibegin() <= __i1 && __i1 <= __i2
+				 && __i2 <= _M_iend());
+	__glibcxx_requires_valid_range(__k1, __k2);
+	return this->replace(__i1 - _M_ibegin(), __i2 - __i1,
+			     __k1.base(), __k2 - __k1);
+      }
+
+      basic_string&
+      replace(iterator __i1, iterator __i2,
+	      const_iterator __k1, const_iterator __k2)
+      {
+	_GLIBCXX_DEBUG_PEDASSERT(_M_ibegin() <= __i1 && __i1 <= __i2
+				 && __i2 <= _M_iend());
+	__glibcxx_requires_valid_range(__k1, __k2);
+	return this->replace(__i1 - _M_ibegin(), __i2 - __i1,
+			     __k1.base(), __k2 - __k1);
+      }
+
+#if __cplusplus >= 201103L
+      /**
+       *  @brief  Replace range of characters with initializer_list.
+       *  @param __i1  Iterator referencing start of range to replace.
+       *  @param __i2  Iterator referencing end of range to replace.
+       *  @param __l  The initializer_list of characters to insert.
+       *  @return  Reference to this string.
+       *  @throw  std::length_error  If new length exceeds @c max_size().
+       *
+       *  Removes the characters in the range [__i1,__i2).  In place,
+       *  characters in the range [__k1,__k2) are inserted.  If the
+       *  length of result exceeds max_size(), length_error is thrown.
+       *  The value of the string doesn't change if an error is
+       *  thrown.
+      */
+      basic_string& replace(iterator __i1, iterator __i2,
+			    initializer_list<_CharT> __l)
+      { return this->replace(__i1, __i2, __l.begin(), __l.end()); }
+#endif // C++11
+
+#if __cplusplus >= 201703L
+      /**
+       *  @brief  Replace range of characters with string_view.
+       *  @param __pos  The position to replace at.
+       *  @param __n    The number of characters to replace.
+       *  @param __svt  The object convertible to string_view to insert.
+       *  @return  Reference to this string.
+      */
+      template<typename _Tp>
+	_If_sv<_Tp, basic_string&>
+	replace(size_type __pos, size_type __n, const _Tp& __svt)
+	{
+	  __sv_type __sv = __svt;
+	  return this->replace(__pos, __n, __sv.data(), __sv.size());
+	}
+
+      /**
+       *  @brief  Replace range of characters with string_view.
+       *  @param __pos1  The position to replace at.
+       *  @param __n1    The number of characters to replace.
+       *  @param __svt   The object convertible to string_view to insert from.
+       *  @param __pos2  The position in the string_view to insert from.
+       *  @param __n2    The number of characters to insert.
+       *  @return  Reference to this string.
+      */
+      template<typename _Tp>
+        _If_sv<_Tp, basic_string&>
+        replace(size_type __pos1, size_type __n1, const _Tp& __svt,
+		size_type __pos2, size_type __n2 = npos)
+	{
+	  __sv_type __sv = __svt;
+	  return this->replace(__pos1, __n1,
+	      __sv.data()
+	      + std::__sv_check(__sv.size(), __pos2, "basic_string::replace"),
+	      std::__sv_limit(__sv.size(), __pos2, __n2));
+	}
+
+      /**
+       *  @brief  Replace range of characters with string_view.
+       *  @param __i1    An iterator referencing the start position
+          to replace at.
+       *  @param __i2    An iterator referencing the end position
+          for the replace.
+       *  @param __svt   The object convertible to string_view to insert from.
+       *  @return  Reference to this string.
+      */
+      template<typename _Tp>
+	_If_sv<_Tp, basic_string&>
+	replace(const_iterator __i1, const_iterator __i2, const _Tp& __svt)
+	{
+	  __sv_type __sv = __svt;
+	  return this->replace(__i1 - begin(), __i2 - __i1, __sv);
+	}
+#endif // C++17
+
+    private:
+      template<class _Integer>
+	basic_string&
+	_M_replace_dispatch(iterator __i1, iterator __i2, _Integer __n,
+			    _Integer __val, __true_type)
+        { return _M_replace_aux(__i1 - _M_ibegin(), __i2 - __i1, __n, __val); }
+
+      template<class _InputIterator>
+	basic_string&
+	_M_replace_dispatch(iterator __i1, iterator __i2, _InputIterator __k1,
+			    _InputIterator __k2, __false_type);
+
+      basic_string&
+      _M_replace_aux(size_type __pos1, size_type __n1, size_type __n2,
+		     _CharT __c);
+
+      basic_string&
+      _M_replace_safe(size_type __pos1, size_type __n1, const _CharT* __s,
+		      size_type __n2);
+
+      // _S_construct_aux is used to implement the 21.3.1 para 15 which
+      // requires special behaviour if _InIter is an integral type
+      template<class _InIterator>
+        static _CharT*
+        _S_construct_aux(_InIterator __beg, _InIterator __end,
+			 const _Alloc& __a, __false_type)
+	{
+          typedef typename iterator_traits<_InIterator>::iterator_category _Tag;
+          return _S_construct(__beg, __end, __a, _Tag());
+	}
+
+      // _GLIBCXX_RESOLVE_LIB_DEFECTS
+      // 438. Ambiguity in the "do the right thing" clause
+      template<class _Integer>
+        static _CharT*
+        _S_construct_aux(_Integer __beg, _Integer __end,
+			 const _Alloc& __a, __true_type)
+        { return _S_construct_aux_2(static_cast<size_type>(__beg),
+				    __end, __a); }
+
+      static _CharT*
+      _S_construct_aux_2(size_type __req, _CharT __c, const _Alloc& __a)
+      { return _S_construct(__req, __c, __a); }
+
+      template<class _InIterator>
+        static _CharT*
+        _S_construct(_InIterator __beg, _InIterator __end, const _Alloc& __a)
+	{
+	  typedef typename std::__is_integer<_InIterator>::__type _Integral;
+	  return _S_construct_aux(__beg, __end, __a, _Integral());
+        }
+
+      // For Input Iterators, used in istreambuf_iterators, etc.
+      template<class _InIterator>
+        static _CharT*
+         _S_construct(_InIterator __beg, _InIterator __end, const _Alloc& __a,
+		      input_iterator_tag);
+
+      // For forward_iterators up to random_access_iterators, used for
+      // string::iterator, _CharT*, etc.
+      template<class _FwdIterator>
+        static _CharT*
+        _S_construct(_FwdIterator __beg, _FwdIterator __end, const _Alloc& __a,
+		     forward_iterator_tag);
+
+      static _CharT*
+      _S_construct(size_type __req, _CharT __c, const _Alloc& __a);
+
+    public:
+
+      /**
+       *  @brief  Copy substring into C string.
+       *  @param __s  C string to copy value into.
+       *  @param __n  Number of characters to copy.
+       *  @param __pos  Index of first character to copy.
+       *  @return  Number of characters actually copied
+       *  @throw  std::out_of_range  If __pos > size().
+       *
+       *  Copies up to @a __n characters starting at @a __pos into the
+       *  C string @a __s.  If @a __pos is %greater than size(),
+       *  out_of_range is thrown.
+      */
+      size_type
+      copy(_CharT* __s, size_type __n, size_type __pos = 0) const;
+
+      /**
+       *  @brief  Swap contents with another string.
+       *  @param __s  String to swap with.
+       *
+       *  Exchanges the contents of this string with that of @a __s in constant
+       *  time.
+      */
+      void
+      swap(basic_string& __s)
+      _GLIBCXX_NOEXCEPT_IF(allocator_traits<_Alloc>::is_always_equal::value);
+
+      // String operations:
+      /**
+       *  @brief  Return const pointer to null-terminated contents.
+       *
+       *  This is a handle to internal data.  Do not modify or dire things may
+       *  happen.
+      */
+      const _CharT*
+      c_str() const _GLIBCXX_NOEXCEPT
+      { return _M_data(); }
+
+      /**
+       *  @brief  Return const pointer to contents.
+       *
+       *  This is a pointer to internal data.  It is undefined to modify
+       *  the contents through the returned pointer. To get a pointer that
+       *  allows modifying the contents use @c &str[0] instead,
+       *  (or in C++17 the non-const @c str.data() overload).
