commit fa2a3fe062ea41d2545ee425cdf0816ee1bd6b36
Author: Jonathan Wakely <jwakely@redhat.com>
Date: Tue Apr 9 12:26:02 2019 +0100
Add comments and style fixes to <variant>
* include/std/variant: Adjust whitespace. Add comments.
(_Multi_array): Leave primary template undefined.
(_Multi_array<_Tp>): Define partial specialization for base case of
recursion.
(__gen_vtable_impl, __gen_vtable): Remove redundant && from type
which is always a reference.
(__gen_vtable::_S_apply()): Remove function, inline body into
default member initializer.
* testsuite/20_util/variant/visit.cc: Test with noncopyable types.
@@ -145,7 +145,8 @@ namespace __variant
__do_visit(_Visitor&& __visitor, _Variants&&... __variants);
template <typename... _Types, typename _Tp>
- decltype(auto) __variant_cast(_Tp&& __rhs)
+ decltype(auto)
+ __variant_cast(_Tp&& __rhs)
{
if constexpr (is_lvalue_reference_v<_Tp>)
{
@@ -197,9 +198,10 @@ namespace __variant
struct _Uninitialized<_Type, true>
{
template<typename... _Args>
- constexpr _Uninitialized(in_place_index_t<0>, _Args&&... __args)
- : _M_storage(std::forward<_Args>(__args)...)
- { }
+ constexpr
+ _Uninitialized(in_place_index_t<0>, _Args&&... __args)
+ : _M_storage(std::forward<_Args>(__args)...)
+ { }
constexpr const _Type& _M_get() const &
{ return _M_storage; }
@@ -220,11 +222,12 @@ namespace __variant
struct _Uninitialized<_Type, false>
{
template<typename... _Args>
- constexpr _Uninitialized(in_place_index_t<0>, _Args&&... __args)
- {
- ::new ((void*)std::addressof(_M_storage))
- _Type(std::forward<_Args>(__args)...);
- }
+ constexpr
+ _Uninitialized(in_place_index_t<0>, _Args&&... __args)
+ {
+ ::new ((void*)std::addressof(_M_storage))
+ _Type(std::forward<_Args>(__args)...);
+ }
const _Type& _M_get() const &
{ return *_M_storage._M_ptr(); }
@@ -360,15 +363,14 @@ namespace __variant
struct _Variant_storage;
template <typename... _Types>
- using __select_index =
- typename __select_int::_Select_int_base<sizeof...(_Types),
- unsigned char,
- unsigned short>::type::value_type;
+ using __select_index =
+ typename __select_int::_Select_int_base<sizeof...(_Types),
+ unsigned char,
+ unsigned short>::type::value_type;
template<typename... _Types>
struct _Variant_storage<false, _Types...>
{
-
constexpr _Variant_storage() : _M_index(variant_npos) { }
template<size_t _Np, typename... _Args>
@@ -387,7 +389,7 @@ namespace __variant
std::_Destroy(std::__addressof(__this_mem));
return {};
}, __variant_cast<_Types...>(*this));
- }
+ }
void _M_reset()
{
@@ -472,7 +474,7 @@ namespace __variant
-> __detail::__variant::__variant_cookie
{
__variant_construct_single(std::forward<_Tp>(__lhs),
- std::forward<decltype(__rhs_mem)>(__rhs_mem));
+ std::forward<decltype(__rhs_mem)>( __rhs_mem));
return {};
}, __variant_cast<_Types...>(std::forward<decltype(__rhs)>(__rhs)));
}
@@ -573,6 +575,7 @@ namespace __variant
__variant_construct_single(*this,
std::forward<_Up>(__rhs));
}
+
template<typename _Up>
void _M_destructive_copy(unsigned short __rhs_index, const _Up& __rhs)
{
@@ -708,8 +711,7 @@ namespace __variant
template<typename... _Types>
using _Move_assign_alias =
- _Move_assign_base<_Traits<_Types...>::_S_trivial_move_assign,
- _Types...>;
+ _Move_assign_base<_Traits<_Types...>::_S_trivial_move_assign, _Types...>;
template<typename... _Types>
struct _Variant_base : _Move_assign_alias<_Types...>
@@ -812,9 +814,13 @@ namespace __variant
&& !_Variant_never_valueless<__remove_cvref_t<_Variant>>::value;
};
- // Used for storing multi-dimensional vtable.
+ // Used for storing a multi-dimensional vtable.
template<typename _Tp, size_t... _Dimensions>
- struct _Multi_array
+ struct _Multi_array;
+
+ // Partial specialization with rank zero, stores a single _Tp element.
