1 // Implementation of std::reference_wrapper -*- C++ -*-
2
3 // Copyright (C) 2004-2023 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 include/bits/refwrap.h
26 * This is an internal header file, included by other library headers.
27 * Do not attempt to use it directly. @headername{functional}
28 */
29
30 #ifndef _GLIBCXX_REFWRAP_H
31 #define _GLIBCXX_REFWRAP_H 1
32
33 #pragma GCC system_header
34
35 #if __cplusplus >= 201103L
36
37 #include <bits/move.h>
38 #include <bits/invoke.h>
39 #include <bits/stl_function.h> // for unary_function and binary_function
40
41 namespace std _GLIBCXX_VISIBILITY(default)
42 {
43 _GLIBCXX_BEGIN_NAMESPACE_VERSION
44
45 /// @cond undocumented
46
47 /**
48 * Derives from @c unary_function or @c binary_function, or perhaps
49 * nothing, depending on the number of arguments provided. The
50 * primary template is the basis case, which derives nothing.
51 */
52 template<typename _Res, typename... _ArgTypes>
53 struct _Maybe_unary_or_binary_function { };
54
55 // Ignore warnings about unary_function and binary_function.
56 #pragma GCC diagnostic push
57 #pragma GCC diagnostic ignored "-Wdeprecated-declarations"
58
59 /// Derives from @c unary_function, as appropriate.
60 template<typename _Res, typename _T1>
61 struct _Maybe_unary_or_binary_function<_Res, _T1>
62 : std::unary_function<_T1, _Res> { };
63
64 /// Derives from @c binary_function, as appropriate.
65 template<typename _Res, typename _T1, typename _T2>
66 struct _Maybe_unary_or_binary_function<_Res, _T1, _T2>
67 : std::binary_function<_T1, _T2, _Res> { };
68
69 #pragma GCC diagnostic pop
70
71 template<typename _Signature>
72 struct _Mem_fn_traits;
73
74 template<typename _Res, typename _Class, typename... _ArgTypes>
75 struct _Mem_fn_traits_base
76 {
77 using __result_type = _Res;
78 using __maybe_type
79 = _Maybe_unary_or_binary_function<_Res, _Class*, _ArgTypes...>;
80 using __arity = integral_constant<size_t, sizeof...(_ArgTypes)>;
81 };
82
83 #define _GLIBCXX_MEM_FN_TRAITS2(_CV, _REF, _LVAL, _RVAL) \
84 template<typename _Res, typename _Class, typename... _ArgTypes> \
85 struct _Mem_fn_traits<_Res (_Class::*)(_ArgTypes...) _CV _REF> \
86 : _Mem_fn_traits_base<_Res, _CV _Class, _ArgTypes...> \
87 { \
88 using __vararg = false_type; \
89 }; \
90 template<typename _Res, typename _Class, typename... _ArgTypes> \
91 struct _Mem_fn_traits<_Res (_Class::*)(_ArgTypes... ...) _CV _REF> \
92 : _Mem_fn_traits_base<_Res, _CV _Class, _ArgTypes...> \
93 { \
94 using __vararg = true_type; \
95 };
96
97 #define _GLIBCXX_MEM_FN_TRAITS(_REF, _LVAL, _RVAL) \
98 _GLIBCXX_MEM_FN_TRAITS2( , _REF, _LVAL, _RVAL) \
99 _GLIBCXX_MEM_FN_TRAITS2(const , _REF, _LVAL, _RVAL) \
100 _GLIBCXX_MEM_FN_TRAITS2(volatile , _REF, _LVAL, _RVAL) \
101 _GLIBCXX_MEM_FN_TRAITS2(const volatile, _REF, _LVAL, _RVAL)
102
103 _GLIBCXX_MEM_FN_TRAITS( , true_type, true_type)
104 _GLIBCXX_MEM_FN_TRAITS(&, true_type, false_type)
105 _GLIBCXX_MEM_FN_TRAITS(&&, false_type, true_type)
106
107 #if __cplusplus > 201402L
108 _GLIBCXX_MEM_FN_TRAITS(noexcept, true_type, true_type)
109 _GLIBCXX_MEM_FN_TRAITS(& noexcept, true_type, false_type)
110 _GLIBCXX_MEM_FN_TRAITS(&& noexcept, false_type, true_type)
111 #endif
112
113 #undef _GLIBCXX_MEM_FN_TRAITS
114 #undef _GLIBCXX_MEM_FN_TRAITS2
115
116 /// If we have found a result_type, extract it.
