1// Aligned memory buffer -*- C++ -*-
2
3// Copyright (C) 2013-2015 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