1 // Implementation of std::reference_wrapper -*- C++ -*-
2
3 // Copyright (C) 2004-2020 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 # include <bits/c++0x_warning.h>
37 #else
38
39 #include <bits/move.h>
40 #include <bits/invoke.h>
41 #include <bits/stl_function.h> // for unary_function and binary_function
42
_GLIBCXX_VISIBILITY(default)43 namespace std _GLIBCXX_VISIBILITY(default)
44 {
45 _GLIBCXX_BEGIN_NAMESPACE_VERSION
46
47 /// @cond undocumented
48
49 /**
50 * Derives from @c unary_function or @c binary_function, or perhaps
51 * nothing, depending on the number of arguments provided. The
52 * primary template is the basis case, which derives nothing.
53 */
54 template<typename _Res, typename... _ArgTypes>
55 struct _Maybe_unary_or_binary_function { };
56
57 /// Derives from @c unary_function, as appropriate.
58 template<typename _Res, typename _T1>
59 struct _Maybe_unary_or_binary_function<_Res, _T1>
60 : std::unary_function<_T1, _Res> { };
61
62 /// Derives from @c binary_function, as appropriate.
63 template<typename _Res, typename _T1, typename _T2>
64 struct _Maybe_unary_or_binary_function<_Res, _T1, _T2>
65 : std::binary_function<_T1, _T2, _Res> { };
66
67 template<typename _Signature>
68 struct _Mem_fn_traits;
69
70 template<typename _Res, typename _Class, typename... _ArgTypes>
71 struct _Mem_fn_traits_base
72 {
73 using __result_type = _Res;
74 using __maybe_type
75 = _Maybe_unary_or_binary_function<_Res, _Class*, _ArgTypes...>;
76 using __arity = integral_constant<size_t, sizeof...(_ArgTypes)>;
77 };
78
79 #define _GLIBCXX_MEM_FN_TRAITS2(_CV, _REF, _LVAL, _RVAL) \
80 template<typename _Res, typename _Class, typename... _ArgTypes> \
81 struct _Mem_fn_traits<_Res (_Class::*)(_ArgTypes...) _CV _REF> \
82 : _Mem_fn_traits_base<_Res, _CV _Class, _ArgTypes...> \
83 { \
84 using __vararg = false_type; \
85 }; \
86 template<typename _Res, typename _Class, typename... _ArgTypes> \
87 struct _Mem_fn_traits<_Res (_Class::*)(_ArgTypes... ...) _CV _REF> \
88 : _Mem_fn_traits_base<_Res, _CV _Class, _ArgTypes...> \
89 { \
90 using __vararg = true_type; \
91 };
92
93 #define _GLIBCXX_MEM_FN_TRAITS(_REF, _LVAL, _RVAL) \
94 _GLIBCXX_MEM_FN_TRAITS2( , _REF, _LVAL, _RVAL) \
95 _GLIBCXX_MEM_FN_TRAITS2(const , _REF, _LVAL, _RVAL) \
96 _GLIBCXX_MEM_FN_TRAITS2(volatile , _REF, _LVAL, _RVAL) \
97 _GLIBCXX_MEM_FN_TRAITS2(const volatile, _REF, _LVAL, _RVAL)
98
99 _GLIBCXX_MEM_FN_TRAITS( , true_type, true_type)
100 _GLIBCXX_MEM_FN_TRAITS(&, true_type, false_type)
101 _GLIBCXX_MEM_FN_TRAITS(&&, false_type, true_type)
102
103 #if __cplusplus > 201402L
104 _GLIBCXX_MEM_FN_TRAITS(noexcept, true_type, true_type)
105 _GLIBCXX_MEM_FN_TRAITS(& noexcept, true_type, false_type)
106 _GLIBCXX_MEM_FN_TRAITS(&& noexcept, false_type, true_type)
107 #endif
108
109 #undef _GLIBCXX_MEM_FN_TRAITS
110 #undef _GLIBCXX_MEM_FN_TRAITS2
111
112 /// If we have found a result_type, extract it.