117 template<typename _Functor, typename = __void_t<>>
118 struct _Maybe_get_result_type
119 { };
120
121 template<typename _Functor>
122 struct _Maybe_get_result_type<_Functor,
123 __void_t<typename _Functor::result_type>>
124 { typedef typename _Functor::result_type result_type; };
125
126 /**
127 * Base class for any function object that has a weak result type, as
128 * defined in 20.8.2 [func.require] of C++11.
129 */
130 template<typename _Functor>
131 struct _Weak_result_type_impl
132 : _Maybe_get_result_type<_Functor>
133 { };
134
135 /// Retrieve the result type for a function type.
136 template<typename _Res, typename... _ArgTypes _GLIBCXX_NOEXCEPT_PARM>
137 struct _Weak_result_type_impl<_Res(_ArgTypes...) _GLIBCXX_NOEXCEPT_QUAL>
138 { typedef _Res result_type; };
139
140 /// Retrieve the result type for a varargs function type.
141 template<typename _Res, typename... _ArgTypes _GLIBCXX_NOEXCEPT_PARM>
142 struct _Weak_result_type_impl<_Res(_ArgTypes......) _GLIBCXX_NOEXCEPT_QUAL>
143 { typedef _Res result_type; };
144
145 /// Retrieve the result type for a function pointer.
146 template<typename _Res, typename... _ArgTypes _GLIBCXX_NOEXCEPT_PARM>
147 struct _Weak_result_type_impl<_Res(*)(_ArgTypes...) _GLIBCXX_NOEXCEPT_QUAL>
148 { typedef _Res result_type; };
149
150 /// Retrieve the result type for a varargs function pointer.
151 template<typename _Res, typename... _ArgTypes _GLIBCXX_NOEXCEPT_PARM>
152 struct
153 _Weak_result_type_impl<_Res(*)(_ArgTypes......) _GLIBCXX_NOEXCEPT_QUAL>
154 { typedef _Res result_type; };
155
156 // Let _Weak_result_type_impl perform the real work.
157 template<typename _Functor,
158 bool = is_member_function_pointer<_Functor>::value>
159 struct _Weak_result_type_memfun
160 : _Weak_result_type_impl<_Functor>
161 { };
162
163 // A pointer to member function has a weak result type.
164 template<typename _MemFunPtr>
165 struct _Weak_result_type_memfun<_MemFunPtr, true>
166 {
167 using result_type = typename _Mem_fn_traits<_MemFunPtr>::__result_type;
168 };
169
170 // A pointer to data member doesn't have a weak result type.
171 template<typename _Func, typename _Class>
172 struct _Weak_result_type_memfun<_Func _Class::*, false>
173 { };
174
175 /**
176 * Strip top-level cv-qualifiers from the function object and let
177 * _Weak_result_type_memfun perform the real work.
178 */
179 template<typename _Functor>
180 struct _Weak_result_type
181 : _Weak_result_type_memfun<typename remove_cv<_Functor>::type>
182 { };
183
184 #if __cplusplus <= 201703L
185 // Detect nested argument_type.
186 template<typename _Tp, typename = __void_t<>>
187 struct _Refwrap_base_arg1
188 { };
189
190 // Nested argument_type.
191 template<typename _Tp>
192 struct _Refwrap_base_arg1<_Tp,
193 __void_t<typename _Tp::argument_type>>
194 {
195 typedef typename _Tp::argument_type argument_type;
196 };
197
198 // Detect nested first_argument_type and second_argument_type.
199 template<typename _Tp, typename = __void_t<>>
200 struct _Refwrap_base_arg2
201 { };
202
203 // Nested first_argument_type and second_argument_type.
