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 // 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