1 // Implementation of std::reference_wrapper -*- C++ -*- 2 3 // Copyright (C) 2004-2019 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 43 namespace std _GLIBCXX_VISIBILITY(default) 44 { 45 _GLIBCXX_BEGIN_NAMESPACE_VERSION 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 /// Derives from @c unary_function, as appropriate. 56 template<typename _Res, typename _T1> 57 struct _Maybe_unary_or_binary_function<_Res, _T1> 58 : std::unary_function<_T1, _Res> { }; 59 60 /// Derives from @c binary_function, as appropriate. 61 template<typename _Res, typename _T1, typename _T2> 62 struct _Maybe_unary_or_binary_function<_Res, _T1, _T2> 63 : std::binary_function<_T1, _T2, _Res> { }; 64 65 template<typename _Signature> 66 struct _Mem_fn_traits; 67 68 template<typename _Res, typename _Class, typename... _ArgTypes> 69 struct _Mem_fn_traits_base 70 { 71 using __result_type = _Res; 72 using __maybe_type 73 = _Maybe_unary_or_binary_function<_Res, _Class*, _ArgTypes...>; 74 using __arity = integral_constant<size_t, sizeof...(_ArgTypes)>; 75 }; 76 77 #define _GLIBCXX_MEM_FN_TRAITS2(_CV, _REF, _LVAL, _RVAL) \ 78 template<typename _Res, typename _Class, typename... _ArgTypes> \ 79 struct _Mem_fn_traits<_Res (_Class::*)(_ArgTypes...) _CV _REF> \ 80 : _Mem_fn_traits_base<_Res, _CV _Class, _ArgTypes...> \ 81 { \ 82 using __vararg = false_type; \ 83 }; \ 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 = true_type; \ 89 }; 90 91 #define _GLIBCXX_MEM_FN_TRAITS(_REF, _LVAL, _RVAL) \ 92 _GLIBCXX_MEM_FN_TRAITS2( , _REF, _LVAL, _RVAL) \ 93 _GLIBCXX_MEM_FN_TRAITS2(const , _REF, _LVAL, _RVAL) \ 94 _GLIBCXX_MEM_FN_TRAITS2(volatile , _REF, _LVAL, _RVAL) \ 95 _GLIBCXX_MEM_FN_TRAITS2(const volatile, _REF, _LVAL, _RVAL) 96 97 _GLIBCXX_MEM_FN_TRAITS( , true_type, true_type) 98 _GLIBCXX_MEM_FN_TRAITS(&, true_type, false_type) 99 _GLIBCXX_MEM_FN_TRAITS(&&, false_type, true_type) 100 101 #if __cplusplus > 201402L 102 _GLIBCXX_MEM_FN_TRAITS(noexcept, true_type, true_type) 103 _GLIBCXX_MEM_FN_TRAITS(& noexcept, true_type, false_type) 104 _GLIBCXX_MEM_FN_TRAITS(&& noexcept, false_type, true_type) 105 #endif 106 107 #undef _GLIBCXX_MEM_FN_TRAITS 108 #undef _GLIBCXX_MEM_FN_TRAITS2 109 110 /// If we have found a result_type, extract it. 111 template<typename _Functor, typename = __void_t<>> 112 struct _Maybe_get_result_type 113 { }; 114 115 template<typename _Functor> 116 struct _Maybe_get_result_type<_Functor, 117 __void_t<typename _Functor::result_type>> 118 { typedef typename _Functor::result_type result_type; }; 119 120 /** 121 * Base class for any function object that has a weak result type, as 122 * defined in 20.8.2 [func.require] of C++11. 123 */ 124 template<typename _Functor> 125 struct _Weak_result_type_impl 126 : _Maybe_get_result_type<_Functor> 127 { }; 128 129 /// Retrieve the result type for a function type. 130 template<typename _Res, typename... _ArgTypes _GLIBCXX_NOEXCEPT_PARM> 131 struct _Weak_result_type_impl<_Res(_ArgTypes...) _GLIBCXX_NOEXCEPT_QUAL> 132 { typedef _Res result_type; }; 133 134 /// Retrieve the result type for a varargs function type. 135 template<typename _Res, typename... _ArgTypes _GLIBCXX_NOEXCEPT_PARM> 136 struct _Weak_result_type_impl<_Res(_ArgTypes......) _GLIBCXX_NOEXCEPT_QUAL> 137 { typedef _Res result_type; }; 138 139 /// Retrieve the result type for a function pointer. 140 template<typename _Res, typename... _ArgTypes _GLIBCXX_NOEXCEPT_PARM> 141 struct _Weak_result_type_impl<_Res(*)(_ArgTypes...) _GLIBCXX_NOEXCEPT_QUAL> 142 { typedef _Res result_type; }; 143 144 /// Retrieve the result type for a varargs function pointer. 