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
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 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 */
120