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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-exception6//7//===----------------------------------------------------------------------===//8 9#ifndef _LIBCPP___UTILITY_EXCEPTION_GUARD_H10#define _LIBCPP___UTILITY_EXCEPTION_GUARD_H11 12#include <__assert>13#include <__config>14#include <__type_traits/is_nothrow_constructible.h>15#include <__utility/move.h>16 17#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)18# pragma GCC system_header19#endif20 21_LIBCPP_PUSH_MACROS22#include <__undef_macros>23 24_LIBCPP_BEGIN_NAMESPACE_STD25 26// __exception_guard is a helper class for writing code with the strong exception guarantee.27//28// When writing code that can throw an exception, one can store rollback instructions in an29// exception guard so that if an exception is thrown at any point during the lifetime of the30// exception guard, it will be rolled back automatically. When the exception guard is done, one31// must mark it as being complete so it isn't rolled back when the exception guard is destroyed.32//33// Exception guards are not default constructible, they can't be copied or assigned to, but34// they can be moved around for convenience.35//36// __exception_guard is a no-op in -fno-exceptions mode to produce better code-gen. This means37// that we don't provide the strong exception guarantees. However, Clang doesn't generate cleanup38// code with exceptions disabled, so even if we wanted to provide the strong exception guarantees39// we couldn't. This is also only relevant for constructs with a stack of40// -fexceptions > -fno-exceptions > -fexceptions code, since the exception can't be caught where41// exceptions are disabled. While -fexceptions > -fno-exceptions is quite common42// (e.g. libc++.dylib > -fno-exceptions), having another layer with exceptions enabled seems a lot43// less common, especially one that tries to catch an exception through -fno-exceptions code.44//45// __exception_guard can help greatly simplify code that would normally be cluttered by46// `#if _LIBCPP_HAS_EXCEPTIONS`. For example:47//48// template <class Iterator, class Size, class OutputIterator>49// Iterator uninitialized_copy_n(Iterator iter, Size n, OutputIterator out) {50// typedef typename iterator_traits<Iterator>::value_type value_type;51// __exception_guard guard([start=out, &out] {52// std::destroy(start, out);53// });54//55// for (; n > 0; ++iter, ++out, --n) {56// ::new ((void*)std::addressof(*out)) value_type(*iter);57// }58// guard.__complete();59// return out;60// }61//62 63template <class _Rollback>64struct __exception_guard_exceptions {65 __exception_guard_exceptions() = delete;66 67 _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 explicit __exception_guard_exceptions(_Rollback __rollback)68 : __rollback_(std::move(__rollback)), __completed_(false) {}69 70 _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX2071 __exception_guard_exceptions(__exception_guard_exceptions&& __other)72 _NOEXCEPT_(is_nothrow_move_constructible<_Rollback>::value)73 : __rollback_(std::move(__other.__rollback_)), __completed_(__other.__completed_) {74 __other.__completed_ = true;75 }76 77 __exception_guard_exceptions(__exception_guard_exceptions const&) = delete;78 __exception_guard_exceptions& operator=(__exception_guard_exceptions const&) = delete;79 __exception_guard_exceptions& operator=(__exception_guard_exceptions&&) = delete;80 81 _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 void __complete() _NOEXCEPT { __completed_ = true; }82 83 _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 ~__exception_guard_exceptions() {84 if (!__completed_)85 __rollback_();86 }87 88private:89 _Rollback __rollback_;90 bool __completed_;91};92 93_LIBCPP_CTAD_SUPPORTED_FOR_TYPE(__exception_guard_exceptions);94 95template <class _Rollback>96struct __exception_guard_noexceptions {97 __exception_guard_noexceptions() = delete;98 _LIBCPP_NODEBUG _LIBCPP_HIDE_FROM_ABI99 _LIBCPP_CONSTEXPR_SINCE_CXX20 explicit __exception_guard_noexceptions(_Rollback) {}100 101 _LIBCPP_NODEBUG _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20102 __exception_guard_noexceptions(__exception_guard_noexceptions&& __other)103 _NOEXCEPT_(is_nothrow_move_constructible<_Rollback>::value)104 : __completed_(__other.__completed_) {105 __other.__completed_ = true;106 }107 108 __exception_guard_noexceptions(__exception_guard_noexceptions const&) = delete;109 __exception_guard_noexceptions& operator=(__exception_guard_noexceptions const&) = delete;110 __exception_guard_noexceptions& operator=(__exception_guard_noexceptions&&) = delete;111 112 _LIBCPP_NODEBUG _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 void __complete() _NOEXCEPT {113 __completed_ = true;114 }115 116 _LIBCPP_NODEBUG _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 ~__exception_guard_noexceptions() {117 _LIBCPP_ASSERT_INTERNAL(__completed_, "__exception_guard not completed with exceptions disabled");118 }119 120private:121 bool __completed_ = false;122};123 124_LIBCPP_CTAD_SUPPORTED_FOR_TYPE(__exception_guard_noexceptions);125 126#if !_LIBCPP_HAS_EXCEPTIONS127template <class _Rollback>128using __exception_guard _LIBCPP_NODEBUG = __exception_guard_noexceptions<_Rollback>;129#else130template <class _Rollback>131using __exception_guard _LIBCPP_NODEBUG = __exception_guard_exceptions<_Rollback>;132#endif133 134template <class _Rollback>135_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR __exception_guard<_Rollback> __make_exception_guard(_Rollback __rollback) {136 return __exception_guard<_Rollback>(std::move(__rollback));137}138 139_LIBCPP_END_NAMESPACE_STD140 141_LIBCPP_POP_MACROS142 143#endif // _LIBCPP___UTILITY_EXCEPTION_GUARD_H144