libstdc++-v3/include/ext/aligned_buffer.h

   1 // Aligned memory buffer -*- C++ -*-
   2
   3 // Copyright (C) 2013-2018 Free Software Foundation, Inc.
   4 //
   5 // This file is part of the GNU ISO C++ Library.  This library is free
   6 // software; you can redistribute it and/or modify it under the
   7 // terms of the GNU General Public License as published by the
   8 // Free Software Foundation; either version 3, or (at your option)
   9 // any later version.
  10
  11 // This library is distributed in the hope that it will be useful,
  12 // but WITHOUT ANY WARRANTY; without even the implied warranty of
  13 // MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  14 // GNU General Public License for more details.
  15
  16 // Under Section 7 of GPL version 3, you are granted additional
  17 // permissions described in the GCC Runtime Library Exception, version
  18 // 3.1, as published by the Free Software Foundation.
  19
  20 // You should have received a copy of the GNU General Public License and
  21 // a copy of the GCC Runtime Library Exception along with this program;
  22 // see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
  23 // <http://www.gnu.org/licenses/>.
  24
  25 /** @file ext/aligned_buffer.h
  26  *  This file is a GNU extension to the Standard C++ Library.
  27  */
  28
  29 #ifndef _ALIGNED_BUFFER_H
  30 #define _ALIGNED_BUFFER_H 1
  31
  32 #pragma GCC system_header
  33
  34 #if __cplusplus >= 201103L
  35 # include <type_traits>
  36 #else
  37 # include <bits/c++0x_warning.h>
  38 #endif
  39
  40 namespace __gnu_cxx
  41 {
  42   // A utility type containing a POD object that can hold an object of type
  43   // _Tp initialized via placement new or allocator_traits::construct.
  44   // Intended for use as a data member subobject, use __aligned_buffer for
  45   // complete objects.
  46   template<typename _Tp>
  47     struct __aligned_membuf
  48     {
  49       // Target macro ADJUST_FIELD_ALIGN can produce different alignment for
  50       // types when used as class members. __aligned_membuf is intended
  51       // for use as a class member, so align the buffer as for a class member.
  52       // Since GCC 8 we could just use alignof(_Tp) instead, but older
  53       // versions of non-GNU compilers might still need this trick.
  54       struct _Tp2 { _Tp _M_t; };
  55
  56       alignas(__alignof__(_Tp2::_M_t)) unsigned char _M_storage[sizeof(_Tp)];
  57
  58       __aligned_membuf() = default;
  59
  60       // Can be used to avoid value-initialization zeroing _M_storage.
  61       __aligned_membuf(std::nullptr_t) { }
  62
  63       void*
  64       _M_addr() noexcept
  65       { return static_cast<void*>(&_M_storage); }
  66
  67       const void*
  68       _M_addr() const noexcept
  69       { return static_cast<const void*>(&_M_storage); }
  70
  71       _Tp*
  72       _M_ptr() noexcept
  73       { return static_cast<_Tp*>(_M_addr()); }
  74
  75       const _Tp*
  76       _M_ptr() const noexcept
  77       { return static_cast<const _Tp*>(_M_addr()); }
  78     };
  79
  80 #if _GLIBCXX_INLINE_VERSION
  81   template<typename _Tp>
  82     using __aligned_buffer = __aligned_membuf<_Tp>;
  83 #else
  84   // Similar to __aligned_membuf but aligned for complete objects, not members.
  85   // This type is used in <forward_list>, <future>, <bits/shared_ptr_base.h>
  86   // and <bits/hashtable_policy.h>, but ideally they would use __aligned_membuf
  87   // instead, as it has smaller size for some types on some targets.
  88   // This type is still used to avoid an ABI change.
  89   template<typename _Tp>
  90     struct __aligned_buffer
  91     : std::aligned_storage<sizeof(_Tp), __alignof__(_Tp)>
  92     {
  93       typename
  94         std::aligned_storage<sizeof(_Tp), __alignof__(_Tp)>::type _M_storage;
  95
  96       __aligned_buffer() = default;
  97
  98       // Can be used to avoid value-initialization
  99       __aligned_buffer(std::nullptr_t) { }
 100
 101       void*
 102       _M_addr() noexcept
 103       {
 104         return static_cast<void*>(&_M_storage);
 105       }
 106
 107       const void*
 108       _M_addr() const noexcept
 109       {
 110         return static_cast<const void*>(&_M_storage);
 111       }
 112
 113       _Tp*
 114       _M_ptr() noexcept
 115       { return static_cast<_Tp*>(_M_addr()); }
 116
 117       const _Tp*
 118       _M_ptr() const noexcept
 119       { return static_cast<const _Tp*>(_M_addr()); }
 120     };
 121 #endif
 122
 123 } // namespace
 124
 125 #endif /* _ALIGNED_BUFFER_H */