@@ -260,6 +260,8 @@ __shared_ptr_pointer<_Tp, _Dp, _Alloc>::__on_zero_shared_weak() _NOEXCEPT
260260 __a.deallocate (_PTraits::pointer_to (*this ), 1 );
261261}
262262
263+ struct __default_initialize_tag {};
264+
263265template <class _Tp , class _Alloc >
264266struct __shared_ptr_emplace
265267 : __shared_weak_count
@@ -278,6 +280,16 @@ struct __shared_ptr_emplace
278280#endif
279281 }
280282
283+
284+ #if _LIBCPP_STD_VER >= 20
285+ _LIBCPP_HIDE_FROM_ABI
286+ explicit __shared_ptr_emplace (__default_initialize_tag, _Alloc __a)
287+ : __storage_(std::move(__a))
288+ {
289+ ::new ((void *)__get_elem ()) _Tp;
290+ }
291+ #endif
292+
281293 _LIBCPP_HIDE_FROM_ABI
282294 _Alloc* __get_alloc () _NOEXCEPT { return __storage_.__get_alloc (); }
283295
@@ -945,6 +957,29 @@ shared_ptr<_Tp> make_shared(_Args&& ...__args)
945957 return _VSTD::allocate_shared<_Tp>(allocator<_Tp>(), _VSTD::forward<_Args>(__args)...);
946958}
947959
960+ #if _LIBCPP_STD_VER >= 20
961+
962+ template <class _Tp , class _Alloc , __enable_if_t <!is_array<_Tp>::value, int > = 0 >
963+ _LIBCPP_HIDE_FROM_ABI
964+ shared_ptr<_Tp> allocate_shared_for_overwrite (const _Alloc& __a)
965+ {
966+ using _ControlBlock = __shared_ptr_emplace<_Tp, _Alloc>;
967+ using _ControlBlockAllocator = typename __allocator_traits_rebind<_Alloc, _ControlBlock>::type;
968+ __allocation_guard<_ControlBlockAllocator> __guard (__a, 1 );
969+ ::new ((void *)_VSTD::addressof (*__guard.__get ())) _ControlBlock (__default_initialize_tag{}, __a);
970+ auto __control_block = __guard.__release_ptr ();
971+ return shared_ptr<_Tp>::__create_with_control_block ((*__control_block).__get_elem (), _VSTD::addressof (*__control_block));
972+ }
973+
974+ template <class _Tp , __enable_if_t <!is_array<_Tp>::value, int > = 0 >
975+ _LIBCPP_HIDE_FROM_ABI
976+ shared_ptr<_Tp> make_shared_for_overwrite ()
977+ {
978+ return std::allocate_shared_for_overwrite<_Tp>(allocator<_Tp>());
979+ }
980+
981+ #endif // _LIBCPP_STD_VER >= 20
982+
948983#if _LIBCPP_STD_VER > 14
949984
950985template <size_t _Alignment>
@@ -975,6 +1010,17 @@ struct __unbounded_array_control_block<_Tp[], _Alloc> : __shared_weak_count
9751010 std::__uninitialized_allocator_value_construct_n (__alloc_, std::begin (__data_), __count_);
9761011 }
9771012
1013+ #if _LIBCPP_STD_VER >= 20
1014+ _LIBCPP_HIDE_FROM_ABI
1015+ explicit __unbounded_array_control_block (_Alloc const & __alloc, size_t __count, __default_initialize_tag)
1016+ : __alloc_(__alloc), __count_(__count)
1017+ {
1018+ // We are purposefully not using an allocator-aware default construction because the spec says so.
1019+ // There's currently no way of expressing default initialization in an allocator-aware manner anyway.
1020+ std::uninitialized_default_construct_n (std::begin (__data_), __count_);
1021+ }
1022+ #endif
1023+
9781024 // Returns the number of bytes required to store a control block followed by the given number
9791025 // of elements of _Tp, with the whole storage being aligned to a multiple of _Tp's alignment.
9801026 _LIBCPP_HIDE_FROM_ABI
@@ -1058,6 +1104,15 @@ struct __bounded_array_control_block<_Tp[_Count], _Alloc>
10581104 std::__uninitialized_allocator_value_construct_n (__alloc_, std::addressof (__data_[0 ]), _Count);
10591105 }
10601106
1107+ #if _LIBCPP_STD_VER >= 20
1108+ _LIBCPP_HIDE_FROM_ABI
1109+ explicit __bounded_array_control_block (_Alloc const & __alloc, __default_initialize_tag) : __alloc_(__alloc) {
1110+ // We are purposefully not using an allocator-aware default construction because the spec says so.
1111+ // There's currently no way of expressing default initialization in an allocator-aware manner anyway.
1112+ std::uninitialized_default_construct_n (std::addressof (__data_[0 ]), _Count);
1113+ }
1114+ #endif
1115+
10611116 _LIBCPP_HIDE_FROM_ABI_VIRTUAL
10621117 ~__bounded_array_control_block () override { } // can't be `= default` because of the sometimes-non-trivial union member __data_
10631118
@@ -1101,6 +1156,7 @@ shared_ptr<_Array> __allocate_shared_bounded_array(const _Alloc& __a, _Arg&& ...