+      */
+      const _CharT*
+      data() const _GLIBCXX_NOEXCEPT
+      { return _M_data(); }
+
+#if __cplusplus >= 201703L
+      /**
+       *  @brief  Return non-const pointer to contents.
+       *
+       *  This is a pointer to the character sequence held by the string.
+       *  Modifying the characters in the sequence is allowed.
+      */
+      _CharT*
+      data() noexcept
+      {
+	_M_leak();
+	return _M_data();
+      }
+#endif
+
+      /**
+       *  @brief  Return copy of allocator used to construct this string.
+      */
+      allocator_type
+      get_allocator() const _GLIBCXX_NOEXCEPT
+      { return _M_dataplus; }
+
+      /**
+       *  @brief  Find position of a C substring.
+       *  @param __s  C string to locate.
+       *  @param __pos  Index of character to search from.
+       *  @param __n  Number of characters from @a s to search for.
+       *  @return  Index of start of first occurrence.
+       *
+       *  Starting from @a __pos, searches forward for the first @a
+       *  __n characters in @a __s within this string.  If found,
+       *  returns the index where it begins.  If not found, returns
+       *  npos.
+      */
+      size_type
+      find(const _CharT* __s, size_type __pos, size_type __n) const
+      _GLIBCXX_NOEXCEPT;
+
+      /**
+       *  @brief  Find position of a string.
+       *  @param __str  String to locate.
+       *  @param __pos  Index of character to search from (default 0).
+       *  @return  Index of start of first occurrence.
+       *
+       *  Starting from @a __pos, searches forward for value of @a __str within
+       *  this string.  If found, returns the index where it begins.  If not
+       *  found, returns npos.
+      */
+      size_type
+      find(const basic_string& __str, size_type __pos = 0) const
+      _GLIBCXX_NOEXCEPT
+      { return this->find(__str.data(), __pos, __str.size()); }
+
+      /**
+       *  @brief  Find position of a C string.
+       *  @param __s  C string to locate.
+       *  @param __pos  Index of character to search from (default 0).
+       *  @return  Index of start of first occurrence.
+       *
+       *  Starting from @a __pos, searches forward for the value of @a
+       *  __s within this string.  If found, returns the index where
+       *  it begins.  If not found, returns npos.
+      */
+      size_type
+      find(const _CharT* __s, size_type __pos = 0) const _GLIBCXX_NOEXCEPT
+      {
+	__glibcxx_requires_string(__s);
+	return this->find(__s, __pos, traits_type::length(__s));
+      }
+
+      /**
+       *  @brief  Find position of a character.
+       *  @param __c  Character to locate.
+       *  @param __pos  Index of character to search from (default 0).
+       *  @return  Index of first occurrence.
+       *
+       *  Starting from @a __pos, searches forward for @a __c within
+       *  this string.  If found, returns the index where it was
+       *  found.  If not found, returns npos.
+      */
+      size_type
+      find(_CharT __c, size_type __pos = 0) const _GLIBCXX_NOEXCEPT;
+
+#if __cplusplus >= 201703L
+      /**
+       *  @brief  Find position of a string_view.
+       *  @param __svt  The object convertible to string_view to locate.
+       *  @param __pos  Index of character to search from (default 0).
+       *  @return  Index of start of first occurrence.
+      */
+      template<typename _Tp>
+	_If_sv<_Tp, size_type>
+	find(const _Tp& __svt, size_type __pos = 0) const
+	noexcept(is_same<_Tp, __sv_type>::value)
+	{
+	  __sv_type __sv = __svt;
+	  return this->find(__sv.data(), __pos, __sv.size());
+	}
+#endif // C++17
+
+      /**
+       *  @brief  Find last position of a string.
+       *  @param __str  String to locate.
+       *  @param __pos  Index of character to search back from (default end).
+       *  @return  Index of start of last occurrence.
+       *
+       *  Starting from @a __pos, searches backward for value of @a
+       *  __str within this string.  If found, returns the index where
+       *  it begins.  If not found, returns npos.
+      */
+      size_type
+      rfind(const basic_string& __str, size_type __pos = npos) const
+      _GLIBCXX_NOEXCEPT
+      { return this->rfind(__str.data(), __pos, __str.size()); }
+
+      /**
+       *  @brief  Find last position of a C substring.
+       *  @param __s  C string to locate.
+       *  @param __pos  Index of character to search back from.
+       *  @param __n  Number of characters from s to search for.
+       *  @return  Index of start of last occurrence.
+       *
+       *  Starting from @a __pos, searches backward for the first @a
+       *  __n characters in @a __s within this string.  If found,
+       *  returns the index where it begins.  If not found, returns
+       *  npos.
+      */
+      size_type
+      rfind(const _CharT* __s, size_type __pos, size_type __n) const
+      _GLIBCXX_NOEXCEPT;
+
+      /**
+       *  @brief  Find last position of a C string.
+       *  @param __s  C string to locate.
+       *  @param __pos  Index of character to start search at (default end).
+       *  @return  Index of start of  last occurrence.
+       *
+       *  Starting from @a __pos, searches backward for the value of
+       *  @a __s within this string.  If found, returns the index
+       *  where it begins.  If not found, returns npos.
+      */
+      size_type
+      rfind(const _CharT* __s, size_type __pos = npos) const _GLIBCXX_NOEXCEPT
+      {
+	__glibcxx_requires_string(__s);
+	return this->rfind(__s, __pos, traits_type::length(__s));
+      }
+
+      /**
+       *  @brief  Find last position of a character.
+       *  @param __c  Character to locate.
+       *  @param __pos  Index of character to search back from (default end).
+       *  @return  Index of last occurrence.
+       *
+       *  Starting from @a __pos, searches backward for @a __c within
+       *  this string.  If found, returns the index where it was
+       *  found.  If not found, returns npos.
+      */
+      size_type
+      rfind(_CharT __c, size_type __pos = npos) const _GLIBCXX_NOEXCEPT;
+
+#if __cplusplus >= 201703L
+      /**
+       *  @brief  Find last position of a string_view.
+       *  @param __svt  The object convertible to string_view to locate.
+       *  @param __pos  Index of character to search back from (default end).
+       *  @return  Index of start of last occurrence.
+      */
+      template<typename _Tp>
+	_If_sv<_Tp, size_type>
+	rfind(const _Tp& __svt, size_type __pos = npos) const
+	noexcept(is_same<_Tp, __sv_type>::value)
+	{
+	  __sv_type __sv = __svt;
+	  return this->rfind(__sv.data(), __pos, __sv.size());
+	}
+#endif // C++17
+
+      /**
+       *  @brief  Find position of a character of string.
+       *  @param __str  String containing characters to locate.
+       *  @param __pos  Index of character to search from (default 0).
+       *  @return  Index of first occurrence.
+       *
+       *  Starting from @a __pos, searches forward for one of the
+       *  characters of @a __str within this string.  If found,
+       *  returns the index where it was found.  If not found, returns
+       *  npos.
+      */
+      size_type
+      find_first_of(const basic_string& __str, size_type __pos = 0) const
+      _GLIBCXX_NOEXCEPT
+      { return this->find_first_of(__str.data(), __pos, __str.size()); }
+
+      /**
+       *  @brief  Find position of a character of C substring.
+       *  @param __s  String containing characters to locate.
+       *  @param __pos  Index of character to search from.
+       *  @param __n  Number of characters from s to search for.
+       *  @return  Index of first occurrence.
+       *
+       *  Starting from @a __pos, searches forward for one of the
+       *  first @a __n characters of @a __s within this string.  If
+       *  found, returns the index where it was found.  If not found,
+       *  returns npos.
+      */
+      size_type
+      find_first_of(const _CharT* __s, size_type __pos, size_type __n) const
+      _GLIBCXX_NOEXCEPT;
+
+      /**
+       *  @brief  Find position of a character of C string.
+       *  @param __s  String containing characters to locate.
+       *  @param __pos  Index of character to search from (default 0).
+       *  @return  Index of first occurrence.
+       *
+       *  Starting from @a __pos, searches forward for one of the
+       *  characters of @a __s within this string.  If found, returns
+       *  the index where it was found.  If not found, returns npos.