113 template<typename _Functor, typename = __void_t<>>
114 struct _Maybe_get_result_type
115 { };
116
117 template<typename _Functor>
118 struct _Maybe_get_result_type<_Functor,
119 __void_t<typename _Functor::result_type>>
120 { typedef typename _Functor::result_type result_type; };
121
122 /**
123 * Base class for any function object that has a weak result type, as
124 * defined in 20.8.2 [func.require] of C++11.
125 */
126 template<typename _Functor>
127 struct _Weak_result_type_impl
128 : _Maybe_get_result_type<_Functor>
129 { };
130
131 /// Retrieve the result type for a function type.
132 template<typename _Res, typename... _ArgTypes _GLIBCXX_NOEXCEPT_PARM>
133 struct _Weak_result_type_impl<_Res(_ArgTypes...) _GLIBCXX_NOEXCEPT_QUAL>
134 { typedef _Res result_type; };
135
136 /// Retrieve the result type for a varargs function type.
137 template<typename _Res, typename... _ArgTypes _GLIBCXX_NOEXCEPT_PARM>
138 struct _Weak_result_type_impl<_Res(_ArgTypes......) _GLIBCXX_NOEXCEPT_QUAL>
139 { typedef _Res result_type; };
140
141 /// Retrieve the result type for a function pointer.
142 template<typename _Res, typename... _ArgTypes _GLIBCXX_NOEXCEPT_PARM>
143 struct _Weak_result_type_impl<_Res(*)(_ArgTypes...) _GLIBCXX_NOEXCEPT_QUAL>
144 { typedef _Res result_type; };
145
146 /// Retrieve the result type for a varargs function pointer.
147 template<typename _Res, typename... _ArgTypes _GLIBCXX_NOEXCEPT_PARM>
148 struct
149 _Weak_result_type_impl<_Res(*)(_ArgTypes......) _GLIBCXX_NOEXCEPT_QUAL>
150 { typedef _Res result_type; };
151
152 // Let _Weak_result_type_impl perform the real work.
153 template<typename _Functor,
154 bool = is_member_function_pointer<_Functor>::value>
155 struct _Weak_result_type_memfun
156 : _Weak_result_type_impl<_Functor>
157 { };
158
159 // A pointer to member function has a weak result type.
160 template<typename _MemFunPtr>
161 struct _Weak_result_type_memfun<_MemFunPtr, true>
162 {
163 using result_type = typename _Mem_fn_traits<_MemFunPtr>::__result_type;
164 };
165
166 // A pointer to data member doesn't have a weak result type.
167 template<typename _Func, typename _Class>
168 struct _Weak_result_type_memfun<_Func _Class::*, false>
169 { };
170
171 /**
172 * Strip top-level cv-qualifiers from the function object and let
173 * _Weak_result_type_memfun perform the real work.
174 */
175 template<typename _Functor>
176 struct _Weak_result_type
177 : _Weak_result_type_memfun<typename remove_cv<_Functor>::type>
178 { };
179
180 #if __cplusplus <= 201703L
181 // Detect nested argument_type.
182 template<typename _Tp, typename = __void_t<>>
183 struct _Refwrap_base_arg1
184 { };
185
186 // Nested argument_type.
187 template<typename _Tp>
188 struct _Refwrap_base_arg1<_Tp,
189 __void_t<typename _Tp::argument_type>>
190 {
191 typedef typename _Tp::argument_type argument_type;
192 };
193
194 // Detect nested first_argument_type and second_argument_type.
195 template<typename _Tp, typename = __void_t<>>
196 struct _Refwrap_base_arg2
197 { };
198
199 // Nested first_argument_type and second_argument_type.
