1// Aligned memory buffer -*- C++ -*-
2
3// Copyright (C) 2013-2021 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
40namespace __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 */
126

source code of include/c++/11/ext/aligned_buffer.h