v1/__utility/small_buffer.h

0001 //===----------------------------------------------------------------------===//
0002 //
0003 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
0004 // See https://llvm.org/LICENSE.txt for license information.
0005 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
0006 //
0007 //===----------------------------------------------------------------------===//
0008
0009 #ifndef _LIBCPP___UTILITY_SMALL_BUFFER_H
0010 #define _LIBCPP___UTILITY_SMALL_BUFFER_H
0011
0012 #include <__config>
0013 #include <__cstddef/byte.h>
0014 #include <__cstddef/size_t.h>
0015 #include <__memory/construct_at.h>
0016 #include <__new/allocate.h>
0017 #include <__new/launder.h>
0018 #include <__type_traits/decay.h>
0019 #include <__type_traits/is_trivially_constructible.h>
0020 #include <__type_traits/is_trivially_destructible.h>
0021 #include <__utility/exception_guard.h>
0022 #include <__utility/forward.h>
0023
0024 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
0025 #  pragma GCC system_header
0026 #endif
0027
0028 #if _LIBCPP_STD_VER >= 23
0029
0030 // __small_buffer is a helper class to perform the well known SBO (small buffer optimization). It is mainly useful to
0031 // allow type-erasing classes like move_only_function to store small objects in a local buffer without requiring an
0032 // allocation.
0033 //
0034 // This small buffer class only allows storing trivially relocatable objects inside the local storage to allow
0035 // __small_buffer to be trivially relocatable itself. Since the buffer doesn't know what's stored inside it, the user
0036 // has to manage the object's lifetime, in particular the destruction of the object.
0037
0038 _LIBCPP_BEGIN_NAMESPACE_STD
0039
0040 template <size_t _BufferSize, size_t _BufferAlignment>
0041   requires(_BufferSize > 0 && _BufferAlignment > 0)
0042 class __small_buffer {
0043 public:
0044   template <class _Tp, class _Decayed = decay_t<_Tp>>
0045   static constexpr bool __fits_in_buffer =
0046       is_trivially_move_constructible_v<_Decayed> && is_trivially_destructible_v<_Decayed> &&
0047       sizeof(_Decayed) <= _BufferSize && alignof(_Decayed) <= _BufferAlignment;
0048
0049   _LIBCPP_HIDE_FROM_ABI __small_buffer()           = default;
0050   __small_buffer(const __small_buffer&)            = delete;
0051   __small_buffer& operator=(const __small_buffer&) = delete;
0052   _LIBCPP_HIDE_FROM_ABI ~__small_buffer()          = default;
0053
0054   // Relocates the buffer - __delete() should never be called on a moved-from __small_buffer
0055   _LIBCPP_HIDE_FROM_ABI __small_buffer(__small_buffer&&)            = default;
0056   _LIBCPP_HIDE_FROM_ABI __small_buffer& operator=(__small_buffer&&) = default;
0057
0058   template <class _Stored>
0059   _LIBCPP_HIDE_FROM_ABI _Stored* __get() {
0060     if constexpr (__fits_in_buffer<_Stored>)
0061       return std::launder(reinterpret_cast<_Stored*>(__buffer_));
0062     else
0063       return *std::launder(reinterpret_cast<_Stored**>(__buffer_));
0064   }
0065
0066   template <class _Stored>
0067   _LIBCPP_HIDE_FROM_ABI _Stored* __alloc() {
0068     if constexpr (__fits_in_buffer<_Stored>) {
0069       return std::launder(reinterpret_cast<_Stored*>(__buffer_));
0070     } else {
0071       byte* __allocation = reinterpret_cast<byte*>(std::__libcpp_allocate<_Stored>(__element_count(1)));
0072       std::construct_at(reinterpret_cast<byte**>(__buffer_), __allocation);
0073       return std::launder(reinterpret_cast<_Stored*>(__allocation));
0074     }
0075   }
0076
0077   template <class _Stored>
0078   _LIBCPP_HIDE_FROM_ABI void __dealloc() noexcept {
0079     if constexpr (!__fits_in_buffer<_Stored>)
0080       std::__libcpp_deallocate<_Stored>(__get<_Stored>(), __element_count(1));
0081   }
0082
0083   template <class _Stored, class... _Args>
0084   _LIBCPP_HIDE_FROM_ABI void __construct(_Args&&... __args) {
0085     _Stored* __buffer = __alloc<_Stored>();
0086     auto __guard      = std::__make_exception_guard([&] { __dealloc<_Stored>(); });
0087     std::construct_at(__buffer, std::forward<_Args>(__args)...);
0088     __guard.__complete();
0089   }
0090
0091 private:
0092   alignas(_BufferAlignment) byte __buffer_[_BufferSize];
0093 };
0094
0095 #  undef _LIBCPP_SMALL_BUFFER_TRIVIAL_ABI
0096
0097 _LIBCPP_END_NAMESPACE_STD
0098
0099 #endif // _LIBCPP_STD_VER >= 23
0100
0101 #endif // _LIBCPP___UTILITY_SMALL_BUFFER_H