200 template<typename _Tp>
201 struct _Refwrap_base_arg2<_Tp,
202 __void_t<typename _Tp::first_argument_type,
203 typename _Tp::second_argument_type>>
204 {
205 typedef typename _Tp::first_argument_type first_argument_type;
206 typedef typename _Tp::second_argument_type second_argument_type;
207 };
208
209 /**
210 * Derives from unary_function or binary_function when it
211 * can. Specializations handle all of the easy cases. The primary
212 * template determines what to do with a class type, which may
213 * derive from both unary_function and binary_function.
214 */
215 template<typename _Tp>
216 struct _Reference_wrapper_base
217 : _Weak_result_type<_Tp>, _Refwrap_base_arg1<_Tp>, _Refwrap_base_arg2<_Tp>
218 { };
219
220 // - a function type (unary)
221 template<typename _Res, typename _T1 _GLIBCXX_NOEXCEPT_PARM>
222 struct _Reference_wrapper_base<_Res(_T1) _GLIBCXX_NOEXCEPT_QUAL>
223 : unary_function<_T1, _Res>
224 { };
225
226 template<typename _Res, typename _T1>
227 struct _Reference_wrapper_base<_Res(_T1) const>
228 : unary_function<_T1, _Res>
229 { };
230
231 template<typename _Res, typename _T1>
232 struct _Reference_wrapper_base<_Res(_T1) volatile>
233 : unary_function<_T1, _Res>
234 { };
235
236 template<typename _Res, typename _T1>
237 struct _Reference_wrapper_base<_Res(_T1) const volatile>
238 : unary_function<_T1, _Res>
239 { };
240
241 // - a function type (binary)
242 template<typename _Res, typename _T1, typename _T2 _GLIBCXX_NOEXCEPT_PARM>
243 struct _Reference_wrapper_base<_Res(_T1, _T2) _GLIBCXX_NOEXCEPT_QUAL>
244 : binary_function<_T1, _T2, _Res>
245 { };
246
247 template<typename _Res, typename _T1, typename _T2>
248 struct _Reference_wrapper_base<_Res(_T1, _T2) const>
249 : binary_function<_T1, _T2, _Res>
250 { };
251
252 template<typename _Res, typename _T1, typename _T2>
253 struct _Reference_wrapper_base<_Res(_T1, _T2) volatile>
254 : binary_function<_T1, _T2, _Res>
255 { };
256
257 template<typename _Res, typename _T1, typename _T2>
258 struct _Reference_wrapper_base<_Res(_T1, _T2) const volatile>
259 : binary_function<_T1, _T2, _Res>
260 { };
261
262 // - a function pointer type (unary)
263 template<typename _Res, typename _T1 _GLIBCXX_NOEXCEPT_PARM>
264 struct _Reference_wrapper_base<_Res(*)(_T1) _GLIBCXX_NOEXCEPT_QUAL>
265 : unary_function<_T1, _Res>
266 { };
267
268 // - a function pointer type (binary)
269 template<typename _Res, typename _T1, typename _T2 _GLIBCXX_NOEXCEPT_PARM>
270 struct _Reference_wrapper_base<_Res(*)(_T1, _T2) _GLIBCXX_NOEXCEPT_QUAL>
271 : binary_function<_T1, _T2, _Res>
272 { };
273
274 template<typename _Tp, bool = is_member_function_pointer<_Tp>::value>
275 struct _Reference_wrapper_base_memfun
276 : _Reference_wrapper_base<_Tp>
277 { };
278
279 template<typename _MemFunPtr>
280 struct _Reference_wrapper_base_memfun<_MemFunPtr, true>
281 : _Mem_fn_traits<_MemFunPtr>::__maybe_type
282 {
283 using result_type = typename _Mem_fn_traits<_MemFunPtr>::__result_type;
284 };
285 #endif // ! C++20
286
287 /// @endcond
288
289 /**
290 * @brief Primary class template for reference_wrapper.
291 * @ingroup functors
292 */
293 template<typename _Tp>
294 class reference_wrapper
295 #if __cplusplus <= 201703L
296 // In C++20 std::reference_wrapper<T> allows T to be incomplete,
297 // so checking for nested types could result in ODR violations.