+      */
+      size_type
+      find_first_of(const _CharT* __s, size_type __pos = 0) const
+      _GLIBCXX_NOEXCEPT
+      {
+	__glibcxx_requires_string(__s);
+	return this->find_first_of(__s, __pos, traits_type::length(__s));
+      }
+
+      /**
+       *  @brief  Find position of a character.
+       *  @param __c  Character to locate.
+       *  @param __pos  Index of character to search from (default 0).
+       *  @return  Index of first occurrence.
+       *
+       *  Starting from @a __pos, searches forward for the character
+       *  @a __c within this string.  If found, returns the index
+       *  where it was found.  If not found, returns npos.
+       *
+       *  Note: equivalent to find(__c, __pos).
+      */
+      size_type
+      find_first_of(_CharT __c, size_type __pos = 0) const _GLIBCXX_NOEXCEPT
+      { return this->find(__c, __pos); }
+
+#if __cplusplus >= 201703L
+      /**
+       *  @brief  Find position of a character of a string_view.
+       *  @param __svt  An object convertible to string_view containing
+       *                characters to locate.
+       *  @param __pos  Index of character to search from (default 0).
+       *  @return  Index of first occurrence.
+      */
+      template<typename _Tp>
+	_If_sv<_Tp, size_type>
+	find_first_of(const _Tp& __svt, size_type __pos = 0) const
+	noexcept(is_same<_Tp, __sv_type>::value)
+	{
+	  __sv_type __sv = __svt;
+	  return this->find_first_of(__sv.data(), __pos, __sv.size());
+	}
+#endif // C++17
+
+      /**
+       *  @brief  Find last position of a character of string.
+       *  @param __str  String containing characters to locate.
+       *  @param __pos  Index of character to search back from (default end).
+       *  @return  Index of last occurrence.
+       *
+       *  Starting from @a __pos, searches backward for one of the
+       *  characters of @a __str within this string.  If found,
+       *  returns the index where it was found.  If not found, returns
+       *  npos.
+      */
+      size_type
+      find_last_of(const basic_string& __str, size_type __pos = npos) const
+      _GLIBCXX_NOEXCEPT
+      { return this->find_last_of(__str.data(), __pos, __str.size()); }
+
+      /**
+       *  @brief  Find last position of a character of C substring.
+       *  @param __s  C string containing characters to locate.
+       *  @param __pos  Index of character to search back from.
+       *  @param __n  Number of characters from s to search for.
+       *  @return  Index of last occurrence.
+       *
+       *  Starting from @a __pos, searches backward for one of the
+       *  first @a __n characters of @a __s within this string.  If
+       *  found, returns the index where it was found.  If not found,
+       *  returns npos.
+      */
+      size_type
+      find_last_of(const _CharT* __s, size_type __pos, size_type __n) const
+      _GLIBCXX_NOEXCEPT;
+
+      /**
+       *  @brief  Find last position of a character of C string.
+       *  @param __s  C string containing characters to locate.
+       *  @param __pos  Index of character to search back from (default end).
+       *  @return  Index of last occurrence.
+       *
+       *  Starting from @a __pos, searches backward for one of the
+       *  characters of @a __s within this string.  If found, returns
+       *  the index where it was found.  If not found, returns npos.
+      */
+      size_type
+      find_last_of(const _CharT* __s, size_type __pos = npos) const
+      _GLIBCXX_NOEXCEPT
+      {
+	__glibcxx_requires_string(__s);
+	return this->find_last_of(__s, __pos, traits_type::length(__s));
+      }
+
+      /**
+       *  @brief  Find last position of a character.
+       *  @param __c  Character to locate.
+       *  @param __pos  Index of character to search back from (default end).
+       *  @return  Index of last occurrence.
+       *
+       *  Starting from @a __pos, searches backward for @a __c within
+       *  this string.  If found, returns the index where it was
+       *  found.  If not found, returns npos.
+       *
+       *  Note: equivalent to rfind(__c, __pos).
+      */
+      size_type
+      find_last_of(_CharT __c, size_type __pos = npos) const _GLIBCXX_NOEXCEPT
+      { return this->rfind(__c, __pos); }
+
+#if __cplusplus >= 201703L
+      /**
+       *  @brief  Find last position of a character of string.
+       *  @param __svt  An object convertible to string_view containing
+       *                characters to locate.
+       *  @param __pos  Index of character to search back from (default end).
+       *  @return  Index of last occurrence.
+      */
+      template<typename _Tp>
+	_If_sv<_Tp, size_type>
+	find_last_of(const _Tp& __svt, size_type __pos = npos) const
+	noexcept(is_same<_Tp, __sv_type>::value)
+	{
+	  __sv_type __sv = __svt;
+	  return this->find_last_of(__sv.data(), __pos, __sv.size());
+	}
+#endif // C++17
+
+      /**
+       *  @brief  Find position of a character not in string.
+       *  @param __str  String containing characters to avoid.
+       *  @param __pos  Index of character to search from (default 0).
+       *  @return  Index of first occurrence.
+       *
+       *  Starting from @a __pos, searches forward for a character not contained
+       *  in @a __str within this string.  If found, returns the index where it
+       *  was found.  If not found, returns npos.
+      */
+      size_type
+      find_first_not_of(const basic_string& __str, size_type __pos = 0) const
+      _GLIBCXX_NOEXCEPT
+      { return this->find_first_not_of(__str.data(), __pos, __str.size()); }
+
+      /**
+       *  @brief  Find position of a character not in C substring.
+       *  @param __s  C string containing characters to avoid.
+       *  @param __pos  Index of character to search from.
+       *  @param __n  Number of characters from __s to consider.
+       *  @return  Index of first occurrence.
+       *
+       *  Starting from @a __pos, searches forward for a character not
+       *  contained in the first @a __n characters of @a __s within
+       *  this string.  If found, returns the index where it was
+       *  found.  If not found, returns npos.
+      */
+      size_type
+      find_first_not_of(const _CharT* __s, size_type __pos,
+			size_type __n) const _GLIBCXX_NOEXCEPT;
+
+      /**
+       *  @brief  Find position of a character not in C string.
+       *  @param __s  C string containing characters to avoid.
+       *  @param __pos  Index of character to search from (default 0).
+       *  @return  Index of first occurrence.
+       *
+       *  Starting from @a __pos, searches forward for a character not
+       *  contained in @a __s within this string.  If found, returns
+       *  the index where it was found.  If not found, returns npos.
+      */
+      size_type
+      find_first_not_of(const _CharT* __s, size_type __pos = 0) const
+      _GLIBCXX_NOEXCEPT
+      {
+	__glibcxx_requires_string(__s);
+	return this->find_first_not_of(__s, __pos, traits_type::length(__s));
+      }
+
+      /**
+       *  @brief  Find position of a different character.
+       *  @param __c  Character to avoid.
+       *  @param __pos  Index of character to search from (default 0).
+       *  @return  Index of first occurrence.
+       *
+       *  Starting from @a __pos, searches forward for a character
+       *  other than @a __c within this string.  If found, returns the
+       *  index where it was found.  If not found, returns npos.
+      */
+      size_type
+      find_first_not_of(_CharT __c, size_type __pos = 0) const
+      _GLIBCXX_NOEXCEPT;
+
+#if __cplusplus >= 201703L
+      /**
+       *  @brief  Find position of a character not in a string_view.
+       *  @param __svt  An object convertible to string_view containing
+       *                characters to avoid.
+       *  @param __pos  Index of character to search from (default 0).
+       *  @return  Index of first occurrence.
+       */
+      template<typename _Tp>
+	_If_sv<_Tp, size_type>
+	find_first_not_of(const _Tp& __svt, size_type __pos = 0) const
+	noexcept(is_same<_Tp, __sv_type>::value)
+	{
+	  __sv_type __sv = __svt;
+	  return this->find_first_not_of(__sv.data(), __pos, __sv.size());
+	}
+#endif // C++17
+
+      /**
+       *  @brief  Find last position of a character not in string.
+       *  @param __str  String containing characters to avoid.
+       *  @param __pos  Index of character to search back from (default end).
+       *  @return  Index of last occurrence.
+       *
+       *  Starting from @a __pos, searches backward for a character
+       *  not contained in @a __str within this string.  If found,
+       *  returns the index where it was found.  If not found, returns
+       *  npos.
