libstdc++: aligned_buffer.h Source File

libstdc++

aligned_buffer.h
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1// Aligned memory buffer -*- C++ -*-
2
3// Copyright (C) 2013-2026 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
42namespace __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 */
type_traits
c++0x_warning.h
__gnu_cxx
GNU extensions for public use.