145 template<typename _Res, typename... _ArgTypes _GLIBCXX_NOEXCEPT_PARM> 146 struct 147 _Weak_result_type_impl<_Res(*)(_ArgTypes......) _GLIBCXX_NOEXCEPT_QUAL> 148 { typedef _Res result_type; }; 149 150 // Let _Weak_result_type_impl perform the real work. 151 template<typename _Functor, 152 bool = is_member_function_pointer<_Functor>::value> 153 struct _Weak_result_type_memfun 154 : _Weak_result_type_impl<_Functor> 155 { }; 156 157 // A pointer to member function has a weak result type. 158 template<typename _MemFunPtr> 159 struct _Weak_result_type_memfun<_MemFunPtr, true> 160 { 161 using result_type = typename _Mem_fn_traits<_MemFunPtr>::__result_type; 162 }; 163 164 // A pointer to data member doesn't have a weak result type. 165 template<typename _Func, typename _Class> 166 struct _Weak_result_type_memfun<_Func _Class::*, false> 167 { }; 168 169 /** 170 * Strip top-level cv-qualifiers from the function object and let 171 * _Weak_result_type_memfun perform the real work. 172 */ 173 template<typename _Functor> 174 struct _Weak_result_type 175 : _Weak_result_type_memfun<typename remove_cv<_Functor>::type> 176 { }; 177 178 #if __cplusplus <= 201703L 179 // Detect nested argument_type. 180 template<typename _Tp, typename = __void_t<>> 181 struct _Refwrap_base_arg1 182 { }; 183 184 // Nested argument_type. 185 template<typename _Tp> 186 struct _Refwrap_base_arg1<_Tp, 187 __void_t<typename _Tp::argument_type>> 188 { 189 typedef typename _Tp::argument_type argument_type; 190 }; 191 192 // Detect nested first_argument_type and second_argument_type. 193 template<typename _Tp, typename = __void_t<>> 194 struct _Refwrap_base_arg2 195 { }; 196 197 // Nested first_argument_type and second_argument_type. 198 template<typename _Tp> 199 struct _Refwrap_base_arg2<_Tp, 200 __void_t<typename _Tp::first_argument_type, 201 typename _Tp::second_argument_type>> 202 { 203 typedef typename _Tp::first_argument_type first_argument_type; 204 typedef typename _Tp::second_argument_type second_argument_type; 205 }; 206 207 /** 208 * Derives from unary_function or binary_function when it 209 * can. Specializations handle all of the easy cases. The primary 210 * template determines what to do with a class type, which may 211 * derive from both unary_function and binary_function. 212 */ 213 template<typename _Tp> 214 struct _Reference_wrapper_base 215 : _Weak_result_type<_Tp>, _Refwrap_base_arg1<_Tp>, _Refwrap_base_arg2<_Tp> 216 { }; 217 218 // - a function type (unary) 219 template<typename _Res, typename _T1 _GLIBCXX_NOEXCEPT_PARM> 220 struct _Reference_wrapper_base<_Res(_T1) _GLIBCXX_NOEXCEPT_QUAL> 221 : unary_function<_T1, _Res> 222 { }; 223 224 template<typename _Res, typename _T1> 225 struct _Reference_wrapper_base<_Res(_T1) const> 226 : unary_function<_T1, _Res> 227 { }; 228 229 template<typename _Res, typename _T1> 230 struct _Reference_wrapper_base<_Res(_T1) volatile> 231 : unary_function<_T1, _Res> 232 { }; 233 234 template<typename _Res, typename _T1> 235 struct _Reference_wrapper_base<_Res(_T1) const volatile> 236 : unary_function<_T1, _Res> 237 { }; 238 239 // - a function type (binary) 240 template<typename _Res, typename _T1, typename _T2 _GLIBCXX_NOEXCEPT_PARM> 241 struct _Reference_wrapper_base<_Res(_T1, _T2) _GLIBCXX_NOEXCEPT_QUAL> 242 : binary_function<_T1, _T2, _Res> 243 { }; 244 245 template<typename _Res, typename _T1, typename _T2> 246 struct _Reference_wrapper_base<_Res(_T1, _T2) const> 247 : binary_function<_T1, _T2, _Res> 248 { }; 249 250 template<typename _Res, typename _T1, typename _T2> 251 struct _Reference_wrapper_base<_Res(_T1, _T2) volatile> 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 volatile> 257 : binary_function<_T1, _T2, _Res> 258 { }; 259 260 // - a function pointer type (unary) 261 template<typename _Res, typename _T1 _GLIBCXX_NOEXCEPT_PARM> 262 struct _Reference_wrapper_base<_Res(*)(_T1) _GLIBCXX_NOEXCEPT_QUAL> 263 : unary_function<_T1, _Res> 