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[libc++] Specialize allocator_traits for std::allocator (#160278)

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This reduces the time to instantiate `allocator_traits<allocator<T>>`.
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Nikolas Klauser 2026-02-05 11:43:47 +01:00 committed by GitHub
parent 205ee34317
commit c9863ceacc
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GPG key ID: B5690EEEBB952194
1 changed files with 61 additions and 1 deletions
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62
libcxx/include/__memory/allocator_traits.h
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@ -11,6 +11,7 @@
#define _LIBCPP___MEMORY_ALLOCATOR_TRAITS_H
#include <__config>
#include <__cstddef/ptrdiff_t.h>
#include <__cstddef/size_t.h>
#include <__fwd/memory.h>
#include <__memory/construct_at.h>
@ -223,8 +224,13 @@ _LIBCPP_CTAD_SUPPORTED_FOR_TYPE(allocation_result);
#endif // _LIBCPP_STD_VER
template <class>
struct allocator_traits;
// We have a base class that can be specialized for different allocators, since the metaprogramming to get the aliases
// is quite expensive and the definition of these aliases is usually quite trivial in the end.
template <class _Alloc>
struct allocator_traits {
struct __allocator_traits_base {
using allocator_type = _Alloc;
using value_type = typename allocator_type::value_type;
using pointer = __pointer<value_type, allocator_type>;
@ -253,6 +259,60 @@ struct allocator_traits {
using other = allocator_traits<typename rebind_alloc<_Tp>::other>;
};
#endif // _LIBCPP_CXX03_LANG
};
template <class _Tp>
struct __allocator_traits_base<allocator<_Tp> > {
using allocator_type = allocator<_Tp>;
using value_type = _Tp;
using pointer = _Tp*;
using const_pointer = const _Tp*;
using void_pointer = void*;
using const_void_pointer = const void*;
using difference_type = ptrdiff_t;
using size_type = size_t;
using propagate_on_container_copy_assignment = false_type;
using propagate_on_container_move_assignment = true_type;
using propagate_on_container_swap = false_type;
using is_always_equal = true_type;
#ifndef _LIBCPP_CXX03_LANG
template <class _Up>
using rebind_alloc = allocator<_Up>;
template <class _Up>
using rebind_traits = allocator_traits<allocator<_Up> >;
#else
template <class _Up>
struct rebind_alloc {
using other = allocator<_Up>;
};
template <class _Up>
struct rebind_traits {
using other = allocator_traits<allocator<_Up> >;
};
#endif
};
template <class _Alloc>
struct allocator_traits : __allocator_traits_base<_Alloc> {
using __base _LIBCPP_NODEBUG = __allocator_traits_base<_Alloc>;
using allocator_type = typename __base::allocator_type;
using value_type = typename __base::value_type;
using pointer = typename __base::pointer;
using const_pointer = typename __base::const_pointer;
using void_pointer = typename __base::void_pointer;
using const_void_pointer = typename __base::const_void_pointer;
using difference_type = typename __base::difference_type;
using size_type = typename __base::size_type;
using propagate_on_container_copy_assignment = typename __base::propagate_on_container_copy_assignment;
using propagate_on_container_move_assignment = typename __base::propagate_on_container_move_assignment;
using is_always_equal = typename __base::is_always_equal;
template <class _Tp>
using rebind_alloc = typename __base::template rebind_alloc<_Tp>;
template <class _Tp>
using rebind_traits = typename __base::template rebind_traits<_Tp>;
[[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 static pointer
allocate(allocator_type& __a, size_type __n) {