1 // Nested Exception support header (nested_exception class) for -*- C++ -*-
2
3 // Copyright (C) 2009-2023 Free Software Foundation, Inc.
4 //
5 // This file is part of the GNU ISO C++ Library. This library is free
6 // software; you can redistribute it and/or modify it under the
7 // terms of the GNU General Public License as published by the
8 // Free Software Foundation; either version 3, or (at your option)
9 // any later version.
10
11 // This library is distributed in the hope that it will be useful,
12 // but WITHOUT ANY WARRANTY; without even the implied warranty of
13 // MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
14 // GNU General Public License for more details.
15
16 // Under Section 7 of GPL version 3, you are granted additional
17 // permissions described in the GCC Runtime Library Exception, version
18 // 3.1, as published by the Free Software Foundation.
19
20 // You should have received a copy of the GNU General Public License and
21 // a copy of the GCC Runtime Library Exception along with this program;
22 // see the files COPYING3 and COPYING.RUNTIME respectively. If not, see
23 // <http://www.gnu.org/licenses/>.
24
25 /** @file bits/nested_exception.h
26 * This is an internal header file, included by other library headers.
27 * Do not attempt to use it directly. @headername{exception}
28 */
29
30 #ifndef _GLIBCXX_NESTED_EXCEPTION_H
31 #define _GLIBCXX_NESTED_EXCEPTION_H 1
32
33 #if __cplusplus < 201103L
34 # include <bits/c++0x_warning.h>
35 #else
36
37 #include <bits/move.h>
38 #include <bits/exception_ptr.h>
39
40 extern "C++" {
41
42 namespace std _GLIBCXX_VISIBILITY(default)
43 {
44 /**
45 * @addtogroup exceptions
46 * @{
47 */
48
49 /** Mixin class that stores the current exception.
50 *
51 * This type can be used via `std::throw_with_nested` to store
52 * the current exception nested within another exception.
53 *
54 * @headerfile exception
55 * @since C++11
56 * @see std::throw_with_nested
57 * @ingroup exceptions
58 */
59 class nested_exception
60 {
61 exception_ptr _M_ptr;
62
63 public:
64 /// The default constructor stores the current exception (if any).
65 nested_exception() noexcept : _M_ptr(current_exception()) { }
66
67 nested_exception(const nested_exception&) noexcept = default;
68
69 nested_exception& operator=(const nested_exception&) noexcept = default;
70
71 virtual ~nested_exception() noexcept;
72
73 /// Rethrow the stored exception, or terminate if none was stored.
74 [[noreturn]]
75 void
76 rethrow_nested() const
77 {
78 if (_M_ptr)
79 rethrow_exception(_M_ptr);
80 std::terminate();
81 }
82
83 /// Access the stored exception.
84 exception_ptr
85 nested_ptr() const noexcept
86 { return _M_ptr; }
87 };
88
89 /// @cond undocumented
90
91 template<typename _Except>
92 struct _Nested_exception : public _Except, public nested_exception
93 {
94 explicit _Nested_exception(const _Except& __ex)
95 : _Except(__ex)
96 { }
97
98 explicit _Nested_exception(_Except&& __ex)
99 : _Except(static_cast<_Except&&>(__ex))
100 { }
101 };
102
103 #if __cplusplus < 201703L || ! defined __cpp_if_constexpr
104 // [except.nested]/8
105 // Throw an exception of unspecified type that is publicly derived from
106 // both remove_reference_t<_Tp> and nested_exception.
107 template<typename _Tp>
108 [[noreturn]]
109 inline void
110 __throw_with_nested_impl(_Tp&& __t, true_type)
111 {
112 throw _Nested_exception<__remove_cvref_t<_Tp>>{std::forward<_Tp>(__t)};
113 }
114
115 template<typename _Tp>
116 [[noreturn]]
117 inline void
118 __throw_with_nested_impl(_Tp&& __t, false_type)
119 { throw std::forward<_Tp>(__t); }
120 #endif
121
122 /// @endcond
123
124 /** Throw an exception that also stores the currently active exception.
125 *
126 * If `_Tp` is derived from `std::nested_exception` or is not usable
127 * as a base-class, throws a copy of `__t`.
128 * Otherwise, throws an object of an implementation-defined type derived
129 * from both `_Tp` and `std::nested_exception`, containing a copy of `__t`
130 * and the result of `std::current_exception()`.
