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1//===----------------------------------------------------------------------===//
2//
3// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4// See https://llvm.org/LICENSE.txt for license information.
5// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6//
7//===----------------------------------------------------------------------===//
8
9#ifndef _LIBCPP___UTILITY_SMALL_BUFFER_H
10#define _LIBCPP___UTILITY_SMALL_BUFFER_H
11
12#include <__config>
13#include <__cstddef/byte.h>
14#include <__cstddef/size_t.h>
15#include <__memory/construct_at.h>
16#include <__new/allocate.h>
17#include <__new/launder.h>
18#include <__type_traits/decay.h>
19#include <__type_traits/is_trivially_constructible.h>
20#include <__type_traits/is_trivially_destructible.h>
21#include <__utility/exception_guard.h>
22#include <__utility/forward.h>
23
24#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
25# pragma GCC system_header
26#endif
27
28#if _LIBCPP_STD_VER >= 23
29
30// __small_buffer is a helper class to perform the well known SBO (small buffer optimization). It is mainly useful to
31// allow type-erasing classes like move_only_function to store small objects in a local buffer without requiring an
32// allocation.
33//
34// This small buffer class only allows storing trivially relocatable objects inside the local storage to allow
35// __small_buffer to be trivially relocatable itself. Since the buffer doesn't know what's stored inside it, the user
36// has to manage the object's lifetime, in particular the destruction of the object.
37
38_LIBCPP_BEGIN_NAMESPACE_STD
39
40template <size_t _BufferSize, size_t _BufferAlignment>
41 requires(_BufferSize > 0 && _BufferAlignment > 0)
42class __small_buffer {
43public:
44 template <class _Tp, class _Decayed = decay_t<_Tp>>
45 static constexpr bool __fits_in_buffer =
46 is_trivially_move_constructible_v<_Decayed> && is_trivially_destructible_v<_Decayed> &&
47 sizeof(_Decayed) <= _BufferSize && alignof(_Decayed) <= _BufferAlignment;
48
49 _LIBCPP_HIDE_FROM_ABI __small_buffer() = default;
50 __small_buffer(const __small_buffer&) = delete;
51 __small_buffer& operator=(const __small_buffer&) = delete;
52 _LIBCPP_HIDE_FROM_ABI ~__small_buffer() = default;
53
54 // Relocates the buffer - __delete() should never be called on a moved-from __small_buffer
55 _LIBCPP_HIDE_FROM_ABI __small_buffer(__small_buffer&&) = default;
56 _LIBCPP_HIDE_FROM_ABI __small_buffer& operator=(__small_buffer&&) = default;
57
58 template <class _Stored>
59 _LIBCPP_HIDE_FROM_ABI _Stored* __get() {
60 if constexpr (__fits_in_buffer<_Stored>)
61 return std::launder(reinterpret_cast<_Stored*>(__buffer_));
62 else
63 return *std::launder(reinterpret_cast<_Stored**>(__buffer_));
64 }
65
66 template <class _Stored>
67 _LIBCPP_HIDE_FROM_ABI _Stored* __alloc() {
68 if constexpr (__fits_in_buffer<_Stored>) {
69 return std::launder(reinterpret_cast<_Stored*>(__buffer_));
70 } else {
71 byte* __allocation = reinterpret_cast<byte*>(std::__libcpp_allocate<_Stored>(__element_count(1)));
72 std::construct_at(reinterpret_cast<byte**>(__buffer_), __allocation);
73 return std::launder(reinterpret_cast<_Stored*>(__allocation));
74 }
75 }
76
77 template <class _Stored>
78 _LIBCPP_HIDE_FROM_ABI void __dealloc() noexcept {
79 if constexpr (!__fits_in_buffer<_Stored>)
80 std::__libcpp_deallocate<_Stored>(__get<_Stored>(), __element_count(1));
81 }
82
83 template <class _Stored, class... _Args>
84 _LIBCPP_HIDE_FROM_ABI void __construct(_Args&&... __args) {
85 _Stored* __buffer = __alloc<_Stored>();
86 auto __guard = std::__make_exception_guard([&] { __dealloc<_Stored>(); });
87 std::construct_at(__buffer, std::forward<_Args>(__args)...);
88 __guard.__complete();
89 }
90
91private:
92 alignas(_BufferAlignment) byte __buffer_[_BufferSize];
93};
94
95# undef _LIBCPP_SMALL_BUFFER_TRIVIAL_ABI
96
97_LIBCPP_END_NAMESPACE_STD
98
99#endif // _LIBCPP_STD_VER >= 23
100
101#endif // _LIBCPP___UTILITY_SMALL_BUFFER_H