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//===----------------------------------------------------------------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
#ifndef _LIBCPP___UTILITY_TRANSACTION_H
#define _LIBCPP___UTILITY_TRANSACTION_H
#include <__assert>
#include <__config>
#include <__type_traits/is_nothrow_move_constructible.h>
#include <__utility/exchange.h>
#include <__utility/move.h>
#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
# pragma GCC system_header
#endif
_LIBCPP_BEGIN_NAMESPACE_STD
// __exception_guard is a helper class for writing code with the strong exception guarantee.
//
// When writing code that can throw an exception, one can store rollback instructions in an
// exception guard so that if an exception is thrown at any point during the lifetime of the
// exception guard, it will be rolled back automatically. When the exception guard is done, one
// must mark it as being complete so it isn't rolled back when the exception guard is destroyed.
//
// Exception guards are not default constructible, they can't be copied or assigned to, but
// they can be moved around for convenience.
//
// __exception_guard is a no-op in -fno-exceptions mode to produce better code-gen. This means
// that we don't provide the strong exception guarantees. However, Clang doesn't generate cleanup
// code with exceptions disabled, so even if we wanted to provide the strong exception guarantees
// we couldn't. This is also only relevant for constructs with a stack of
// -fexceptions > -fno-exceptions > -fexceptions code, since the exception can't be caught where
// exceptions are disabled. While -fexceptions > -fno-exceptions is quite common
// (e.g. libc++.dylib > -fno-exceptions), having another layer with exceptions enabled seems a lot
// less common, especially one that tries to catch an exception through -fno-exceptions code.
//
// __exception_guard can help greatly simplify code that would normally be cluttered by
// `#if _LIBCPP_NO_EXCEPTIONS`. For example:
//
// template <class Iterator, class Size, class OutputIterator>
// Iterator uninitialized_copy_n(Iterator iter, Size n, OutputIterator out) {
// typedef typename iterator_traits<Iterator>::value_type value_type;
// __exception_guard guard([start=out, &out] {
// std::destroy(start, out);
// });
//
// for (; n > 0; ++iter, ++out, --n) {
// ::new ((void*)std::addressof(*out)) value_type(*iter);
// }
// guard.__complete();
// return out;
// }
//
#ifndef _LIBCPP_NO_EXCEPTIONS
template <class _Rollback>
struct __exception_guard_exceptions {
__exception_guard_exceptions() = delete;
_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 explicit __exception_guard_exceptions(_Rollback __rollback)
: __rollback_(std::move(__rollback)), __completed_(false) {}
_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20
__exception_guard_exceptions(__exception_guard_exceptions&& __other)
_NOEXCEPT_(is_nothrow_move_constructible<_Rollback>::value)
: __rollback_(std::move(__other.__rollback_)), __completed_(__other.__completed_) {
__other.__completed_ = true;
}
__exception_guard_exceptions(__exception_guard_exceptions const&) = delete;
__exception_guard_exceptions& operator=(__exception_guard_exceptions const&) = delete;
__exception_guard_exceptions& operator=(__exception_guard_exceptions&&) = delete;
_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 void __complete() _NOEXCEPT { __completed_ = true; }
_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 ~__exception_guard_exceptions() {
if (!__completed_)
__rollback_();
}
private:
_Rollback __rollback_;
bool __completed_;
};
_LIBCPP_CTAD_SUPPORTED_FOR_TYPE(__exception_guard_exceptions);
template <class _Rollback>
using __exception_guard = __exception_guard_exceptions<_Rollback>;
#else // _LIBCPP_NO_EXCEPTIONS
template <class _Rollback>
struct __exception_guard_noexceptions {
__exception_guard_noexceptions() = delete;
_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20
_LIBCPP_NODEBUG explicit __exception_guard_noexceptions(_Rollback) {}
_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 _LIBCPP_NODEBUG
__exception_guard_noexceptions(__exception_guard_noexceptions&& __other)
_NOEXCEPT_(is_nothrow_move_constructible<_Rollback>::value)
: __completed_(__other.__completed_) {
__other.__completed_ = true;
}
__exception_guard_noexceptions(__exception_guard_noexceptions const&) = delete;
__exception_guard_noexceptions& operator=(__exception_guard_noexceptions const&) = delete;
__exception_guard_noexceptions& operator=(__exception_guard_noexceptions&&) = delete;
_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 _LIBCPP_NODEBUG void __complete() _NOEXCEPT {
__completed_ = true;
}
_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 _LIBCPP_NODEBUG ~__exception_guard_noexceptions() {
_LIBCPP_ASSERT(__completed_, "__exception_guard not completed with exceptions disabled");
}
private:
bool __completed_ = false;
};
_LIBCPP_CTAD_SUPPORTED_FOR_TYPE(__exception_guard_noexceptions);
template <class _Rollback>
using __exception_guard = __exception_guard_noexceptions<_Rollback>;
#endif // _LIBCPP_NO_EXCEPTIONS
template <class _Rollback>
_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR __exception_guard<_Rollback> __make_exception_guard(_Rollback __rollback) {
return __exception_guard<_Rollback>(std::move(__rollback));
}
_LIBCPP_END_NAMESPACE_STD
#endif // _LIBCPP___UTILITY_TRANSACTION_H
|
