1// Aligned memory buffer -*- C++ -*- 
2 
3// Copyright (C) 2013-2019 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