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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-exception
6//
7//===----------------------------------------------------------------------===//
8
9#ifndef _LIBCPP___UTILITY_EXCEPTION_GUARD_H
10#define _LIBCPP___UTILITY_EXCEPTION_GUARD_H
11
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_header
19#endif
20
21_LIBCPP_PUSH_MACROS
22#include <__undef_macros>
23
24_LIBCPP_BEGIN_NAMESPACE_STD
25
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 an
29// exception guard so that if an exception is thrown at any point during the lifetime of the
30// exception guard, it will be rolled back automatically. When the exception guard is done, one
31// 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, but
34// 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 means
37// that we don't provide the strong exception guarantees. However, Clang doesn't generate cleanup
38// code with exceptions disabled, so even if we wanted to provide the strong exception guarantees
39// we couldn't. This is also only relevant for constructs with a stack of
40// -fexceptions > -fno-exceptions > -fexceptions code, since the exception can't be caught where
41// exceptions are disabled. While -fexceptions > -fno-exceptions is quite common
42// (e.g. libc++.dylib > -fno-exceptions), having another layer with exceptions enabled seems a lot
43// 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 by
46// `#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_CXX20
71 __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_ABI
99 _LIBCPP_CONSTEXPR_SINCE_CXX20 explicit __exception_guard_noexceptions(_Rollback) {}
100
101 _LIBCPP_NODEBUG _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20
102 __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_EXCEPTIONS
127template <class _Rollback>
128using __exception_guard _LIBCPP_NODEBUG = __exception_guard_noexceptions<_Rollback>;
129#else
130template <class _Rollback>
131using __exception_guard _LIBCPP_NODEBUG = __exception_guard_exceptions<_Rollback>;
132#endif
133
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_STD
140
141_LIBCPP_POP_MACROS
142
143#endif // _LIBCPP___UTILITY_EXCEPTION_GUARD_H