libstdc++
aligned_buffer.h
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1 // Aligned memory buffer -*- C++ -*-
2 
3 // Copyright (C) 2013-2025 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 #ifdef _GLIBCXX_SYSHDR
33 #pragma GCC system_header
34 #endif
35 
36 #if __cplusplus >= 201103L
37 # include <type_traits>
38 #else
39 # include <bits/c++0x_warning.h>
40 #endif
41 
42 namespace __gnu_cxx
43 {
44  // A utility type containing a POD object that can hold an object of type
45  // _Tp initialized via placement new or allocator_traits::construct.
46  // Intended for use as a data member subobject, use __aligned_buffer for
47  // complete objects.
48  template<typename _Tp>
49  struct __aligned_membuf
50  {
51  // Target macro ADJUST_FIELD_ALIGN can produce different alignment for
52  // types when used as class members. __aligned_membuf is intended
53  // for use as a class member, so align the buffer as for a class member.
54  // Since GCC 8 we can just use alignas(_Tp) to get the right alignment.
55 #ifdef __EDG__
56  // The EDG front end does not implement the PR c++/69560 alignof change.
57  struct _Tp2 { _Tp _M_t; };
58  alignas(__alignof__(_Tp2::_M_t))
59 #else
60  alignas(_Tp)
61 #endif
62  unsigned char _M_storage[sizeof(_Tp)];
63 
64  __aligned_membuf() = default;
65 
66  // Can be used to avoid value-initialization zeroing _M_storage.
67  __aligned_membuf(std::nullptr_t) { }
68 
69  void*
70  _M_addr() noexcept
71  { return static_cast<void*>(&_M_storage); }
72 
73  const void*
74  _M_addr() const noexcept
75  { return static_cast<const void*>(&_M_storage); }
76 
77  _Tp*
78  _M_ptr() noexcept
79  { return static_cast<_Tp*>(_M_addr()); }
80 
81  const _Tp*
82  _M_ptr() const noexcept
83  { return static_cast<const _Tp*>(_M_addr()); }
84  };
85 
86 #if _GLIBCXX_INLINE_VERSION
87  template<typename _Tp>
88  using __aligned_buffer = __aligned_membuf<_Tp>;
89 #else
90  // Similar to __aligned_membuf but aligned for complete objects, not members.
91  // This type is used in <forward_list>, <future>, <bits/shared_ptr_base.h>
92  // and <bits/hashtable_policy.h>, but ideally they would use __aligned_membuf
93  // instead, as it has smaller size for some types on some targets.
94  // This type is still used to avoid an ABI change.
95  template<typename _Tp>
96  struct __aligned_buffer
97  {
98  // Using __alignof__ gives the alignment for a complete object.
99  alignas(__alignof__(_Tp)) unsigned char _M_storage[sizeof(_Tp)];
100 
101  __aligned_buffer() = default;
102 
103  // Can be used to avoid value-initialization
104  __aligned_buffer(std::nullptr_t) { }
105 
106  void*
107  _M_addr() noexcept
108  {
109  return static_cast<void*>(&_M_storage);
110  }
111 
112  const void*
113  _M_addr() const noexcept
114  {
115  return static_cast<const void*>(&_M_storage);
116  }
117 
118  _Tp*
119  _M_ptr() noexcept
120  { return static_cast<_Tp*>(_M_addr()); }
121 
122  const _Tp*
123  _M_ptr() const noexcept
124  { return static_cast<const _Tp*>(_M_addr()); }
125  };
126 #endif
127 
128 } // namespace
129 
130 #endif /* _ALIGNED_BUFFER_H */
GNU extensions for public use.