298 : public _Reference_wrapper_base_memfun<typename remove_cv<_Tp>::type>
299 #endif
300 {
301 _Tp* _M_data;
302
303 _GLIBCXX20_CONSTEXPR
304 static _Tp* _S_fun(_Tp& __r) noexcept { return std::__addressof(__r); }
305
306 static void _S_fun(_Tp&&) = delete;
307
308 template<typename _Up, typename _Up2 = __remove_cvref_t<_Up>>
309 using __not_same
310 = typename enable_if<!is_same<reference_wrapper, _Up2>::value>::type;
311
312 public:
313 typedef _Tp type;
314
315 // _GLIBCXX_RESOLVE_LIB_DEFECTS
316 // 2993. reference_wrapper<T> conversion from T&&
317 // 3041. Unnecessary decay in reference_wrapper
318 template<typename _Up, typename = __not_same<_Up>, typename
319 = decltype(reference_wrapper::_S_fun(std::declval<_Up>()))>
320 _GLIBCXX20_CONSTEXPR
321 reference_wrapper(_Up&& __uref)
322 noexcept(noexcept(reference_wrapper::_S_fun(std::declval<_Up>())))
323 : _M_data(reference_wrapper::_S_fun(std::forward<_Up>(__uref)))
324 { }
325
326 reference_wrapper(const reference_wrapper&) = default;
327
328 reference_wrapper&
329 operator=(const reference_wrapper&) = default;
330
331 _GLIBCXX20_CONSTEXPR
332 operator _Tp&() const noexcept
333 { return this->get(); }
334
335 _GLIBCXX20_CONSTEXPR
336 _Tp&
337 get() const noexcept
338 { return *_M_data; }
339
340 template<typename... _Args>
341 _GLIBCXX20_CONSTEXPR
342 typename result_of<_Tp&(_Args&&...)>::type
343 operator()(_Args&&... __args) const
344 {
345 #if __cplusplus > 201703L
346 if constexpr (is_object_v<type>)
347 static_assert(sizeof(type), "type must be complete");
348 #endif
349 return std::__invoke(get(), std::forward<_Args>(__args)...);
350 }
351 };
352
353 #if __cpp_deduction_guides
354 template<typename _Tp>
355 reference_wrapper(_Tp&) -> reference_wrapper<_Tp>;
356 #endif
357
358 /// @relates reference_wrapper @{
359
360 /// Denotes a reference should be taken to a variable.
361 template<typename _Tp>
362 _GLIBCXX20_CONSTEXPR
363 inline reference_wrapper<_Tp>
364 ref(_Tp& __t) noexcept
365 { return reference_wrapper<_Tp>(__t); }
366
367 /// Denotes a const reference should be taken to a variable.
368 template<typename _Tp>
369 _GLIBCXX20_CONSTEXPR
370 inline reference_wrapper<const _Tp>
371 cref(const _Tp& __t) noexcept
372 { return reference_wrapper<const _Tp>(__t); }
373
374 template<typename _Tp>
375 void ref(const _Tp&&) = delete;
376
377 template<typename _Tp>
378 void cref(const _Tp&&) = delete;
379
380 /// std::ref overload to prevent wrapping a reference_wrapper
381 template<typename _Tp>
382 _GLIBCXX20_CONSTEXPR
383 inline reference_wrapper<_Tp>
384 ref(reference_wrapper<_Tp> __t) noexcept
385 { return __t; }
386
387 /// std::cref overload to prevent wrapping a reference_wrapper
388 template<typename _Tp>
389 _GLIBCXX20_CONSTEXPR
390 inline reference_wrapper<const _Tp>
391 cref(reference_wrapper<_Tp> __t) noexcept
392 { return { __t.get() }; }
393
394 /// @}
395
396 _GLIBCXX_END_NAMESPACE_VERSION
397 } // namespace std
398
399 #endif // C++11
400
401 #endif // _GLIBCXX_REFWRAP_H
402