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

   1 // Aligned memory buffer -*- C++ -*-
   2
   3 // Copyright (C) 2013-2017 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       struct _Tp2 { _Tp _M_t; };
  53
  54       alignas(__alignof__(_Tp2::_M_t)) unsigned char _M_storage[sizeof(_Tp)];
  55
  56       __aligned_membuf() = default;
  57
  58       // Can be used to avoid value-initialization zeroing _M_storage.
  59       __aligned_membuf(std::nullptr_t) { }
  60
  61       void*
  62       _M_addr() noexcept
  63       { return static_cast<void*>(&_M_storage); }
  64
  65       const void*
  66       _M_addr() const noexcept
  67       { return static_cast<const void*>(&_M_storage); }
  68
  69       _Tp*
  70       _M_ptr() noexcept
  71       { return static_cast<_Tp*>(_M_addr()); }
  72
  73       const _Tp*
  74       _M_ptr() const noexcept
  75       { return static_cast<const _Tp*>(_M_addr()); }
  76     };
  77
  78   // Similar to __aligned_membuf but aligned for complete objects, not members.
  79   // This type is used in <forward_list>, <future>, <bits/shared_ptr_base.h>
  80   // and <bits/hashtable_policy.h>, but ideally they would use __aligned_membuf
  81   // instead, as it has smaller size for some types on some targets.
  82   // This type is still used to avoid an ABI change.
  83   template<typename _Tp>
  84     struct __aligned_buffer
  85     : std::aligned_storage<sizeof(_Tp), std::alignment_of<_Tp>::value>
  86     {
  87       typename
  88         std::aligned_storage<sizeof(_Tp), std::alignment_of<_Tp>::value>::type
  89         _M_storage;
  90
  91       __aligned_buffer() = default;
  92
  93       // Can be used to avoid value-initialization
  94       __aligned_buffer(std::nullptr_t) { }
  95
  96       void*
  97       _M_addr() noexcept
  98       {
  99         return static_cast<void*>(&_M_storage);
 100       }
 101
 102       const void*
 103       _M_addr() const noexcept
 104       {
 105         return static_cast<const void*>(&_M_storage);
 106       }
 107
 108       _Tp*
 109       _M_ptr() noexcept
 110       { return static_cast<_Tp*>(_M_addr()); }
 111
 112       const _Tp*
 113       _M_ptr() const noexcept
 114       { return static_cast<const _Tp*>(_M_addr()); }
 115     };
 116
 117 } // namespace
 118
 119 #endif /* _ALIGNED_BUFFER_H */