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___CXX03___UTILITY_TRANSACTION_H
10 #define _LIBCPP___CXX03___UTILITY_TRANSACTION_H
11
12 #include <__cxx03/__assert>
13 #include <__cxx03/__config>
14 #include <__cxx03/__type_traits/is_nothrow_constructible.h>
15 #include <__cxx03/__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 <__cxx03/__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_NO_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
63 template <class _Rollback>
64 struct __exception_guard_exceptions {
65 __exception_guard_exceptions() = delete;
66
__exception_guard_exceptions__exception_guard_exceptions67 _LIBCPP_HIDE_FROM_ABI explicit __exception_guard_exceptions(_Rollback __rollback)
68 : __rollback_(std::move(__rollback)), __completed_(false) {}
69
__exception_guard_exceptions__exception_guard_exceptions70 _LIBCPP_HIDE_FROM_ABI __exception_guard_exceptions(__exception_guard_exceptions&& __other)
71 : __rollback_(std::move(__other.__rollback_)), __completed_(__other.__completed_) {
72 __other.__completed_ = true;
73 }
74
75 __exception_guard_exceptions(__exception_guard_exceptions const&) = delete;
76 __exception_guard_exceptions& operator=(__exception_guard_exceptions const&) = delete;
77 __exception_guard_exceptions& operator=(__exception_guard_exceptions&&) = delete;
78
__complete__exception_guard_exceptions79 _LIBCPP_HIDE_FROM_ABI void __complete() _NOEXCEPT { __completed_ = true; }
80
~__exception_guard_exceptions__exception_guard_exceptions81 _LIBCPP_HIDE_FROM_ABI ~__exception_guard_exceptions() {
82 if (!__completed_)
83 __rollback_();
84 }
85
86 private:
87 _Rollback __rollback_;
88 bool __completed_;
89 };
90
91 _LIBCPP_CTAD_SUPPORTED_FOR_TYPE(__exception_guard_exceptions);
92
93 template <class _Rollback>
94 struct __exception_guard_noexceptions {
95 __exception_guard_noexceptions() = delete;
__exception_guard_noexceptions__exception_guard_noexceptions96 _LIBCPP_HIDE_FROM_ABI _LIBCPP_NODEBUG explicit __exception_guard_noexceptions(_Rollback) {}
97
__exception_guard_noexceptions__exception_guard_noexceptions98 _LIBCPP_HIDE_FROM_ABI _LIBCPP_NODEBUG __exception_guard_noexceptions(__exception_guard_noexceptions&& __other)
99 : __completed_(__other.__completed_) {
100 __other.__completed_ = true;
101 }
102
103 __exception_guard_noexceptions(__exception_guard_noexceptions const&) = delete;
104 __exception_guard_noexceptions& operator=(__exception_guard_noexceptions const&) = delete;
105 __exception_guard_noexceptions& operator=(__exception_guard_noexceptions&&) = delete;
106
__complete__exception_guard_noexceptions107 _LIBCPP_HIDE_FROM_ABI _LIBCPP_NODEBUG void __complete() _NOEXCEPT { __completed_ = true; }
108
~__exception_guard_noexceptions__exception_guard_noexceptions109 _LIBCPP_HIDE_FROM_ABI _LIBCPP_NODEBUG ~__exception_guard_noexceptions() {
110 _LIBCPP_ASSERT_INTERNAL(__completed_, "__exception_guard not completed with exceptions disabled");
111 }
112
113 private:
114 bool __completed_ = false;
115 };
116
117 _LIBCPP_CTAD_SUPPORTED_FOR_TYPE(__exception_guard_noexceptions);
118
119 #ifdef _LIBCPP_HAS_NO_EXCEPTIONS
120 template <class _Rollback>
121 using __exception_guard = __exception_guard_noexceptions<_Rollback>;
122 #else
123 template <class _Rollback>
124 using __exception_guard = __exception_guard_exceptions<_Rollback>;
125 #endif
126
127 template <class _Rollback>
__make_exception_guard(_Rollback __rollback)128 _LIBCPP_HIDE_FROM_ABI __exception_guard<_Rollback> __make_exception_guard(_Rollback __rollback) {
129 return __exception_guard<_Rollback>(std::move(__rollback));
130 }
131
132 _LIBCPP_END_NAMESPACE_STD
133
134 _LIBCPP_POP_MACROS
135
136 #endif // _LIBCPP___CXX03___UTILITY_TRANSACTION_H
137