+      */
+      size_type
+      find_last_not_of(const basic_string& __str, size_type __pos = npos) const
+      _GLIBCXX_NOEXCEPT
+      { return this->find_last_not_of(__str.data(), __pos, __str.size()); }
+
+      /**
+       *  @brief  Find last position of a character not in C substring.
+       *  @param __s  C string containing characters to avoid.
+       *  @param __pos  Index of character to search back from.
+       *  @param __n  Number of characters from s to consider.
+       *  @return  Index of last occurrence.
+       *
+       *  Starting from @a __pos, searches backward for a character not
+       *  contained in the first @a __n characters of @a __s within this string.
+       *  If found, returns the index where it was found.  If not found,
+       *  returns npos.
+      */
+      size_type
+      find_last_not_of(const _CharT* __s, size_type __pos,
+		       size_type __n) const _GLIBCXX_NOEXCEPT;
+      /**
+       *  @brief  Find last position of a character not in C string.
+       *  @param __s  C string containing characters to avoid.
+       *  @param __pos  Index of character to search back from (default end).
+       *  @return  Index of last occurrence.
+       *
+       *  Starting from @a __pos, searches backward for a character
+       *  not contained in @a __s within this string.  If found,
+       *  returns the index where it was found.  If not found, returns
+       *  npos.
+      */
+      size_type
+      find_last_not_of(const _CharT* __s, size_type __pos = npos) const
+      _GLIBCXX_NOEXCEPT
+      {
+	__glibcxx_requires_string(__s);
+	return this->find_last_not_of(__s, __pos, traits_type::length(__s));
+      }
+
+      /**
+       *  @brief  Find last position of a different character.
+       *  @param __c  Character to avoid.
+       *  @param __pos  Index of character to search back from (default end).
+       *  @return  Index of last occurrence.
+       *
+       *  Starting from @a __pos, searches backward for a character other than
+       *  @a __c within this string.  If found, returns the index where it was
+       *  found.  If not found, returns npos.
+      */
+      size_type
+      find_last_not_of(_CharT __c, size_type __pos = npos) const
+      _GLIBCXX_NOEXCEPT;
+
+#if __cplusplus >= 201703L
+      /**
+       *  @brief  Find last position of a character not in a string_view.
+       *  @param __svt  An object convertible to string_view containing
+       *                characters to avoid.
+       *  @param __pos  Index of character to search back from (default end).
+       *  @return  Index of last occurrence.
+       */
+      template<typename _Tp>
+	_If_sv<_Tp, size_type>
+	find_last_not_of(const _Tp& __svt, size_type __pos = npos) const
+	noexcept(is_same<_Tp, __sv_type>::value)
+	{
+	  __sv_type __sv = __svt;
+	  return this->find_last_not_of(__sv.data(), __pos, __sv.size());
+	}
+#endif // C++17
+
+      /**
+       *  @brief  Get a substring.
+       *  @param __pos  Index of first character (default 0).
+       *  @param __n  Number of characters in substring (default remainder).
+       *  @return  The new string.
+       *  @throw  std::out_of_range  If __pos > size().
+       *
+       *  Construct and return a new string using the @a __n
+       *  characters starting at @a __pos.  If the string is too
+       *  short, use the remainder of the characters.  If @a __pos is
+       *  beyond the end of the string, out_of_range is thrown.
+      */
+      basic_string
+      substr(size_type __pos = 0, size_type __n = npos) const
+      { return basic_string(*this,
+			    _M_check(__pos, "basic_string::substr"), __n); }
+
+      /**
+       *  @brief  Compare to a string.
+       *  @param __str  String to compare against.
+       *  @return  Integer < 0, 0, or > 0.
+       *
+       *  Returns an integer < 0 if this string is ordered before @a
+       *  __str, 0 if their values are equivalent, or > 0 if this
+       *  string is ordered after @a __str.  Determines the effective
+       *  length rlen of the strings to compare as the smallest of
+       *  size() and str.size().  The function then compares the two
+       *  strings by calling traits::compare(data(), str.data(),rlen).
+       *  If the result of the comparison is nonzero returns it,
+       *  otherwise the shorter one is ordered first.
+      */
+      int
+      compare(const basic_string& __str) const
+      {
+	const size_type __size = this->size();
+	const size_type __osize = __str.size();
+	const size_type __len = std::min(__size, __osize);
+
+	int __r = traits_type::compare(_M_data(), __str.data(), __len);
+	if (!__r)
+	  __r = _S_compare(__size, __osize);
+	return __r;
+      }
+
+#if __cplusplus >= 201703L
+      /**
+       *  @brief  Compare to a string_view.
+       *  @param __svt An object convertible to string_view to compare against.
+       *  @return  Integer < 0, 0, or > 0.
+       */
+      template<typename _Tp>
+	_If_sv<_Tp, int>
+	compare(const _Tp& __svt) const
+	noexcept(is_same<_Tp, __sv_type>::value)
+	{
+	   __sv_type __sv = __svt;
+	  const size_type __size = this->size();
+	  const size_type __osize = __sv.size();
+	  const size_type __len = std::min(__size, __osize);
+
+	  int __r = traits_type::compare(_M_data(), __sv.data(), __len);
+	  if (!__r)
+	    __r = _S_compare(__size, __osize);
+	  return __r;
+	}
+
+      /**
+       *  @brief  Compare to a string_view.
+       *  @param __pos  A position in the string to start comparing from.
+       *  @param __n  The number of characters to compare.
+       *  @param __svt  An object convertible to string_view to compare
+       *                against.
+       *  @return  Integer < 0, 0, or > 0.
+       */
+      template<typename _Tp>
+	_If_sv<_Tp, int>
+	compare(size_type __pos, size_type __n, const _Tp& __svt) const
+	noexcept(is_same<_Tp, __sv_type>::value)
+	{
+	  __sv_type __sv = __svt;
+	  return __sv_type(*this).substr(__pos, __n).compare(__sv);
+	}
+
+      /**
+       *  @brief  Compare to a string_view.
+       *  @param __pos1  A position in the string to start comparing from.
+       *  @param __n1  The number of characters to compare.
+       *  @param __svt   An object convertible to string_view to compare
+       *                 against.
+       *  @param __pos2  A position in the string_view to start comparing from.
+       *  @param __n2  The number of characters to compare.
+       *  @return  Integer < 0, 0, or > 0.
+       */
+      template<typename _Tp>
+	_If_sv<_Tp, int>
+	compare(size_type __pos1, size_type __n1, const _Tp& __svt,
+		size_type __pos2, size_type __n2 = npos) const
+	noexcept(is_same<_Tp, __sv_type>::value)
+	{
+	  __sv_type __sv = __svt;
+	  return __sv_type(*this)
+	    .substr(__pos1, __n1).compare(__sv.substr(__pos2, __n2));
+	}
+#endif // C++17
+
+      /**
+       *  @brief  Compare substring to a string.
+       *  @param __pos  Index of first character of substring.
+       *  @param __n  Number of characters in substring.
+       *  @param __str  String to compare against.
+       *  @return  Integer < 0, 0, or > 0.
+       *
+       *  Form the substring of this string from the @a __n characters
+       *  starting at @a __pos.  Returns an integer < 0 if the
+       *  substring is ordered before @a __str, 0 if their values are
+       *  equivalent, or > 0 if the substring is ordered after @a
+       *  __str.  Determines the effective length rlen of the strings
+       *  to compare as the smallest of the length of the substring
+       *  and @a __str.size().  The function then compares the two
+       *  strings by calling
+       *  traits::compare(substring.data(),str.data(),rlen).  If the
+       *  result of the comparison is nonzero returns it, otherwise
+       *  the shorter one is ordered first.
+      */
+      int
+      compare(size_type __pos, size_type __n, const basic_string& __str) const;
+
+      /**
+       *  @brief  Compare substring to a substring.
+       *  @param __pos1  Index of first character of substring.
+       *  @param __n1  Number of characters in substring.
+       *  @param __str  String to compare against.
+       *  @param __pos2  Index of first character of substring of str.
+       *  @param __n2  Number of characters in substring of str.
+       *  @return  Integer < 0, 0, or > 0.
+       *
+       *  Form the substring of this string from the @a __n1
+       *  characters starting at @a __pos1.  Form the substring of @a
+       *  __str from the @a __n2 characters starting at @a __pos2.