204 template<typename _Tp>
205 struct _Refwrap_base_arg2<_Tp,
206 __void_t<typename _Tp::first_argument_type,
207 typename _Tp::second_argument_type>>
208 {
209 typedef typename _Tp::first_argument_type first_argument_type;
210 typedef typename _Tp::second_argument_type second_argument_type;
211 };
212
213 /**
214 * Derives from unary_function or binary_function when it
215 * can. Specializations handle all of the easy cases. The primary
216 * template determines what to do with a class type, which may
217 * derive from both unary_function and binary_function.
218 */
219 template<typename _Tp>
220 struct _Reference_wrapper_base
221 : _Weak_result_type<_Tp>, _Refwrap_base_arg1<_Tp>, _Refwrap_base_arg2<_Tp>
222 { };
223
224 // Ignore warnings about unary_function and binary_function.
225 #pragma GCC diagnostic push
226 #pragma GCC diagnostic ignored "-Wdeprecated-declarations"
227
228 // - a function type (unary)
229 template<typename _Res, typename _T1 _GLIBCXX_NOEXCEPT_PARM>
230 struct _Reference_wrapper_base<_Res(_T1) _GLIBCXX_NOEXCEPT_QUAL>
231 : unary_function<_T1, _Res>
232 { };
233
234 template<typename _Res, typename _T1>
235 struct _Reference_wrapper_base<_Res(_T1) const>
236 : unary_function<_T1, _Res>
237 { };
238
239 template<typename _Res, typename _T1>
240 struct _Reference_wrapper_base<_Res(_T1) volatile>
241 : unary_function<_T1, _Res>
242 { };
243
244 template<typename _Res, typename _T1>
245 struct _Reference_wrapper_base<_Res(_T1) const volatile>
246 : unary_function<_T1, _Res>
247 { };
248
249 // - a function type (binary)
250 template<typename _Res, typename _T1, typename _T2 _GLIBCXX_NOEXCEPT_PARM>
251 struct _Reference_wrapper_base<_Res(_T1, _T2) _GLIBCXX_NOEXCEPT_QUAL>
252 : binary_function<_T1, _T2, _Res>
253 { };
254
255 template<typename _Res, typename _T1, typename _T2>
256 struct _Reference_wrapper_base<_Res(_T1, _T2) const>
257 : binary_function<_T1, _T2, _Res>
258 { };
259
260 template<typename _Res, typename _T1, typename _T2>
261 struct _Reference_wrapper_base<_Res(_T1, _T2) volatile>
262 : binary_function<_T1, _T2, _Res>
263 { };
264
265 template<typename _Res, typename _T1, typename _T2>
266 struct _Reference_wrapper_base<_Res(_T1, _T2) const volatile>
267 : binary_function<_T1, _T2, _Res>
268 { };
269
270 // - a function pointer type (unary)
271 template<typename _Res, typename _T1 _GLIBCXX_NOEXCEPT_PARM>
272 struct _Reference_wrapper_base<_Res(*)(_T1) _GLIBCXX_NOEXCEPT_QUAL>
273 : unary_function<_T1, _Res>
274 { };
275
276 // - a function pointer type (binary)
277 template<typename _Res, typename _T1, typename _T2 _GLIBCXX_NOEXCEPT_PARM>
278 struct _Reference_wrapper_base<_Res(*)(_T1, _T2) _GLIBCXX_NOEXCEPT_QUAL>
279 : binary_function<_T1, _T2, _Res>
280 { };
281
282 template<typename _Tp, bool = is_member_function_pointer<_Tp>::value>
283 struct _Reference_wrapper_base_memfun
284 : _Reference_wrapper_base<_Tp>
285 { };
286
287 template<typename _MemFunPtr>
288 struct _Reference_wrapper_base_memfun<_MemFunPtr, true>
289 : _Mem_fn_traits<_MemFunPtr>::__maybe_type
290 {
291 using result_type = typename _Mem_fn_traits<_MemFunPtr>::__result_type;
292 };
293 #pragma GCC diagnostic pop
294 #endif // ! C++20
295
296 /// @endcond
297
298 /**
299 * @brief Primary class template for reference_wrapper.