11011156
11021157#if _LIBCPP_STD_VER > 17
11031158
1159+ // bounded array variants
11041160template <class _Tp , class _Alloc , class = __enable_if_t <is_bounded_array<_Tp>::value>>
11051161_LIBCPP_HIDE_FROM_ABI
11061162shared_ptr<_Tp> allocate_shared (const _Alloc& __a)
@@ -1115,18 +1171,11 @@ shared_ptr<_Tp> allocate_shared(const _Alloc& __a, const remove_extent_t<_Tp>& _
11151171 return std::__allocate_shared_bounded_array<_Tp>(__a, __u);
11161172}
11171173
1118- template <class _Tp , class _Alloc , class = __enable_if_t <is_unbounded_array<_Tp>::value>>
1119- _LIBCPP_HIDE_FROM_ABI
1120- shared_ptr<_Tp> allocate_shared (const _Alloc& __a, size_t __n)
1121- {
1122- return std::__allocate_shared_unbounded_array<_Tp>(__a, __n);
1123- }
1124-
1125- template <class _Tp , class _Alloc , class = __enable_if_t <is_unbounded_array<_Tp>::value>>
1174+ template <class _Tp , class _Alloc , __enable_if_t <is_bounded_array<_Tp>::value, int > = 0 >
11261175_LIBCPP_HIDE_FROM_ABI
1127- shared_ptr<_Tp> allocate_shared (const _Alloc& __a, size_t __n, const remove_extent_t <_Tp>& __u )
1176+ shared_ptr<_Tp> allocate_shared_for_overwrite (const _Alloc& __a)
11281177{
1129- return std::__allocate_shared_unbounded_array <_Tp>(__a, __n, __u );
1178+ return std::__allocate_shared_bounded_array <_Tp>(__a, __default_initialize_tag{} );
11301179}
11311180
11321181template <class _Tp , class = __enable_if_t <is_bounded_array<_Tp>::value>>
@@ -1143,6 +1192,35 @@ shared_ptr<_Tp> make_shared(const remove_extent_t<_Tp>& __u)
11431192 return std::__allocate_shared_bounded_array<_Tp>(allocator<_Tp>(), __u);
11441193}
11451194
1195+ template <class _Tp , __enable_if_t <is_bounded_array<_Tp>::value, int > = 0 >
1196+ _LIBCPP_HIDE_FROM_ABI
1197+ shared_ptr<_Tp> make_shared_for_overwrite ()
1198+ {
1199+ return std::__allocate_shared_bounded_array<_Tp>(allocator<_Tp>(), __default_initialize_tag{});
1200+ }
1201+
1202+ // unbounded array variants
1203+ template <class _Tp , class _Alloc , class = __enable_if_t <is_unbounded_array<_Tp>::value>>
1204+ _LIBCPP_HIDE_FROM_ABI
1205+ shared_ptr<_Tp> allocate_shared (const _Alloc& __a, size_t __n)
1206+ {
1207+ return std::__allocate_shared_unbounded_array<_Tp>(__a, __n);
1208+ }
1209+
1210+ template <class _Tp , class _Alloc , class = __enable_if_t <is_unbounded_array<_Tp>::value>>
1211+ _LIBCPP_HIDE_FROM_ABI
1212+ shared_ptr<_Tp> allocate_shared (const _Alloc& __a, size_t __n, const remove_extent_t <_Tp>& __u)
1213+ {
1214+ return std::__allocate_shared_unbounded_array<_Tp>(__a, __n, __u);
1215+ }
1216+
1217+ template <class _Tp , class _Alloc , __enable_if_t <is_unbounded_array<_Tp>::value, int > = 0 >
1218+ _LIBCPP_HIDE_FROM_ABI
1219+ shared_ptr<_Tp> allocate_shared_for_overwrite (const _Alloc& __a, size_t __n)
1220+ {
1221+ return std::__allocate_shared_unbounded_array<_Tp>(__a, __n, __default_initialize_tag{});
1222+ }
1223+
11461224template <class _Tp , class = __enable_if_t <is_unbounded_array<_Tp>::value>>
11471225_LIBCPP_HIDE_FROM_ABI
11481226shared_ptr<_Tp> make_shared (size_t __n)
@@ -1157,6 +1235,13 @@ shared_ptr<_Tp> make_shared(size_t __n, const remove_extent_t<_Tp>& __u)
11571235 return std::__allocate_shared_unbounded_array<_Tp>(allocator<_Tp>(), __n, __u);
11581236}
11591237
1238+ template <class _Tp , __enable_if_t <is_unbounded_array<_Tp>::value, int > = 0 >
1239+ _LIBCPP_HIDE_FROM_ABI
1240+ shared_ptr<_Tp> make_shared_for_overwrite (size_t __n)
1241+ {
1242+ return std::__allocate_shared_unbounded_array<_Tp>(allocator<_Tp>(), __n, __default_initialize_tag{});
1243+ }
1244+
11601245#endif // _LIBCPP_STD_VER > 17
11611246
11621247template <class _Tp , class _Up >
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