+       *  Returns an integer < 0 if this substring is ordered before
+       *  the substring of @a __str, 0 if their values are equivalent,
+       *  or > 0 if this substring is ordered after the substring of
+       *  @a __str.  Determines the effective length rlen of the
+       *  strings to compare as the smallest of the lengths of the
+       *  substrings.  The function then compares the two strings by
+       *  calling
+       *  traits::compare(substring.data(),str.substr(pos2,n2).data(),rlen).
+       *  If the result of the comparison is nonzero returns it,
+       *  otherwise the shorter one is ordered first.
+      */
+      int
+      compare(size_type __pos1, size_type __n1, const basic_string& __str,
+	      size_type __pos2, size_type __n2 = npos) const;
+
+      /**
+       *  @brief  Compare to a C string.
+       *  @param __s  C string to compare against.
+       *  @return  Integer < 0, 0, or > 0.
+       *
+       *  Returns an integer < 0 if this string is ordered before @a __s, 0 if
+       *  their values are equivalent, or > 0 if this string is ordered after
+       *  @a __s.  Determines the effective length rlen of the strings to
+       *  compare as the smallest of size() and the length of a string
+       *  constructed from @a __s.  The function then compares the two strings
+       *  by calling traits::compare(data(),s,rlen).  If the result of the
+       *  comparison is nonzero returns it, otherwise the shorter one is
+       *  ordered first.
+      */
+      int
+      compare(const _CharT* __s) const _GLIBCXX_NOEXCEPT;
+
+      // _GLIBCXX_RESOLVE_LIB_DEFECTS
+      // 5 String::compare specification questionable
+      /**
+       *  @brief  Compare substring to a C string.
+       *  @param __pos  Index of first character of substring.
+       *  @param __n1  Number of characters in substring.
+       *  @param __s  C string to compare against.
+       *  @return  Integer < 0, 0, or > 0.
+       *
+       *  Form the substring of this string from the @a __n1
+       *  characters starting at @a pos.  Returns an integer < 0 if
+       *  the substring is ordered before @a __s, 0 if their values
+       *  are equivalent, or > 0 if the substring is ordered after @a
+       *  __s.  Determines the effective length rlen of the strings to
+       *  compare as the smallest of the length of the substring and
+       *  the length of a string constructed from @a __s.  The
+       *  function then compares the two string by calling
+       *  traits::compare(substring.data(),__s,rlen).  If the result of
+       *  the comparison is nonzero returns it, otherwise the shorter
+       *  one is ordered first.
+      */
+      int
+      compare(size_type __pos, size_type __n1, const _CharT* __s) const;
+
+      /**
+       *  @brief  Compare substring against a character %array.
+       *  @param __pos  Index of first character of substring.
+       *  @param __n1  Number of characters in substring.
+       *  @param __s  character %array to compare against.
+       *  @param __n2  Number of characters of s.
+       *  @return  Integer < 0, 0, or > 0.
+       *
+       *  Form the substring of this string from the @a __n1
+       *  characters starting at @a __pos.  Form a string from the
+       *  first @a __n2 characters of @a __s.  Returns an integer < 0
+       *  if this substring is ordered before the string from @a __s,
+       *  0 if their values are equivalent, or > 0 if this substring
+       *  is ordered after the string from @a __s.  Determines the
+       *  effective length rlen of the strings to compare as the
+       *  smallest of the length of the substring and @a __n2.  The
+       *  function then compares the two strings by calling
+       *  traits::compare(substring.data(),s,rlen).  If the result of
+       *  the comparison is nonzero returns it, otherwise the shorter
+       *  one is ordered first.
+       *
+       *  NB: s must have at least n2 characters, &apos;\\0&apos; has
+       *  no special meaning.
+      */
+      int
+      compare(size_type __pos, size_type __n1, const _CharT* __s,
+	      size_type __n2) const;
+
+#if __cplusplus > 201703L
+      bool
+      starts_with(basic_string_view<_CharT, _Traits> __x) const noexcept
+      { return __sv_type(this->data(), this->size()).starts_with(__x); }
+
+      bool
+      starts_with(_CharT __x) const noexcept
+      { return __sv_type(this->data(), this->size()).starts_with(__x); }
+
+      bool
+      starts_with(const _CharT* __x) const noexcept
+      { return __sv_type(this->data(), this->size()).starts_with(__x); }
+
+      bool
+      ends_with(basic_string_view<_CharT, _Traits> __x) const noexcept
+      { return __sv_type(this->data(), this->size()).ends_with(__x); }
+
+      bool
+      ends_with(_CharT __x) const noexcept
+      { return __sv_type(this->data(), this->size()).ends_with(__x); }
+
+      bool
+      ends_with(const _CharT* __x) const noexcept
+      { return __sv_type(this->data(), this->size()).ends_with(__x); }
+#endif // C++20
+
+#if __cplusplus >= 202011L \
+  || (__cplusplus == 202002L && !defined __STRICT_ANSI__)
+      bool
+      contains(basic_string_view<_CharT, _Traits> __x) const noexcept
+      { return __sv_type(this->data(), this->size()).contains(__x); }
+
+      bool
+      contains(_CharT __x) const noexcept
+      { return __sv_type(this->data(), this->size()).contains(__x); }
+
+      bool
+      contains(const _CharT* __x) const noexcept
+      { return __sv_type(this->data(), this->size()).contains(__x); }
+#endif // C++23
+
+# ifdef _GLIBCXX_TM_TS_INTERNAL
+      friend void
+      ::_txnal_cow_string_C1_for_exceptions(void* that, const char* s,
+					    void* exc);
+      friend const char*
+      ::_txnal_cow_string_c_str(const void *that);
+      friend void
+      ::_txnal_cow_string_D1(void *that);
+      friend void
+      ::_txnal_cow_string_D1_commit(void *that);
+# endif
+  };
+
+  template<typename _CharT, typename _Traits, typename _Alloc>
+    const typename basic_string<_CharT, _Traits, _Alloc>::size_type
+    basic_string<_CharT, _Traits, _Alloc>::
+    _Rep::_S_max_size = (((npos - sizeof(_Rep_base))/sizeof(_CharT)) - 1) / 4;
+
+  template<typename _CharT, typename _Traits, typename _Alloc>
+    const _CharT
+    basic_string<_CharT, _Traits, _Alloc>::
+    _Rep::_S_terminal = _CharT();
+
+  template<typename _CharT, typename _Traits, typename _Alloc>
+    const typename basic_string<_CharT, _Traits, _Alloc>::size_type
+    basic_string<_CharT, _Traits, _Alloc>::npos;
+
+  // Linker sets _S_empty_rep_storage to all 0s (one reference, empty string)
+  // at static init time (before static ctors are run).
+  template<typename _CharT, typename _Traits, typename _Alloc>
+    typename basic_string<_CharT, _Traits, _Alloc>::size_type
+    basic_string<_CharT, _Traits, _Alloc>::_Rep::_S_empty_rep_storage[
+    (sizeof(_Rep_base) + sizeof(_CharT) + sizeof(size_type) - 1) /
+      sizeof(size_type)];
+
+  // NB: This is the special case for Input Iterators, used in
+  // istreambuf_iterators, etc.
+  // Input Iterators have a cost structure very different from
+  // pointers, calling for a different coding style.
+  template<typename _CharT, typename _Traits, typename _Alloc>
+    template<typename _InIterator>
+      _CharT*
+      basic_string<_CharT, _Traits, _Alloc>::
+      _S_construct(_InIterator __beg, _InIterator __end, const _Alloc& __a,
+		   input_iterator_tag)
+      {
+#if _GLIBCXX_FULLY_DYNAMIC_STRING == 0
+	if (__beg == __end && __a == _Alloc())
+	  return _S_empty_rep()._M_refdata();
+#endif
+	// Avoid reallocation for common case.
+	_CharT __buf[128];
+	size_type __len = 0;
+	while (__beg != __end && __len < sizeof(__buf) / sizeof(_CharT))
+	  {
+	    __buf[__len++] = *__beg;
+	    ++__beg;
+	  }
+	_Rep* __r = _Rep::_S_create(__len, size_type(0), __a);
+	_M_copy(__r->_M_refdata(), __buf, __len);
+	__try
+	  {
+	    while (__beg != __end)
+	      {
+		if (__len == __r->_M_capacity)
+		  {
+		    // Allocate more space.