+ template<typename _Tp>
+ struct _Multi_array<_Tp>
{
constexpr const _Tp&
_M_access() const
@@ -823,6 +829,7 @@ namespace __variant
_Tp _M_data;
};
+ // Partial specialization with rank >= 1.
template<typename _Ret,
typename _Visitor,
typename... _Variants,
@@ -831,42 +838,53 @@ namespace __variant
{
static constexpr size_t __index =
sizeof...(_Variants) - sizeof...(__rest) - 1;
+
using _Variant = typename _Nth_type<__index, _Variants...>::type;
+
static constexpr int __do_cookie =
_Extra_visit_slot_needed<_Ret, _Variant>::value ? 1 : 0;
+
using _Tp = _Ret(*)(_Visitor, _Variants...);
+
template<typename... _Args>
constexpr const _Tp&
_M_access(size_t __first_index, _Args... __rest_indices) const
- { return _M_arr[__first_index + __do_cookie]._M_access(__rest_indices...); }
+ {
+ return _M_arr[__first_index + __do_cookie]
+ ._M_access(__rest_indices...);
+ }
_Multi_array<_Tp, __rest...> _M_arr[__first + __do_cookie];
};
// Creates a multi-dimensional vtable recursively.
//
+ // The __same_return_types non-type template parameter specifies whether
+ // to enforce that all visitor invocations return the same type. This is
+ // required by std::visit but not std::visit<R>.
+ //
// For example,
// visit([](auto, auto){},
// variant<int, char>(), // typedef'ed as V1
// variant<float, double, long double>()) // typedef'ed as V2
// will trigger instantiations of:
- // __gen_vtable_impl<_Multi_array<void(*)(V1&&, V2&&), 2, 3>,
+ // __gen_vtable_impl<true, _Multi_array<void(*)(V1&&, V2&&), 2, 3>,
// tuple<V1&&, V2&&>, std::index_sequence<>>
- // __gen_vtable_impl<_Multi_array<void(*)(V1&&, V2&&), 3>,
+ // __gen_vtable_impl<true, _Multi_array<void(*)(V1&&, V2&&), 3>,
// tuple<V1&&, V2&&>, std::index_sequence<0>>
- // __gen_vtable_impl<_Multi_array<void(*)(V1&&, V2&&)>,
+ // __gen_vtable_impl<true, _Multi_array<void(*)(V1&&, V2&&)>,
// tuple<V1&&, V2&&>, std::index_sequence<0, 0>>
- // __gen_vtable_impl<_Multi_array<void(*)(V1&&, V2&&)>,
+ // __gen_vtable_impl<true, _Multi_array<void(*)(V1&&, V2&&)>,
// tuple<V1&&, V2&&>, std::index_sequence<0, 1>>
- // __gen_vtable_impl<_Multi_array<void(*)(V1&&, V2&&)>,
+ // __gen_vtable_impl<true, _Multi_array<void(*)(V1&&, V2&&)>,
// tuple<V1&&, V2&&>, std::index_sequence<0, 2>>
- // __gen_vtable_impl<_Multi_array<void(*)(V1&&, V2&&), 3>,
+ // __gen_vtable_impl<true, _Multi_array<void(*)(V1&&, V2&&), 3>,
// tuple<V1&&, V2&&>, std::index_sequence<1>>
- // __gen_vtable_impl<_Multi_array<void(*)(V1&&, V2&&)>,
+ // __gen_vtable_impl<true, _Multi_array<void(*)(V1&&, V2&&)>,
// tuple<V1&&, V2&&>, std::index_sequence<1, 0>>
- // __gen_vtable_impl<_Multi_array<void(*)(V1&&, V2&&)>,
+ // __gen_vtable_impl<true, _Multi_array<void(*)(V1&&, V2&&)>,
// tuple<V1&&, V2&&>, std::index_sequence<1, 1>>
- // __gen_vtable_impl<_Multi_array<void(*)(V1&&, V2&&)>,
+ // __gen_vtable_impl<true, _Multi_array<void(*)(V1&&, V2&&)>,
// tuple<V1&&, V2&&>, std::index_sequence<1, 2>>
// The returned multi-dimensional vtable can be fast accessed by the visitor
// using index calculation.
@@ -874,6 +892,13 @@ namespace __variant
typename _Array_type, typename _Variant_tuple, typename _Index_seq>
struct __gen_vtable_impl;
+ // Defines the _S_apply() member that returns a _Multi_array populated
+ // with function pointers that perform the visitation expressions e(m)
+ // for each valid pack of indexes into the variant types _Variants.
