1 //===----------------------------------------------------------------------===// 2 // 3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions. 4 // See https://llvm.org/LICENSE.txt for license information. 5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception 6 // 7 //===----------------------------------------------------------------------===// 8 9 #ifndef _LIBCPP___UTILITY_TRANSACTION_H 10 #define _LIBCPP___UTILITY_TRANSACTION_H 11 12 #include <__assert> 13 #include <__config> 14 #include <__type_traits/is_nothrow_move_constructible.h> 15 #include <__utility/exchange.h> 16 #include <__utility/move.h> 17 18 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER) 19 # pragma GCC system_header 20 #endif 21 22 _LIBCPP_PUSH_MACROS 23 #include <__undef_macros> 24 25 _LIBCPP_BEGIN_NAMESPACE_STD 26 27 // __exception_guard is a helper class for writing code with the strong exception guarantee. 28 // 29 // When writing code that can throw an exception, one can store rollback instructions in an 30 // exception guard so that if an exception is thrown at any point during the lifetime of the 31 // exception guard, it will be rolled back automatically. When the exception guard is done, one 32 // must mark it as being complete so it isn't rolled back when the exception guard is destroyed. 33 // 34 // Exception guards are not default constructible, they can't be copied or assigned to, but 35 // they can be moved around for convenience. 36 // 37 // __exception_guard is a no-op in -fno-exceptions mode to produce better code-gen. This means 38 // that we don't provide the strong exception guarantees. However, Clang doesn't generate cleanup 39 // code with exceptions disabled, so even if we wanted to provide the strong exception guarantees 40 // we couldn't. This is also only relevant for constructs with a stack of 41 // -fexceptions > -fno-exceptions > -fexceptions code, since the exception can't be caught where 42 // exceptions are disabled. While -fexceptions > -fno-exceptions is quite common 43 // (e.g. libc++.dylib > -fno-exceptions), having another layer with exceptions enabled seems a lot 44 // less common, especially one that tries to catch an exception through -fno-exceptions code. 45 // 46 // __exception_guard can help greatly simplify code that would normally be cluttered by 47 // `#if _LIBCPP_HAS_NO_EXCEPTIONS`. For example: 48 // 49 // template <class Iterator, class Size, class OutputIterator> 50 // Iterator uninitialized_copy_n(Iterator iter, Size n, OutputIterator out) { 51 // typedef typename iterator_traits<Iterator>::value_type value_type; 52 // __exception_guard guard([start=out, &out] { 53 // std::destroy(start, out); 54 // }); 55 // 56 // for (; n > 0; ++iter, ++out, --n) { 57 // ::new ((void*)std::addressof(*out)) value_type(*iter); 58 // } 59 // guard.__complete(); 60 // return out; 61 // } 62 // 63 64 template <class _Rollback> 65 struct __exception_guard_exceptions { 66 __exception_guard_exceptions() = delete; 67 68 _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 explicit __exception_guard_exceptions(_Rollback __rollback) 69 : __rollback_(std::move(__rollback)), __completed_(false) {} 70 71 _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 72 __exception_guard_exceptions(__exception_guard_exceptions&& __other) 73 _NOEXCEPT_(is_nothrow_move_constructible<_Rollback>::value) 74 : __rollback_(std::move(__other.__rollback_)), __completed_(__other.__completed_) { 75 __other.__completed_ = true; 76 } 77 78 __exception_guard_exceptions(__exception_guard_exceptions const&) = delete; 79 __exception_guard_exceptions& operator=(__exception_guard_exceptions const&) = delete; 80 __exception_guard_exceptions& operator=(__exception_guard_exceptions&&) = delete; 81 82 _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 void __complete() _NOEXCEPT { __completed_ = true; } 83 84 _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 ~__exception_guard_exceptions() { 85 if (!__completed_) 86 __rollback_(); 87 } 88 89 private: 90 _Rollback __rollback_; 91 bool __completed_; 92 }; 93 94 _LIBCPP_CTAD_SUPPORTED_FOR_TYPE(__exception_guard_exceptions); 95 96 template <class _Rollback> 97 struct __exception_guard_noexceptions { 98 __exception_guard_noexceptions() = delete; 99 _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 100 _LIBCPP_NODEBUG explicit __exception_guard_noexceptions(_Rollback) {} 101 102 _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 _LIBCPP_NODEBUG 103 __exception_guard_noexceptions(__exception_guard_noexceptions&& __other) 104 _NOEXCEPT_(is_nothrow_move_constructible<_Rollback>::value) 105 : __completed_(__other.__completed_) { 106 __other.__completed_ = true; 107 } 108 109 __exception_guard_noexceptions(__exception_guard_noexceptions const&) = delete; 110 __exception_guard_noexceptions& operator=(__exception_guard_noexceptions const&) = delete; 111 __exception_guard_noexceptions& operator=(__exception_guard_noexceptions&&) = delete; 112 113 _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 _LIBCPP_NODEBUG void __complete() _NOEXCEPT { 114 __completed_ = true; 115 } 116 117 _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 _LIBCPP_NODEBUG ~__exception_guard_noexceptions() { 118 _LIBCPP_ASSERT_UNCATEGORIZED(__completed_, "__exception_guard not completed with exceptions disabled"); 119 } 120 121 private: 122 bool __completed_ = false; 123 }; 124 125 _LIBCPP_CTAD_SUPPORTED_FOR_TYPE(__exception_guard_noexceptions); 126 127 #ifdef _LIBCPP_HAS_NO_EXCEPTIONS 128 template <class _Rollback> 129 using __exception_guard = __exception_guard_noexceptions<_Rollback>; 130 #else 131 template <class _Rollback> 132 using __exception_guard = __exception_guard_exceptions<_Rollback>; 133 #endif 134 135 template <class _Rollback> 136 _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR __exception_guard<_Rollback> __make_exception_guard(_Rollback __rollback) { 137 return __exception_guard<_Rollback>(std::move(__rollback)); 138 } 139 140 _LIBCPP_END_NAMESPACE_STD 141 142 _LIBCPP_POP_MACROS 143 144 #endif // _LIBCPP___UTILITY_TRANSACTION_H 145