+		    _Rep* __another = _Rep::_S_create(__len + 1, __len, __a);
+		    _M_copy(__another->_M_refdata(), __r->_M_refdata(), __len);
+		    __r->_M_destroy(__a);
+		    __r = __another;
+		  }
+		__r->_M_refdata()[__len++] = *__beg;
+		++__beg;
+	      }
+	  }
+	__catch(...)
+	  {
+	    __r->_M_destroy(__a);
+	    __throw_exception_again;
+	  }
+	__r->_M_set_length_and_sharable(__len);
+	return __r->_M_refdata();
+      }
+
+  template<typename _CharT, typename _Traits, typename _Alloc>
+    template <typename _InIterator>
+      _CharT*
+      basic_string<_CharT, _Traits, _Alloc>::
+      _S_construct(_InIterator __beg, _InIterator __end, const _Alloc& __a,
+		   forward_iterator_tag)
+      {
+#if _GLIBCXX_FULLY_DYNAMIC_STRING == 0
+	if (__beg == __end && __a == _Alloc())
+	  return _S_empty_rep()._M_refdata();
+#endif
+	// NB: Not required, but considered best practice.
+	if (__gnu_cxx::__is_null_pointer(__beg) && __beg != __end)
+	  __throw_logic_error(__N("basic_string::_S_construct null not valid"));
+
+	const size_type __dnew = static_cast<size_type>(std::distance(__beg,
+								      __end));
+	// Check for out_of_range and length_error exceptions.
+	_Rep* __r = _Rep::_S_create(__dnew, size_type(0), __a);
+	__try
+	  { _S_copy_chars(__r->_M_refdata(), __beg, __end); }
+	__catch(...)
+	  {
+	    __r->_M_destroy(__a);
+	    __throw_exception_again;
+	  }
+	__r->_M_set_length_and_sharable(__dnew);
+	return __r->_M_refdata();
+      }
+
+  template<typename _CharT, typename _Traits, typename _Alloc>
+    _CharT*
+    basic_string<_CharT, _Traits, _Alloc>::
+    _S_construct(size_type __n, _CharT __c, const _Alloc& __a)
+    {
+#if _GLIBCXX_FULLY_DYNAMIC_STRING == 0
+      if (__n == 0 && __a == _Alloc())
+	return _S_empty_rep()._M_refdata();
+#endif
+      // Check for out_of_range and length_error exceptions.
+      _Rep* __r = _Rep::_S_create(__n, size_type(0), __a);
+      if (__n)
+	_M_assign(__r->_M_refdata(), __n, __c);
+
+      __r->_M_set_length_and_sharable(__n);
+      return __r->_M_refdata();
+    }
+
+  template<typename _CharT, typename _Traits, typename _Alloc>
+    basic_string<_CharT, _Traits, _Alloc>::
+    basic_string(const basic_string& __str, size_type __pos, const _Alloc& __a)
+    : _M_dataplus(_S_construct(__str._M_data()
+			       + __str._M_check(__pos,
+						"basic_string::basic_string"),
+			       __str._M_data() + __str._M_limit(__pos, npos)
+			       + __pos, __a), __a)
+    { }
+
+  template<typename _CharT, typename _Traits, typename _Alloc>
+    basic_string<_CharT, _Traits, _Alloc>::
+    basic_string(const basic_string& __str, size_type __pos, size_type __n)
+    : _M_dataplus(_S_construct(__str._M_data()
+			       + __str._M_check(__pos,
+						"basic_string::basic_string"),
+			       __str._M_data() + __str._M_limit(__pos, __n)
+			       + __pos, _Alloc()), _Alloc())
+    { }
+
+  template<typename _CharT, typename _Traits, typename _Alloc>
+    basic_string<_CharT, _Traits, _Alloc>::
+    basic_string(const basic_string& __str, size_type __pos,
+		 size_type __n, const _Alloc& __a)
+    : _M_dataplus(_S_construct(__str._M_data()
+			       + __str._M_check(__pos,
+						"basic_string::basic_string"),
+			       __str._M_data() + __str._M_limit(__pos, __n)
+			       + __pos, __a), __a)
+    { }
+
+  template<typename _CharT, typename _Traits, typename _Alloc>
+    basic_string<_CharT, _Traits, _Alloc>&
+    basic_string<_CharT, _Traits, _Alloc>::
+    assign(const basic_string& __str)
+    {
+      if (_M_rep() != __str._M_rep())
+	{
+	  // XXX MT
+	  const allocator_type __a = this->get_allocator();
+	  _CharT* __tmp = __str._M_rep()->_M_grab(__a, __str.get_allocator());
+	  _M_rep()->_M_dispose(__a);
+	  _M_data(__tmp);
+	}
+      return *this;
+    }
+
+  template<typename _CharT, typename _Traits, typename _Alloc>
+    basic_string<_CharT, _Traits, _Alloc>&
+    basic_string<_CharT, _Traits, _Alloc>::
+    assign(const _CharT* __s, size_type __n)
+    {
+      __glibcxx_requires_string_len(__s, __n);
+      _M_check_length(this->size(), __n, "basic_string::assign");
+      if (_M_disjunct(__s) || _M_rep()->_M_is_shared())
+	return _M_replace_safe(size_type(0), this->size(), __s, __n);
+      else
+	{
+	  // Work in-place.
+	  const size_type __pos = __s - _M_data();
+	  if (__pos >= __n)
+	    _M_copy(_M_data(), __s, __n);
+	  else if (__pos)
+	    _M_move(_M_data(), __s, __n);
+	  _M_rep()->_M_set_length_and_sharable(__n);
+	  return *this;
+	}
+     }
+
+  template<typename _CharT, typename _Traits, typename _Alloc>
+    basic_string<_CharT, _Traits, _Alloc>&
+    basic_string<_CharT, _Traits, _Alloc>::
+    append(size_type __n, _CharT __c)
+    {
+      if (__n)
+	{
+	  _M_check_length(size_type(0), __n, "basic_string::append");
+	  const size_type __len = __n + this->size();
+	  if (__len > this->capacity() || _M_rep()->_M_is_shared())
+	    this->reserve(__len);
+	  _M_assign(_M_data() + this->size(), __n, __c);
+	  _M_rep()->_M_set_length_and_sharable(__len);
+	}
+      return *this;
+    }
+
+  template<typename _CharT, typename _Traits, typename _Alloc>
+    basic_string<_CharT, _Traits, _Alloc>&
+    basic_string<_CharT, _Traits, _Alloc>::
+    append(const _CharT* __s, size_type __n)
+    {
+      __glibcxx_requires_string_len(__s, __n);
+      if (__n)
+	{
+	  _M_check_length(size_type(0), __n, "basic_string::append");
+	  const size_type __len = __n + this->size();
+	  if (__len > this->capacity() || _M_rep()->_M_is_shared())
+	    {
+	      if (_M_disjunct(__s))
+		this->reserve(__len);
+	      else
+		{
+		  const size_type __off = __s - _M_data();
+		  this->reserve(__len);
+		  __s = _M_data() + __off;
+		}
+	    }
+	  _M_copy(_M_data() + this->size(), __s, __n);
+	  _M_rep()->_M_set_length_and_sharable(__len);
+	}
+      return *this;
+    }
+
+  template<typename _CharT, typename _Traits, typename _Alloc>
+    basic_string<_CharT, _Traits, _Alloc>&
+    basic_string<_CharT, _Traits, _Alloc>::
+    append(const basic_string& __str)
+    {
+      const size_type __size = __str.size();
+      if (__size)
+	{
+	  const size_type __len = __size + this->size();
+	  if (__len > this->capacity() || _M_rep()->_M_is_shared())
+	    this->reserve(__len);
+	  _M_copy(_M_data() + this->size(), __str._M_data(), __size);
+	  _M_rep()->_M_set_length_and_sharable(__len);
+	}
+      return *this;
+    }
+
+  template<typename _CharT, typename _Traits, typename _Alloc>
+    basic_string<_CharT, _Traits, _Alloc>&
+    basic_string<_CharT, _Traits, _Alloc>::
+    append(const basic_string& __str, size_type __pos, size_type __n)
+    {
+      __str._M_check(__pos, "basic_string::append");
+      __n = __str._M_limit(__pos, __n);
+      if (__n)
+	{
+	  const size_type __len = __n + this->size();
+	  if (__len > this->capacity() || _M_rep()->_M_is_shared())
+	    this->reserve(__len);
+	  _M_copy(_M_data() + this->size(), __str._M_data() + __pos, __n);
+	  _M_rep()->_M_set_length_and_sharable(__len);
+	}
+      return *this;
+    }
+
+   template<typename _CharT, typename _Traits, typename _Alloc>
+     basic_string<_CharT, _Traits, _Alloc>&
+     basic_string<_CharT, _Traits, _Alloc>::
+     insert(size_type __pos, const _CharT* __s, size_type __n)
+     {
+       __glibcxx_requires_string_len(__s, __n);
+       _M_check(__pos, "basic_string::insert");
+       _M_check_length(size_type(0), __n, "basic_string::insert");
+       if (_M_disjunct(__s) || _M_rep()->_M_is_shared())
+         return _M_replace_safe(__pos, size_type(0), __s, __n);
+       else
+         {
+           // Work in-place.
