1 // Aligned memory buffer -*- C++ -*- 2 3 // Copyright (C) 2013-2018 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 40 namespace __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 #if _GLIBCXX_INLINE_VERSION 79 template<typename _Tp> 80 using __aligned_buffer = __aligned_membuf<_Tp>; 81 #else 82 // Similar to __aligned_membuf but aligned for complete objects, not members. 83 // This type is used in <forward_list>, <future>, <bits/shared_ptr_base.h> 84 // and <bits/hashtable_policy.h>, but ideally they would use __aligned_membuf 85 // instead, as it has smaller size for some types on some targets. 86 // This type is still used to avoid an ABI change. 87 template<typename _Tp> 88 struct __aligned_buffer 89 : std::aligned_storage<sizeof(_Tp), std::alignment_of<_Tp>::value> 90 { 91 typename 92 std::aligned_storage<sizeof(_Tp), std::alignment_of<_Tp>::value>::type 93 _M_storage; 94 95 __aligned_buffer() = default; 96 97 // Can be used to avoid value-initialization 98 __aligned_buffer(std::nullptr_t) { } 99 100 void* 101 _M_addr() noexcept 102 { 103 return static_cast<void*>(&_M_storage); 104 } 105 106 const void* 107 _M_addr() const noexcept 108 { 109 return static_cast<const void*>(&_M_storage); 110 } 111 112 _Tp* 113 _M_ptr() noexcept 114 { return static_cast<_Tp*>(_M_addr()); } 115 116 const _Tp* 117 _M_ptr() const noexcept 118 { return static_cast<const _Tp*>(_M_addr()); } 119 }; 120 #endif 121 122 } // namespace 123 124 #endif /* _ALIGNED_BUFFER_H */ 125