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