264 { }; 265 266 // - a function pointer type (binary) 267 template<typename _Res, typename _T1, typename _T2 _GLIBCXX_NOEXCEPT_PARM> 268 struct _Reference_wrapper_base<_Res(*)(_T1, _T2) _GLIBCXX_NOEXCEPT_QUAL> 269 : binary_function<_T1, _T2, _Res> 270 { }; 271 272 template<typename _Tp, bool = is_member_function_pointer<_Tp>::value> 273 struct _Reference_wrapper_base_memfun 274 : _Reference_wrapper_base<_Tp> 275 { }; 276 277 template<typename _MemFunPtr> 278 struct _Reference_wrapper_base_memfun<_MemFunPtr, true> 279 : _Mem_fn_traits<_MemFunPtr>::__maybe_type 280 { 281 using result_type = typename _Mem_fn_traits<_MemFunPtr>::__result_type; 282 }; 283 #endif // ! C++20 284 285 /** 286 * @brief Primary class template for reference_wrapper. 287 * @ingroup functors 288 * @{ 289 */ 290 template<typename _Tp> 291 class reference_wrapper 292 #if __cplusplus <= 201703L 293 // In C++20 std::reference_wrapper<T> allows T to be incomplete, 294 // so checking for nested types could result in ODR violations. 295 : public _Reference_wrapper_base_memfun<typename remove_cv<_Tp>::type> 296 #endif 297 { 298 _Tp* _M_data; 299 300 static _Tp* _S_fun(_Tp& __r) noexcept { return std::__addressof(__r); } 301 static void _S_fun(_Tp&&) = delete; 302 303 template<typename _Up, typename _Up2 = __remove_cvref_t<_Up>> 304 using __not_same 305 = typename enable_if<!is_same<reference_wrapper, _Up2>::value>::type; 306 307 public: 308 typedef _Tp type; 309 310 // _GLIBCXX_RESOLVE_LIB_DEFECTS 311 // 2993. reference_wrapper<T> conversion from T&& 312 // 3041. Unnecessary decay in reference_wrapper 313 template<typename _Up, typename = __not_same<_Up>, typename 314 = decltype(reference_wrapper::_S_fun(std::declval<_Up>()))> 315 reference_wrapper(_Up&& __uref) 316 noexcept(noexcept(reference_wrapper::_S_fun(std::declval<_Up>()))) 317 : _M_data(reference_wrapper::_S_fun(std::forward<_Up>(__uref))) 318 { } 319 320 reference_wrapper(const reference_wrapper&) = default; 321 322 reference_wrapper& 323 operator=(const reference_wrapper&) = default; 324 325 operator _Tp&() const noexcept 326 { return this->get(); } 327 328 _Tp& 329 get() const noexcept 330 { return *_M_data; } 331 332 template<typename... _Args> 333 typename result_of<_Tp&(_Args&&...)>::type 334 operator()(_Args&&... __args) const 335 { 336 #if __cplusplus > 201703L 337 if constexpr (is_object_v<type>) 338 static_assert(sizeof(type), "type must be complete"); 339 #endif 340 return std::__invoke(get(), std::forward<_Args>(__args)...); 341 } 342 }; 343 344 #if __cpp_deduction_guides 345 template<typename _Tp> 346 reference_wrapper(_Tp&) -> reference_wrapper<_Tp>; 347 #endif 348 349 /// Denotes a reference should be taken to a variable. 350 template<typename _Tp> 351 inline reference_wrapper<_Tp> 352 ref(_Tp& __t) noexcept 353 { return reference_wrapper<_Tp>(__t); } 354 355 /// Denotes a const reference should be taken to a variable. 356 template<typename _Tp> 357 inline reference_wrapper<const _Tp> 358 cref(const _Tp& __t) noexcept 359 { return reference_wrapper<const _Tp>(__t); } 360 361 template<typename _Tp> 362 void ref(const _Tp&&) = delete; 363 364 template<typename _Tp> 365 void cref(const _Tp&&) = delete; 366 367 /// std::ref overload to prevent wrapping a reference_wrapper 368 template<typename _Tp> 369 inline reference_wrapper<_Tp> 370 ref(reference_wrapper<_Tp> __t) noexcept 371 { return __t; } 372 373 /// std::cref overload to prevent wrapping a reference_wrapper 374 template<typename _Tp> 375 inline reference_wrapper<const _Tp> 376 cref(reference_wrapper<_Tp> __t) noexcept 377 { return { __t.get() }; } 378 379 // @} group functors 380 381 _GLIBCXX_END_NAMESPACE_VERSION 382 } // namespace std 383 384 #endif // C++11 385 386 #endif // _GLIBCXX_REFWRAP_H 387