+ //
+ // This partial specialization builds up the index sequences by recursively
+ // calling _S_apply() on the next specialization of __gen_vtable_impl.
+ // The base case of the recursion defines the actual function pointers.
template<bool __same_return_types,
typename _Result_type, typename _Visitor, size_t... __dimensions,
typename... _Variants, size_t... __indices>
@@ -940,6 +965,9 @@ namespace __variant
}
};
+ // This partial specialization is the base case for the recursion.
+ // It populates a _Multi_array element with the address of a function
+ // that invokes the visitor with the alternatives specified by __indices.
template<bool __same_return_types,
typename _Result_type, typename _Visitor, typename... _Variants,
size_t... __indices>
@@ -949,7 +977,7 @@ namespace __variant
tuple<_Variants...>, std::index_sequence<__indices...>>
{
using _Array_type =
- _Multi_array<_Result_type (*)(_Visitor&&, _Variants...)>;
+ _Multi_array<_Result_type (*)(_Visitor, _Variants...)>;
template<size_t __index, typename _Variant>
static constexpr decltype(auto)
@@ -964,20 +992,22 @@ namespace __variant
static constexpr decltype(auto)
__visit_invoke_impl(_Visitor&& __visitor, _Variants... __vars)
{
- if constexpr (is_same_v<_Result_type, __variant_idx_cookie>)
- return std::__invoke(std::forward<_Visitor>(__visitor),
- __element_by_index_or_cookie<__indices>(
- std::forward<_Variants>(__vars))...,
- integral_constant<size_t, __indices>()...);
- else if constexpr (!__same_return_types &&
+ // For raw visitation using indices, pass the indices to the visitor:
+ if constexpr (is_same_v<_Result_type, __variant_idx_cookie>)
+ return std::__invoke(std::forward<_Visitor>(__visitor),
+ __element_by_index_or_cookie<__indices>(
+ std::forward<_Variants>(__vars))...,
+ integral_constant<size_t, __indices>()...);
+ // For std::visit<cv void>, cast the result to void:
+ else if constexpr (!__same_return_types &&
std::is_void_v<_Result_type>)
return (void)std::__invoke(std::forward<_Visitor>(__visitor),
- __element_by_index_or_cookie<__indices>(
- std::forward<_Variants>(__vars))...);
+ __element_by_index_or_cookie<__indices>(
+ std::forward<_Variants>(__vars))...);
else
return std::__invoke(std::forward<_Visitor>(__visitor),
- __element_by_index_or_cookie<__indices>(
- std::forward<_Variants>(__vars))...);
+ __element_by_index_or_cookie<__indices>(
+ std::forward<_Variants>(__vars))...);
}
static constexpr decltype(auto)
@@ -987,6 +1017,7 @@ namespace __variant
std::forward<_Variants>(__vars)...);
}
+ // Perform the implicit conversion to _Result_type for std::visit<R>.
static constexpr _Result_type
__do_visit_invoke_r(_Visitor&& __visitor, _Variants... __vars)
{
@@ -1014,20 +1045,14 @@ namespace __variant
typename _Result_type, typename _Visitor, typename... _Variants>
struct __gen_vtable
{
- using _Func_ptr = _Result_type (*)(_Visitor&&, _Variants...);
using _Array_type =
- _Multi_array<_Func_ptr,
+ _Multi_array<_Result_type (*)(_Visitor, _Variants...),
variant_size_v<remove_reference_t<_Variants>>...>;
- static constexpr _Array_type
- _S_apply()
- {
- return __gen_vtable_impl<__same_return_types,
- _Array_type, tuple<_Variants...>,
- std::index_sequence<>>::_S_apply();
- }
-
- static constexpr auto _S_vtable = _S_apply();
+ static constexpr _Array_type _S_vtable
+ = __gen_vtable_impl<__same_return_types,
+ _Array_type, tuple<_Variants...>,
+ std::index_sequence<>>::_S_apply();
};
template<size_t _Np, typename _Tp>
@@ -66,8 +66,30 @@ test01()
VERIFY( res == 35 );
}
+void
+test02()
+{
+ struct NoCopy
+ {
+ NoCopy() { }
+ NoCopy(const NoCopy&) = delete;
+ NoCopy(NoCopy&&) = delete;
+ ~NoCopy() { }
+
+ int operator()(int i) { return i; }
+ int operator()(const NoCopy&) { return 0; }
+ };
+
+ std::variant<NoCopy, int> v{1};
+ NoCopy f;
+ // Visit should not need arguments to be copyable:
+ int res = std::visit(f, v);
+ VERIFY( res == 1 );
+}
+
int
main()
{
test01();
+ test02();
}