+           const size_type __off = __s - _M_data();
+           _M_mutate(__pos, 0, __n);
+           __s = _M_data() + __off;
+           _CharT* __p = _M_data() + __pos;
+           if (__s  + __n <= __p)
+             _M_copy(__p, __s, __n);
+           else if (__s >= __p)
+             _M_copy(__p, __s + __n, __n);
+           else
+             {
+	       const size_type __nleft = __p - __s;
+               _M_copy(__p, __s, __nleft);
+               _M_copy(__p + __nleft, __p + __n, __n - __nleft);
+             }
+           return *this;
+         }
+     }
+
+   template<typename _CharT, typename _Traits, typename _Alloc>
+     typename basic_string<_CharT, _Traits, _Alloc>::iterator
+     basic_string<_CharT, _Traits, _Alloc>::
+     erase(iterator __first, iterator __last)
+     {
+       _GLIBCXX_DEBUG_PEDASSERT(__first >= _M_ibegin() && __first <= __last
+				&& __last <= _M_iend());
+
+       // NB: This isn't just an optimization (bail out early when
+       // there is nothing to do, really), it's also a correctness
+       // issue vs MT, see libstdc++/40518.
+       const size_type __size = __last - __first;
+       if (__size)
+	 {
+	   const size_type __pos = __first - _M_ibegin();
+	   _M_mutate(__pos, __size, size_type(0));
+	   _M_rep()->_M_set_leaked();
+	   return iterator(_M_data() + __pos);
+	 }
+       else
+	 return __first;
+     }
+
+   template<typename _CharT, typename _Traits, typename _Alloc>
+     basic_string<_CharT, _Traits, _Alloc>&
+     basic_string<_CharT, _Traits, _Alloc>::
+     replace(size_type __pos, size_type __n1, const _CharT* __s,
+	     size_type __n2)
+     {
+       __glibcxx_requires_string_len(__s, __n2);
+       _M_check(__pos, "basic_string::replace");
+       __n1 = _M_limit(__pos, __n1);
+       _M_check_length(__n1, __n2, "basic_string::replace");
+       bool __left;
+       if (_M_disjunct(__s) || _M_rep()->_M_is_shared())
+         return _M_replace_safe(__pos, __n1, __s, __n2);
+       else if ((__left = __s + __n2 <= _M_data() + __pos)
+		|| _M_data() + __pos + __n1 <= __s)
+	 {
+	   // Work in-place: non-overlapping case.
+	   size_type __off = __s - _M_data();
+	   __left ? __off : (__off += __n2 - __n1);
+	   _M_mutate(__pos, __n1, __n2);
+	   _M_copy(_M_data() + __pos, _M_data() + __off, __n2);
+	   return *this;
+	 }
+       else
+	 {
+	   // Todo: overlapping case.
+	   const basic_string __tmp(__s, __n2);
+	   return _M_replace_safe(__pos, __n1, __tmp._M_data(), __n2);
+	 }
+     }
+
+  template<typename _CharT, typename _Traits, typename _Alloc>
+    void
+    basic_string<_CharT, _Traits, _Alloc>::_Rep::
+    _M_destroy(const _Alloc& __a) throw ()
+    {
+      const size_type __size = sizeof(_Rep_base) +
+	                       (this->_M_capacity + 1) * sizeof(_CharT);
+      _Raw_bytes_alloc(__a).deallocate(reinterpret_cast<char*>(this), __size);
+    }
+
+  template<typename _CharT, typename _Traits, typename _Alloc>
+    void
+    basic_string<_CharT, _Traits, _Alloc>::
+    _M_leak_hard()
+    {
+#if _GLIBCXX_FULLY_DYNAMIC_STRING == 0
+      if (_M_rep() == &_S_empty_rep())
+	return;
+#endif
+      if (_M_rep()->_M_is_shared())
+	_M_mutate(0, 0, 0);
+      _M_rep()->_M_set_leaked();
+    }
+
+  template<typename _CharT, typename _Traits, typename _Alloc>
+    void
+    basic_string<_CharT, _Traits, _Alloc>::
+    _M_mutate(size_type __pos, size_type __len1, size_type __len2)
+    {
+      const size_type __old_size = this->size();
+      const size_type __new_size = __old_size + __len2 - __len1;
+      const size_type __how_much = __old_size - __pos - __len1;
+
+      if (__new_size > this->capacity() || _M_rep()->_M_is_shared())
+	{
+	  // Must reallocate.
+	  const allocator_type __a = get_allocator();
+	  _Rep* __r = _Rep::_S_create(__new_size, this->capacity(), __a);
+
+	  if (__pos)
+	    _M_copy(__r->_M_refdata(), _M_data(), __pos);
+	  if (__how_much)
+	    _M_copy(__r->_M_refdata() + __pos + __len2,
+		    _M_data() + __pos + __len1, __how_much);
+
+	  _M_rep()->_M_dispose(__a);
+	  _M_data(__r->_M_refdata());
+	}
+      else if (__how_much && __len1 != __len2)
+	{
+	  // Work in-place.
+	  _M_move(_M_data() + __pos + __len2,
+		  _M_data() + __pos + __len1, __how_much);
+	}
+      _M_rep()->_M_set_length_and_sharable(__new_size);
+    }
+
+  template<typename _CharT, typename _Traits, typename _Alloc>
+    void
+    basic_string<_CharT, _Traits, _Alloc>::
+    reserve(size_type __res)
+    {
+      const size_type __capacity = capacity();
+
+      // _GLIBCXX_RESOLVE_LIB_DEFECTS
+      // 2968. Inconsistencies between basic_string reserve and
+      // vector/unordered_map/unordered_set reserve functions
+      // P0966 reserve should not shrink
+      if (__res <= __capacity)
+	{
+	  if (!_M_rep()->_M_is_shared())
+	    return;
+
+	  // unshare, but keep same capacity
+	  __res = __capacity;
+	}
+
+      const allocator_type __a = get_allocator();
+      _CharT* __tmp = _M_rep()->_M_clone(__a, __res - this->size());
+      _M_rep()->_M_dispose(__a);
+      _M_data(__tmp);
+    }
+
+  template<typename _CharT, typename _Traits, typename _Alloc>
+    void
+    basic_string<_CharT, _Traits, _Alloc>::
+    swap(basic_string& __s)
+    _GLIBCXX_NOEXCEPT_IF(allocator_traits<_Alloc>::is_always_equal::value)
+    {
+      if (_M_rep()->_M_is_leaked())
+	_M_rep()->_M_set_sharable();
+      if (__s._M_rep()->_M_is_leaked())
+	__s._M_rep()->_M_set_sharable();
+      if (this->get_allocator() == __s.get_allocator())
+	{
+	  _CharT* __tmp = _M_data();
+	  _M_data(__s._M_data());
+	  __s._M_data(__tmp);
+	}
+      // The code below can usually be optimized away.
