@@ -123,6 +123,40 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
// Inherit everything else.
};
+ /// Partial specialization for const types
+ template<typename _Tp>
+ class allocator<const _Tp> : public allocator<_Tp>
+ {
+#if __cplusplus >= 201103L
+ public:
+ allocator()
+ {
+ static_assert( !is_const<const _Tp>::value,
+ "allocator's value_type must be non-const" );
+ }
+#else
+ private:
+ allocator();
+#endif
+ };
+
+ /// Partial specialization for reference types
+ template<typename _Tp>
+ class allocator<_Tp&> : public allocator<_Tp>
+ {
+#if __cplusplus >= 201103L
+ public:
+ allocator()
+ {
+ static_assert( is_object<_Tp&>::value,
+ "allocator's value_type must be an object type" );
+ }
+#else
+ private:
+ allocator();
+#endif
+ };
+
template<typename _T1, typename _T2>
inline bool
operator==(const allocator<_T1>&, const allocator<_T2>&)
new file mode 100644
@@ -0,0 +1,27 @@
+// Copyright (C) 2013 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.
+
+// You should have received a copy of the GNU General Public License along
+// with this library; see the file COPYING3. If not see
+// <http://www.gnu.org/licenses/>.
+
+// 17.6.3.5 Allocator requirements
+// { dg-do compile }
+// { dg-options "-std=gnu++11" }
+
+#include <memory>
+
+std::allocator<int&> a1; // { dg-error "here" }
+// { dg-prune-output "must be an object type" }
+std::allocator<const int> a2; // { dg-error "here" }
+// { dg-prune-output "must be non-const" }
For PR libstdc++/55963 I said I'd improve the diagnostics when you try to use std::vector<const T>, which is invalid because the C++ Allocator requirements require the value_type to be a non-const object type. I tried adding static assertions to the primary std::allocator template, but the diagnostics are still not very clear, because const types cause ambiguous overloads and reference types cause invalid pointer-to-reference types to be formed. This patch adds std::allocator<const T> and std::allocator<T&> partial specializations instead, which inherit from std::allocator<T> so they can be used in limited ways (most importantly, you can rebind them to a valid specialization) but attempting to construct such an allocator will fail with a helpful message in C++11 mode (and just fail due to using a private constructor in C++03 mode.) The change passes testing, but I haven't committed yet as I'd like to hear other opinions on the approach, or if this is even worth doing at all. I think the better diagnostics are helpful, e.g. with this patch instantiating std::vector<const int> gives the following, rather than 33 far more cryptic lines: In file included from /home/jwakely/gcc/4.x/include/c++/4.9.0/vector:61:0, from a.cc:1: /home/jwakely/gcc/4.x/include/c++/4.9.0/bits/allocator.h: In instantiation of ‘std::allocator<const _Tp>::allocator() [with _Tp = int]’: a.cc:5:26: required from here /home/jwakely/gcc/4.x/include/c++/4.9.0/bits/allocator.h:134:2: error: static assertion failed: allocator's value_type must be non-const static_assert( !is_const<const _Tp>::value, ^ commit b63fee8ad1f7ee67dd68954272f73e02d3fa6e70 Author: Jonathan Wakely <jwakely.gcc@gmail.com> Date: Sat Nov 9 13:23:46 2013 +0000 PR libstdc++/55963 * include/bits/allocator.h (allocator<const _Tp>, allocator<_Tp&>): Add partial specializations to improve diagnostics for invalid uses. * testsuite/20_util/allocator/55963.cc: New.