300 * @ingroup functors
301 */
302 template<typename _Tp>
303 class reference_wrapper
304 #if __cplusplus <= 201703L
305 // In C++20 std::reference_wrapper<T> allows T to be incomplete,
306 // so checking for nested types could result in ODR violations.
307 : public _Reference_wrapper_base_memfun<typename remove_cv<_Tp>::type>
308 #endif
309 {
310 _Tp* _M_data;
311
312 _GLIBCXX20_CONSTEXPR
313 static _Tp* _S_fun(_Tp& __r) noexcept { return std::__addressof(__r); }
314
315 static void _S_fun(_Tp&&) = delete;
316
317 template<typename _Up, typename _Up2 = __remove_cvref_t<_Up>>
318 using __not_same
319 = typename enable_if<!is_same<reference_wrapper, _Up2>::value>::type;
320
321 public:
322 typedef _Tp type;
323
324 // _GLIBCXX_RESOLVE_LIB_DEFECTS
325 // 2993. reference_wrapper<T> conversion from T&&
326 // 3041. Unnecessary decay in reference_wrapper
327 template<typename _Up, typename = __not_same<_Up>, typename
328 = decltype(reference_wrapper::_S_fun(std::declval<_Up>()))>
329 _GLIBCXX20_CONSTEXPR
330 reference_wrapper(_Up&& __uref)
331 noexcept(noexcept(reference_wrapper::_S_fun(std::declval<_Up>())))
332 : _M_data(reference_wrapper::_S_fun(std::forward<_Up>(__uref)))
333 { }
334
335 reference_wrapper(const reference_wrapper&) = default;
336
337 reference_wrapper&
338 operator=(const reference_wrapper&) = default;
339
340 _GLIBCXX20_CONSTEXPR
341 operator _Tp&() const noexcept
342 { return this->get(); }
343
344 _GLIBCXX20_CONSTEXPR
345 _Tp&
346 get() const noexcept
347 { return *_M_data; }
348
349 template<typename... _Args>
350 _GLIBCXX20_CONSTEXPR
351 typename __invoke_result<_Tp&, _Args...>::type
352 operator()(_Args&&... __args) const
353 noexcept(__is_nothrow_invocable<_Tp&, _Args...>::value)
354 {
355 #if __cplusplus > 201703L
356 if constexpr (is_object_v<type>)
357 static_assert(sizeof(type), "type must be complete");
358 #endif
359 return std::__invoke(get(), std::forward<_Args>(__args)...);
360 }
361 };
362
363 #if __cpp_deduction_guides
364 template<typename _Tp>
365 reference_wrapper(_Tp&) -> reference_wrapper<_Tp>;
366 #endif
367
368 /// @relates reference_wrapper @{
369
370 /// Denotes a reference should be taken to a variable.
371 template<typename _Tp>
372 _GLIBCXX20_CONSTEXPR
373 inline reference_wrapper<_Tp>
374 ref(_Tp& __t) noexcept
375 { return reference_wrapper<_Tp>(__t); }
376
377 /// Denotes a const reference should be taken to a variable.
378 template<typename _Tp>
379 _GLIBCXX20_CONSTEXPR
380 inline reference_wrapper<const _Tp>
381 cref(const _Tp& __t) noexcept
382 { return reference_wrapper<const _Tp>(__t); }
383
384 template<typename _Tp>
385 void ref(const _Tp&&) = delete;
386
387 template<typename _Tp>
388 void cref(const _Tp&&) = delete;
389
390 /// std::ref overload to prevent wrapping a reference_wrapper
391 template<typename _Tp>
392 _GLIBCXX20_CONSTEXPR
393 inline reference_wrapper<_Tp>
394 ref(reference_wrapper<_Tp> __t) noexcept
395 { return __t; }
396
397 /// std::cref overload to prevent wrapping a reference_wrapper
398 template<typename _Tp>
399 _GLIBCXX20_CONSTEXPR
400 inline reference_wrapper<const _Tp>
401 cref(reference_wrapper<_Tp> __t) noexcept
402 { return { __t.get() }; }
403
404 /// @}
405
406 _GLIBCXX_END_NAMESPACE_VERSION
407 } // namespace std
408
409 #endif // C++11
410
411 #endif // _GLIBCXX_REFWRAP_H