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_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
64template <class _Rollback>
65struct __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
89private:
90 _Rollback __rollback_;
91 bool __completed_;
92};
93
94_LIBCPP_CTAD_SUPPORTED_FOR_TYPE(__exception_guard_exceptions);
95
96template <class _Rollback>
97struct __exception_guard_noexceptions {
98 __exception_guard_noexceptions() = delete;
99 _LIBCPP_HIDE_FROM_ABI
100 _LIBCPP_CONSTEXPR_SINCE_CXX20 _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_INTERNAL(__completed_, "__exception_guard not completed with exceptions disabled");
119 }
120
121private:
122 bool __completed_ = false;
123};
124
125_LIBCPP_CTAD_SUPPORTED_FOR_TYPE(__exception_guard_noexceptions);
126
127#ifdef _LIBCPP_HAS_NO_EXCEPTIONS
128template <class _Rollback>
129using __exception_guard = __exception_guard_noexceptions<_Rollback>;
130#else
131template <class _Rollback>
132using __exception_guard = __exception_guard_exceptions<_Rollback>;
133#endif
134
135template <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