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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_TRANSACTION_H
10 : #define _LIBCPP___UTILITY_TRANSACTION_H
11 :
12 : #include <__assert>
13 : #include <__config>
14 : #include <__type_traits/is_nothrow_move_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_BEGIN_NAMESPACE_STD
23 :
24 : // __exception_guard is a helper class for writing code with the strong exception guarantee.
25 : //
26 : // When writing code that can throw an exception, one can store rollback instructions in an
27 : // exception guard so that if an exception is thrown at any point during the lifetime of the
28 : // exception guard, it will be rolled back automatically. When the exception guard is done, one
29 : // must mark it as being complete so it isn't rolled back when the exception guard is destroyed.
30 : //
31 : // Exception guards are not default constructible, they can't be copied or assigned to, but
32 : // they can be moved around for convenience.
33 : //
34 : // __exception_guard is a no-op in -fno-exceptions mode to produce better code-gen. This means
35 : // that we don't provide the strong exception guarantees. However, Clang doesn't generate cleanup
36 : // code with exceptions disabled, so even if we wanted to provide the strong exception guarantees
37 : // we couldn't. This is also only relevant for constructs with a stack of
38 : // -fexceptions > -fno-exceptions > -fexceptions code, since the exception can't be caught where
39 : // exceptions are disabled. While -fexceptions > -fno-exceptions is quite common
40 : // (e.g. libc++.dylib > -fno-exceptions), having another layer with exceptions enabled seems a lot
41 : // less common, especially one that tries to catch an exception through -fno-exceptions code.
42 : //
43 : // __exception_guard can help greatly simplify code that would normally be cluttered by
44 : // `#if _LIBCPP_NO_EXCEPTIONS`. For example:
45 : //
46 : // template <class Iterator, class Size, class OutputIterator>
47 : // Iterator uninitialized_copy_n(Iterator iter, Size n, OutputIterator out) {
48 : // typedef typename iterator_traits<Iterator>::value_type value_type;
49 : // __exception_guard guard([start=out, &out] {
50 : // std::destroy(start, out);
51 : // });
52 : //
53 : // for (; n > 0; ++iter, ++out, --n) {
54 : // ::new ((void*)std::addressof(*out)) value_type(*iter);
55 : // }
56 : // guard.__complete();
57 : // return out;
58 : // }
59 : //
60 :
61 : #ifndef _LIBCPP_NO_EXCEPTIONS
62 : template <class _Rollback>
63 : struct __exception_guard_exceptions {
64 : __exception_guard_exceptions() = delete;
65 :
66 30 : _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 explicit __exception_guard_exceptions(_Rollback __rollback)
67 30 : : __rollback_(std::move(__rollback)), __completed_(false) {}
68 :
69 : _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20
70 : __exception_guard_exceptions(__exception_guard_exceptions&& __other)
71 : _NOEXCEPT_(is_nothrow_move_constructible<_Rollback>::value)
72 : : __rollback_(std::move(__other.__rollback_)), __completed_(__other.__completed_) {
73 : __other.__completed_ = true;
74 : }
75 :
76 : __exception_guard_exceptions(__exception_guard_exceptions const&) = delete;
77 : __exception_guard_exceptions& operator=(__exception_guard_exceptions const&) = delete;
78 : __exception_guard_exceptions& operator=(__exception_guard_exceptions&&) = delete;
79 :
80 15 : _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 void __complete() _NOEXCEPT { __completed_ = true; }
81 :
82 30 : _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 ~__exception_guard_exceptions() {
83 15 : if (!__completed_)
84 0 : __rollback_();
85 30 : }
86 :
87 : private:
88 : _Rollback __rollback_;
89 : bool __completed_;
90 : };
91 :
92 : _LIBCPP_CTAD_SUPPORTED_FOR_TYPE(__exception_guard_exceptions);
93 :
94 : template <class _Rollback>
95 : using __exception_guard = __exception_guard_exceptions<_Rollback>;
96 : #else // _LIBCPP_NO_EXCEPTIONS
97 : template <class _Rollback>
98 : struct __exception_guard_noexceptions {
99 : __exception_guard_noexceptions() = delete;
100 : _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20
101 : _LIBCPP_NODEBUG explicit __exception_guard_noexceptions(_Rollback) {}
102 :
103 : _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 _LIBCPP_NODEBUG
104 : __exception_guard_noexceptions(__exception_guard_noexceptions&& __other)
105 : _NOEXCEPT_(is_nothrow_move_constructible<_Rollback>::value)
106 : : __completed_(__other.__completed_) {
107 : __other.__completed_ = true;
108 : }
109 :
110 : __exception_guard_noexceptions(__exception_guard_noexceptions const&) = delete;
111 : __exception_guard_noexceptions& operator=(__exception_guard_noexceptions const&) = delete;
112 : __exception_guard_noexceptions& operator=(__exception_guard_noexceptions&&) = delete;
113 :
114 : _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 _LIBCPP_NODEBUG void __complete() _NOEXCEPT {
115 : __completed_ = true;
116 : }
117 :
118 : _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 _LIBCPP_NODEBUG ~__exception_guard_noexceptions() {
119 : _LIBCPP_ASSERT(__completed_, "__exception_guard not completed with exceptions disabled");
120 : }
121 :
122 : private:
123 : bool __completed_ = false;
124 : };
125 :
126 : _LIBCPP_CTAD_SUPPORTED_FOR_TYPE(__exception_guard_noexceptions);
127 :
128 : template <class _Rollback>
129 : using __exception_guard = __exception_guard_noexceptions<_Rollback>;
130 : #endif // _LIBCPP_NO_EXCEPTIONS
131 :
132 : template <class _Rollback>
133 15 : _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR __exception_guard<_Rollback> __make_exception_guard(_Rollback __rollback) {
134 15 : return __exception_guard<_Rollback>(std::move(__rollback));
135 : }
136 :
137 : _LIBCPP_END_NAMESPACE_STD
138 :
139 : #endif // _LIBCPP___UTILITY_TRANSACTION_H
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