libcxx/include/__utility/small_buffer.h

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