libstdc++
aligned_buffer.h
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1 // Aligned memory buffer -*- C++ -*-
2 
3 // Copyright (C) 2013-2020 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 */
GNU extensions for public use.
Alignment type.
Definition: type_traits:2070