__cxx03/__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___CXX03___UTILITY_SMALL_BUFFER_H
0010 #define _LIBCPP___CXX03___UTILITY_SMALL_BUFFER_H
0011
0012 #include <__cxx03/__config>
0013 #include <__cxx03/__memory/construct_at.h>
0014 #include <__cxx03/__type_traits/decay.h>
0015 #include <__cxx03/__type_traits/is_trivially_constructible.h>
0016 #include <__cxx03/__type_traits/is_trivially_destructible.h>
0017 #include <__cxx03/__utility/exception_guard.h>
0018 #include <__cxx03/__utility/forward.h>
0019 #include <__cxx03/cstddef>
0020 #include <__cxx03/new>
0021
0022 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
0023 #  pragma GCC system_header
0024 #endif
0025
0026 #if _LIBCPP_STD_VER >= 23
0027
0028 // __small_buffer is a helper class to perform the well known SBO (small buffer optimization). It is mainly useful to
0029 // allow type-erasing classes like move_only_function to store small objects in a local buffer without requiring an
0030 // allocation.
0031 //
0032 // This small buffer class only allows storing trivially relocatable objects inside the local storage to allow
0033 // __small_buffer to be trivially relocatable itself. Since the buffer doesn't know what's stored inside it, the user
0034 // has to manage the object's lifetime, in particular the destruction of the object.
0035
0036 _LIBCPP_BEGIN_NAMESPACE_STD
0037
0038 template <size_t _BufferSize, size_t _BufferAlignment>
0039   requires(_BufferSize > 0 && _BufferAlignment > 0)
0040 class __small_buffer {
0041 public:
0042   template <class _Tp, class _Decayed = decay_t<_Tp>>
0043   static constexpr bool __fits_in_buffer =
0044       is_trivially_move_constructible_v<_Decayed> && is_trivially_destructible_v<_Decayed> &&
0045       sizeof(_Decayed) <= _BufferSize && alignof(_Decayed) <= _BufferAlignment;
0046
0047   _LIBCPP_HIDE_FROM_ABI __small_buffer()           = default;
0048   __small_buffer(const __small_buffer&)            = delete;
0049   __small_buffer& operator=(const __small_buffer&) = delete;
0050   _LIBCPP_HIDE_FROM_ABI ~__small_buffer()          = default;
0051
0052   // Relocates the buffer - __delete() should never be called on a moved-from __small_buffer
0053   _LIBCPP_HIDE_FROM_ABI __small_buffer(__small_buffer&&)            = default;
0054   _LIBCPP_HIDE_FROM_ABI __small_buffer& operator=(__small_buffer&&) = default;
0055
0056   template <class _Stored>
0057   _LIBCPP_HIDE_FROM_ABI _Stored* __get() {
0058     if constexpr (__fits_in_buffer<_Stored>)
0059       return std::launder(reinterpret_cast<_Stored*>(__buffer_));
0060     else
0061       return *std::launder(reinterpret_cast<_Stored**>(__buffer_));
0062   }
0063
0064   template <class _Stored>
0065   _LIBCPP_HIDE_FROM_ABI _Stored* __alloc() {
0066     if constexpr (__fits_in_buffer<_Stored>) {
0067       return std::launder(reinterpret_cast<_Stored*>(__buffer_));
0068     } else {
0069       byte* __allocation = static_cast<byte*>(::operator new[](sizeof(_Stored), align_val_t{alignof(_Stored)}));
0070       std::construct_at(reinterpret_cast<byte**>(__buffer_), __allocation);
0071       return std::launder(reinterpret_cast<_Stored*>(__allocation));
0072     }
0073   }
0074
0075   template <class _Stored>
0076   _LIBCPP_HIDE_FROM_ABI void __dealloc() noexcept {
0077     if constexpr (!__fits_in_buffer<_Stored>)
0078       ::operator delete[](*reinterpret_cast<void**>(__buffer_), sizeof(_Stored), align_val_t{alignof(_Stored)});
0079   }
0080
0081   template <class _Stored, class... _Args>
0082   _LIBCPP_HIDE_FROM_ABI void __construct(_Args&&... __args) {
0083     _Stored* __buffer = __alloc<_Stored>();
0084     auto __guard      = std::__make_exception_guard([&] { __dealloc<_Stored>(); });
0085     std::construct_at(__buffer, std::forward<_Args>(__args)...);
0086     __guard.__complete();
0087   }
0088
0089 private:
0090   alignas(_BufferAlignment) byte __buffer_[_BufferSize];
0091 };
0092
0093 #  undef _LIBCPP_SMALL_BUFFER_TRIVIAL_ABI
0094
0095 _LIBCPP_END_NAMESPACE_STD
0096
0097 #endif // _LIBCPP_STD_VER >= 23
0098
0099 #endif // _LIBCPP___CXX03___UTILITY_SMALL_BUFFER_H