aligned_buffer.h

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// Aligned memory buffer -*- C++ -*-
 
// Copyright (C) 2013-2020 Free Software Foundation, Inc.
//
// This file is part of the GNU ISO C++ Library.  This library is free
// software; you can redistribute it and/or modify it under the
// terms of the GNU General Public License as published by the
// Free Software Foundation; either version 3, or (at your option)
// any later version.
 
// This library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU General Public License for more details.
 
// Under Section 7 of GPL version 3, you are granted additional
// permissions described in the GCC Runtime Library Exception, version
// 3.1, as published by the Free Software Foundation.
 
// You should have received a copy of the GNU General Public License and
// a copy of the GCC Runtime Library Exception along with this program;
// see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
// <http://www.gnu.org/licenses/>.
 
/** @file ext/aligned_buffer.h
 *  This file is a GNU extension to the Standard C++ Library.
 */
 
#ifndef _ALIGNED_BUFFER_H
#define _ALIGNED_BUFFER_H 1
 
#pragma GCC system_header
 
#if __cplusplus >= 201103L
# include <type_traits>
#else
# include <bits/c++0x_warning.h>
#endif
 
namespace __gnu_cxx
{
  // A utility type containing a POD object that can hold an object of type
  // _Tp initialized via placement new or allocator_traits::construct.
  // Intended for use as a data member subobject, use __aligned_buffer for
  // complete objects.
  template<typename _Tp>
    struct __aligned_membuf
    {
      // Target macro ADJUST_FIELD_ALIGN can produce different alignment for
      // types when used as class members. __aligned_membuf is intended
      // for use as a class member, so align the buffer as for a class member.
      // Since GCC 8 we could just use alignof(_Tp) instead, but older
      // versions of non-GNU compilers might still need this trick.
      struct _Tp2 { _Tp _M_t; };
 
      alignas(__alignof__(_Tp2::_M_t)) unsigned char _M_storage[sizeof(_Tp)];
 
      __aligned_membuf() = default;
 
      // Can be used to avoid value-initialization zeroing _M_storage.
      __aligned_membuf(std::nullptr_t) { }
 
      void*
      _M_addr() noexcept
      { return static_cast<void*>(&_M_storage); }
 
      const void*
      _M_addr() const noexcept
      { return static_cast<const void*>(&_M_storage); }
 
      _Tp*
      _M_ptr() noexcept
      { return static_cast<_Tp*>(_M_addr()); }
 
      const _Tp*
      _M_ptr() const noexcept
      { return static_cast<const _Tp*>(_M_addr()); }
    };
 
#if _GLIBCXX_INLINE_VERSION
  template<typename _Tp>
    using __aligned_buffer = __aligned_membuf<_Tp>;
#else
  // Similar to __aligned_membuf but aligned for complete objects, not members.
  // This type is used in <forward_list>, <future>, <bits/shared_ptr_base.h>
  // and <bits/hashtable_policy.h>, but ideally they would use __aligned_membuf
  // instead, as it has smaller size for some types on some targets.
  // This type is still used to avoid an ABI change.
  template<typename _Tp>
    struct __aligned_buffer
    : std::aligned_storage<sizeof(_Tp), __alignof__(_Tp)>
    {
      typename
        std::aligned_storage<sizeof(_Tp), __alignof__(_Tp)>::type _M_storage;
 
      __aligned_buffer() = default;
 
      // Can be used to avoid value-initialization
      __aligned_buffer(std::nullptr_t) { }
 
      void*
      _M_addr() noexcept
      {
        return static_cast<void*>(&_M_storage);
      }
 
      const void*
      _M_addr() const noexcept
      {
        return static_cast<const void*>(&_M_storage);
      }
 
      _Tp*
      _M_ptr() noexcept
      { return static_cast<_Tp*>(_M_addr()); }
 
      const _Tp*
      _M_ptr() const noexcept
      { return static_cast<const _Tp*>(_M_addr()); }
    };
#endif
 
} // namespace
 
#endif /* _ALIGNED_BUFFER_H */