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_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
64 template <class _Rollback>
65 struct __exception_guard_exceptions {
66 __exception_guard_exceptions() = delete;
67
__exception_guard_exceptions__exception_guard_exceptions68 _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
__exception_guard_exceptions__exception_guard_exceptions72 __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
__complete__exception_guard_exceptions82 _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 void __complete() _NOEXCEPT { __completed_ = true; }
83
~__exception_guard_exceptions__exception_guard_exceptions84 _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 ~__exception_guard_exceptions() {
85 if (!__completed_)
86 __rollback_();
87 }
88
89 private:
90 _Rollback __rollback_;
91 bool __completed_;
92 };
93
94 _LIBCPP_CTAD_SUPPORTED_FOR_TYPE(__exception_guard_exceptions);
95
96 template <class _Rollback>
97 struct __exception_guard_noexceptions {
98 __exception_guard_noexceptions() = delete;
99 _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20
__exception_guard_noexceptions__exception_guard_noexceptions100 _LIBCPP_NODEBUG explicit __exception_guard_noexceptions(_Rollback) {}
101
102 _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 _LIBCPP_NODEBUG
__exception_guard_noexceptions__exception_guard_noexceptions103 __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
__complete__exception_guard_noexceptions113 _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 _LIBCPP_NODEBUG void __complete() _NOEXCEPT {
114 __completed_ = true;
115 }
116
~__exception_guard_noexceptions__exception_guard_noexceptions117 _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
121 private:
122 bool __completed_ = false;
123 };
124
125 _LIBCPP_CTAD_SUPPORTED_FOR_TYPE(__exception_guard_noexceptions);
126
127 #ifdef _LIBCPP_HAS_NO_EXCEPTIONS
128 template <class _Rollback>
129 using __exception_guard = __exception_guard_noexceptions<_Rollback>;
130 #else
131 template <class _Rollback>
132 using __exception_guard = __exception_guard_exceptions<_Rollback>;
133 #endif
134
135 template <class _Rollback>
__make_exception_guard(_Rollback __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