forked from OSchip/llvm-project
92 lines
3.0 KiB
C
92 lines
3.0 KiB
C
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//===----------------------------------------------------------------------===//
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//
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// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
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// See https://llvm.org/LICENSE.txt for license information.
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// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
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//
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//===----------------------------------------------------------------------===//
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#ifndef _LIBCPP___UTILITY_TRANSACTION_H
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#define _LIBCPP___UTILITY_TRANSACTION_H
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#include <__config>
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#include <__utility/exchange.h>
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#include <__utility/move.h>
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#include <type_traits>
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#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
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#pragma GCC system_header
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#endif
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_LIBCPP_BEGIN_NAMESPACE_STD
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// __transaction is a helper class for writing code with the strong exception guarantee.
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//
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// When writing code that can throw an exception, one can store rollback instructions in a
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// transaction so that if an exception is thrown at any point during the lifetime of the
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// transaction, it will be rolled back automatically. When the transaction is done, one
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// must mark it as being complete so it isn't rolled back when the transaction is destroyed.
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//
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// Transactions are not default constructible, they can't be copied or assigned to, but
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// they can be moved around for convenience.
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//
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// __transaction can help greatly simplify code that would normally be cluttered by
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// `#if _LIBCPP_NO_EXCEPTIONS`. For example:
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//
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// template <class Iterator, class Size, class OutputIterator>
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// Iterator uninitialized_copy_n(Iterator iter, Size n, OutputIterator out) {
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// typedef typename iterator_traits<Iterator>::value_type value_type;
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// __transaction transaction([start=out, &out] {
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// std::destroy(start, out);
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// });
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//
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// for (; n > 0; ++iter, ++out, --n) {
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// ::new ((void*)std::addressof(*out)) value_type(*iter);
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// }
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// transaction.__complete();
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// return out;
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// }
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//
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template <class _Rollback>
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struct __transaction {
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__transaction() = delete;
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_LIBCPP_HIDE_FROM_ABI
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_LIBCPP_CONSTEXPR_AFTER_CXX17 explicit __transaction(_Rollback __rollback)
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: __rollback_(_VSTD::move(__rollback))
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, __completed_(false)
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{ }
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_LIBCPP_HIDE_FROM_ABI
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_LIBCPP_CONSTEXPR_AFTER_CXX17 __transaction(__transaction&& __other)
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_NOEXCEPT_(is_nothrow_move_constructible<_Rollback>::value)
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: __rollback_(_VSTD::move(__other.__rollback_))
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, __completed_(__other.__completed_)
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{
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__other.__completed_ = true;
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}
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__transaction(__transaction const&) = delete;
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__transaction& operator=(__transaction const&) = delete;
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__transaction& operator=(__transaction&&) = delete;
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_LIBCPP_HIDE_FROM_ABI
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_LIBCPP_CONSTEXPR_AFTER_CXX17 void __complete() _NOEXCEPT {
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__completed_ = true;
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}
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_LIBCPP_HIDE_FROM_ABI
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_LIBCPP_CONSTEXPR_AFTER_CXX17 ~__transaction() {
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if (!__completed_)
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__rollback_();
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}
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private:
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_Rollback __rollback_;
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bool __completed_;
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};
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_LIBCPP_END_NAMESPACE_STD
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#endif // _LIBCPP___UTILITY_TRANSACTION_H
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