+      else
+	{
+	  const basic_string __tmp1(_M_ibegin(), _M_iend(),
+				    __s.get_allocator());
+	  const basic_string __tmp2(__s._M_ibegin(), __s._M_iend(),
+				    this->get_allocator());
+	  *this = __tmp2;
+	  __s = __tmp1;
+	}
+    }
+
+  template<typename _CharT, typename _Traits, typename _Alloc>
+    typename basic_string<_CharT, _Traits, _Alloc>::_Rep*
+    basic_string<_CharT, _Traits, _Alloc>::_Rep::
+    _S_create(size_type __capacity, size_type __old_capacity,
+	      const _Alloc& __alloc)
+    {
+      // _GLIBCXX_RESOLVE_LIB_DEFECTS
+      // 83.  String::npos vs. string::max_size()
+      if (__capacity > _S_max_size)
+	__throw_length_error(__N("basic_string::_S_create"));
+
+      // The standard places no restriction on allocating more memory
+      // than is strictly needed within this layer at the moment or as
+      // requested by an explicit application call to reserve(n).
+
+      // Many malloc implementations perform quite poorly when an
+      // application attempts to allocate memory in a stepwise fashion
+      // growing each allocation size by only 1 char.  Additionally,
+      // it makes little sense to allocate less linear memory than the
+      // natural blocking size of the malloc implementation.
+      // Unfortunately, we would need a somewhat low-level calculation
+      // with tuned parameters to get this perfect for any particular
+      // malloc implementation.  Fortunately, generalizations about
+      // common features seen among implementations seems to suffice.
+
+      // __pagesize need not match the actual VM page size for good
+      // results in practice, thus we pick a common value on the low
+      // side.  __malloc_header_size is an estimate of the amount of
+      // overhead per memory allocation (in practice seen N * sizeof
+      // (void*) where N is 0, 2 or 4).  According to folklore,
+      // picking this value on the high side is better than
+      // low-balling it (especially when this algorithm is used with
+      // malloc implementations that allocate memory blocks rounded up
+      // to a size which is a power of 2).
+      const size_type __pagesize = 4096;
+      const size_type __malloc_header_size = 4 * sizeof(void*);
+
+      // The below implements an exponential growth policy, necessary to
+      // meet amortized linear time requirements of the library: see
+      // http://gcc.gnu.org/ml/libstdc++/2001-07/msg00085.html.
+      // It's active for allocations requiring an amount of memory above
+      // system pagesize. This is consistent with the requirements of the
+      // standard: http://gcc.gnu.org/ml/libstdc++/2001-07/msg00130.html
+      if (__capacity > __old_capacity && __capacity < 2 * __old_capacity)
+	__capacity = 2 * __old_capacity;
+
+      // NB: Need an array of char_type[__capacity], plus a terminating
+      // null char_type() element, plus enough for the _Rep data structure.
+      // Whew. Seemingly so needy, yet so elemental.
+      size_type __size = (__capacity + 1) * sizeof(_CharT) + sizeof(_Rep);
+
+      const size_type __adj_size = __size + __malloc_header_size;
+      if (__adj_size > __pagesize && __capacity > __old_capacity)
+	{
+	  const size_type __extra = __pagesize - __adj_size % __pagesize;
+	  __capacity += __extra / sizeof(_CharT);
+	  // Never allocate a string bigger than _S_max_size.
+	  if (__capacity > _S_max_size)
+	    __capacity = _S_max_size;
+	  __size = (__capacity + 1) * sizeof(_CharT) + sizeof(_Rep);
+	}
+
+      // NB: Might throw, but no worries about a leak, mate: _Rep()
+      // does not throw.
+      void* __place = _Raw_bytes_alloc(__alloc).allocate(__size);
+      _Rep *__p = new (__place) _Rep;
+      __p->_M_capacity = __capacity;
+      // ABI compatibility - 3.4.x set in _S_create both
+      // _M_refcount and _M_length.  All callers of _S_create
+      // in basic_string.tcc then set just _M_length.
+      // In 4.0.x and later both _M_refcount and _M_length
+      // are initialized in the callers, unfortunately we can
+      // have 3.4.x compiled code with _S_create callers inlined
+      // calling 4.0.x+ _S_create.
+      __p->_M_set_sharable();
+      return __p;
+    }
+
+  template<typename _CharT, typename _Traits, typename _Alloc>
+    _CharT*
+    basic_string<_CharT, _Traits, _Alloc>::_Rep::
+    _M_clone(const _Alloc& __alloc, size_type __res)
+    {
+      // Requested capacity of the clone.
+      const size_type __requested_cap = this->_M_length + __res;
+      _Rep* __r = _Rep::_S_create(__requested_cap, this->_M_capacity,
+				  __alloc);
+      if (this->_M_length)
+	_M_copy(__r->_M_refdata(), _M_refdata(), this->_M_length);
+
+      __r->_M_set_length_and_sharable(this->_M_length);
+      return __r->_M_refdata();
+    }
+
+  template<typename _CharT, typename _Traits, typename _Alloc>
+    void
+    basic_string<_CharT, _Traits, _Alloc>::
+    resize(size_type __n, _CharT __c)
+    {
+      const size_type __size = this->size();
+      _M_check_length(__size, __n, "basic_string::resize");
+      if (__size < __n)
+	this->append(__n - __size, __c);
+      else if (__n < __size)
+	this->erase(__n);
+      // else nothing (in particular, avoid calling _M_mutate() unnecessarily.)
+    }
+
+  template<typename _CharT, typename _Traits, typename _Alloc>
+    template<typename _InputIterator>
+      basic_string<_CharT, _Traits, _Alloc>&
+      basic_string<_CharT, _Traits, _Alloc>::
+      _M_replace_dispatch(iterator __i1, iterator __i2, _InputIterator __k1,
+			  _InputIterator __k2, __false_type)
+      {
+	const basic_string __s(__k1, __k2);
+	const size_type __n1 = __i2 - __i1;
+	_M_check_length(__n1, __s.size(), "basic_string::_M_replace_dispatch");
+	return _M_replace_safe(__i1 - _M_ibegin(), __n1, __s._M_data(),
+			       __s.size());
+      }
+
+  template<typename _CharT, typename _Traits, typename _Alloc>
+    basic_string<_CharT, _Traits, _Alloc>&
+    basic_string<_CharT, _Traits, _Alloc>::
+    _M_replace_aux(size_type __pos1, size_type __n1, size_type __n2,
+		   _CharT __c)
+    {
+      _M_check_length(__n1, __n2, "basic_string::_M_replace_aux");
+      _M_mutate(__pos1, __n1, __n2);
+      if (__n2)
+	_M_assign(_M_data() + __pos1, __n2, __c);
+      return *this;
+    }
+
+  template<typename _CharT, typename _Traits, typename _Alloc>
+    basic_string<_CharT, _Traits, _Alloc>&
+    basic_string<_CharT, _Traits, _Alloc>::
+    _M_replace_safe(size_type __pos1, size_type __n1, const _CharT* __s,
+		    size_type __n2)
+    {
+      _M_mutate(__pos1, __n1, __n2);
+      if (__n2)
+	_M_copy(_M_data() + __pos1, __s, __n2);
+      return *this;
+    }
+
+  template<typename _CharT, typename _Traits, typename _Alloc>
+    void
+    basic_string<_CharT, _Traits, _Alloc>::
+    reserve()
+    {
+#if __cpp_exceptions
+      if (length() < capacity() || _M_rep()->_M_is_shared())
+	try
+	  {
+	    const allocator_type __a = get_allocator();
+	    _CharT* __tmp = _M_rep()->_M_clone(__a);
+	    _M_rep()->_M_dispose(__a);
+	    _M_data(__tmp);
+	  }
+	catch (const __cxxabiv1::__forced_unwind&)
+	  { throw; }
+	catch (...)
+	  { /* swallow the exception */ }
+#endif
+    }
+
+    template<typename _CharT, typename _Traits, typename _Alloc>
+    typename basic_string<_CharT, _Traits, _Alloc>::size_type
+    basic_string<_CharT, _Traits, _Alloc>::
+    copy(_CharT* __s, size_type __n, size_type __pos) const
+    {
+      _M_check(__pos, "basic_string::copy");
+      __n = _M_limit(__pos, __n);
+      __glibcxx_requires_string_len(__s, __n);
+      if (__n)
+	_M_copy(__s, _M_data() + __pos, __n);
+      // 21.3.5.7 par 3: do not append null.  (good.)
+      return __n;
+    }
+_GLIBCXX_END_NAMESPACE_VERSION
+} // namespace std
+#endif  // ! _GLIBCXX_USE_CXX11_ABI
+#endif  // _COW_STRING_H