131 *
132 * In other words, throws the argument as a new exception that contains
133 * the currently active exception nested within it. This is intended for
134 * use in a catch handler to replace the caught exception with a different
135 * type, while still preserving the original exception. When the new
136 * exception is caught, the nested exception can be rethrown by using
137 * `std::rethrow_if_nested`.
138 *
139 * This can be used at API boundaries, for example to catch a library's
140 * internal exception type and rethrow it nested with a `std::runtime_error`,
141 * or vice versa.
142 *
143 * @since C++11
144 */
145 template<typename _Tp>
146 [[noreturn]]
147 inline void
148 throw_with_nested(_Tp&& __t)
149 {
150 using _Up = typename decay<_Tp>::type;
151 using _CopyConstructible
152 = __and_<is_copy_constructible<_Up>, is_move_constructible<_Up>>;
153 static_assert(_CopyConstructible::value,
154 "throw_with_nested argument must be CopyConstructible");
155
156 #if __cplusplus >= 201703L && __cpp_if_constexpr
157 if constexpr (is_class_v<_Up>)
158 if constexpr (!is_final_v<_Up>)
159 if constexpr (!is_base_of_v<nested_exception, _Up>)
160 throw _Nested_exception<_Up>{std::forward<_Tp>(__t)};
161 throw std::forward<_Tp>(__t);
162 #else
163 using __nest = __and_<is_class<_Up>, __bool_constant<!__is_final(_Up)>,
164 __not_<is_base_of<nested_exception, _Up>>>;
165 std::__throw_with_nested_impl(std::forward<_Tp>(__t), __nest{});
166 #endif
167 }
168
169 #if __cplusplus < 201703L || ! defined __cpp_if_constexpr
170 /// @cond undocumented
171
172 // Attempt dynamic_cast to nested_exception and call rethrow_nested().
173 template<typename _Ex>
174 inline void
175 __rethrow_if_nested_impl(const _Ex* __ptr, true_type)
176 {
177 if (auto __ne_ptr = dynamic_cast<const nested_exception*>(__ptr))
178 __ne_ptr->rethrow_nested();
179 }
180
181 // Otherwise, no effects.
182 inline void
183 __rethrow_if_nested_impl(const void*, false_type)
184 { }
185
186 /// @endcond
187 #endif
188
189 /** Rethrow a nested exception
190 *
191 * If `__ex` contains a `std::nested_exception` object, call its
192 * `rethrow_nested()` member to rethrow the stored exception.
193 *
194 * After catching an exception thrown by a call to `std::throw_with_nested`
195 * this function can be used to rethrow the exception that was active when
196 * `std::throw_with_nested` was called.
197 *
198 * @since C++11
199 */
200 // _GLIBCXX_RESOLVE_LIB_DEFECTS
201 // 2484. rethrow_if_nested() is doubly unimplementable
202 // 2784. Resolution to LWG 2484 is missing "otherwise, no effects" and [...]
203 template<typename _Ex>
204 # if ! __cpp_rtti
205 [[__gnu__::__always_inline__]]
206 #endif
207 inline void
208 rethrow_if_nested(const _Ex& __ex)
209 {
210 const _Ex* __ptr = __builtin_addressof(__ex);
211 #if __cplusplus < 201703L || ! defined __cpp_if_constexpr
212 # if __cpp_rtti
213 using __cast = __and_<is_polymorphic<_Ex>,
214 __or_<__not_<is_base_of<nested_exception, _Ex>>,
215 is_convertible<_Ex*, nested_exception*>>>;
216 # else
217 using __cast = __and_<is_polymorphic<_Ex>,
218 is_base_of<nested_exception, _Ex>,
219 is_convertible<_Ex*, nested_exception*>>;
220 # endif
221 std::__rethrow_if_nested_impl(__ptr, __cast{});
222 #else
223 if constexpr (!is_polymorphic_v<_Ex>)
224 return;
225 else if constexpr (is_base_of_v<nested_exception, _Ex>
226 && !is_convertible_v<_Ex*, nested_exception*>)
227 return; // nested_exception base class is inaccessible or ambiguous.
228 # if ! __cpp_rtti
229 else if constexpr (!is_base_of_v<nested_exception, _Ex>)
230 return; // Cannot do polymorphic casts without RTTI.
231 # endif
232 else if (auto __ne_ptr = dynamic_cast<const nested_exception*>(__ptr))
233 __ne_ptr->rethrow_nested();
234 #endif
235 }
236
237 /// @} group exceptions
238 } // namespace std
239
240 } // extern "C++"
241
242 #endif // C++11
243 #endif // _GLIBCXX_NESTED_EXCEPTION_H