1 // 2 // Copyright 2017 The Abseil Authors. 3 // 4 // Licensed under the Apache License, Version 2.0 (the "License"); 5 // you may not use this file except in compliance with the License. 6 // You may obtain a copy of the License at 7 // 8 // https://www.apache.org/licenses/LICENSE-2.0 9 // 10 // Unless required by applicable law or agreed to in writing, software 11 // distributed under the License is distributed on an "AS IS" BASIS, 12 // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 13 // See the License for the specific language governing permissions and 14 // limitations under the License. 15 // 16 // ----------------------------------------------------------------------------- 17 // type_traits.h 18 // ----------------------------------------------------------------------------- 19 // 20 // This file contains C++11-compatible versions of standard <type_traits> API 21 // functions for determining the characteristics of types. Such traits can 22 // support type inference, classification, and transformation, as well as 23 // make it easier to write templates based on generic type behavior. 24 // 25 // See https://en.cppreference.com/w/cpp/header/type_traits 26 // 27 // WARNING: use of many of the constructs in this header will count as "complex 28 // template metaprogramming", so before proceeding, please carefully consider 29 // https://google.github.io/styleguide/cppguide.html#Template_metaprogramming 30 // 31 // WARNING: using template metaprogramming to detect or depend on API 32 // features is brittle and not guaranteed. Neither the standard library nor 33 // Abseil provides any guarantee that APIs are stable in the face of template 34 // metaprogramming. Use with caution. 35 #ifndef ABSL_META_TYPE_TRAITS_H_ 36 #define ABSL_META_TYPE_TRAITS_H_ 37 38 #include <stddef.h> 39 #include <functional> 40 #include <type_traits> 41 42 #include "absl/base/config.h" 43 44 // MSVC constructibility traits do not detect destructor properties and so our 45 // implementations should not use them as a source-of-truth. 46 #if defined(_MSC_VER) && !defined(__clang__) && !defined(__GNUC__) 47 #define ABSL_META_INTERNAL_STD_CONSTRUCTION_TRAITS_DONT_CHECK_DESTRUCTION 1 48 #endif 49 50 namespace absl { 51 ABSL_NAMESPACE_BEGIN 52 53 // Defined and documented later on in this file. 54 template <typename T> 55 struct is_trivially_destructible; 56 57 // Defined and documented later on in this file. 58 template <typename T> 59 struct is_trivially_move_assignable; 60 61 namespace type_traits_internal { 62 63 // Silence MSVC warnings about the destructor being defined as deleted. 64 #if defined(_MSC_VER) && !defined(__GNUC__) 65 #pragma warning(push) 66 #pragma warning(disable : 4624) 67 #endif // defined(_MSC_VER) && !defined(__GNUC__) 68 69 template <class T> 70 union SingleMemberUnion { 71 T t; 72 }; 73 74 // Restore the state of the destructor warning that was silenced above. 75 #if defined(_MSC_VER) && !defined(__GNUC__) 76 #pragma warning(pop) 77 #endif // defined(_MSC_VER) && !defined(__GNUC__) 78 79 template <class T> 80 struct IsTriviallyMoveConstructibleObject 81 : std::integral_constant< 82 bool, std::is_move_constructible< 83 type_traits_internal::SingleMemberUnion<T>>::value && 84 absl::is_trivially_destructible<T>::value> {}; 85 86 template <class T> 87 struct IsTriviallyCopyConstructibleObject 88 : std::integral_constant< 89 bool, std::is_copy_constructible< 90 type_traits_internal::SingleMemberUnion<T>>::value && 91 absl::is_trivially_destructible<T>::value> {}; 92 93 template <class T> 94 struct IsTriviallyMoveAssignableReference : std::false_type {}; 95 96 template <class T> 97 struct IsTriviallyMoveAssignableReference<T&> 98 : absl::is_trivially_move_assignable<T>::type {}; 99 100 template <class T> 101 struct IsTriviallyMoveAssignableReference<T&&> 102 : absl::is_trivially_move_assignable<T>::type {}; 103 104 template <typename... Ts> 105 struct VoidTImpl { 106 using type = void; 107 }; 108 109 // This trick to retrieve a default alignment is necessary for our 110 // implementation of aligned_storage_t to be consistent with any implementation 111 // of std::aligned_storage. 112 template <size_t Len, typename T = std::aligned_storage<Len>> 113 struct default_alignment_of_aligned_storage; 114 115 template <size_t Len, size_t Align> 116 struct default_alignment_of_aligned_storage<Len, 117 std::aligned_storage<Len, Align>> { 118 static constexpr size_t value = Align; 119 }; 120 121 //////////////////////////////// 122 // Library Fundamentals V2 TS // 123 //////////////////////////////// 124 125 // NOTE: The `is_detected` family of templates here differ from the library 126 // fundamentals specification in that for library fundamentals, `Op<Args...>` is 127 // evaluated as soon as the type `is_detected<Op, Args...>` undergoes 128 // substitution, regardless of whether or not the `::value` is accessed. That 129 // is inconsistent with all other standard traits and prevents lazy evaluation 130 // in larger contexts (such as if the `is_detected` check is a trailing argument 131 // of a `conjunction`. This implementation opts to instead be lazy in the same 132 // way that the standard traits are (this "defect" of the detection idiom 133 // specifications has been reported). 134 135 template <class Enabler, template <class...> class Op, class... Args> 136 struct is_detected_impl { 137 using type = std::false_type; 138 }; 139 140 template <template <class...> class Op, class... Args> 141 struct is_detected_impl<typename VoidTImpl<Op<Args...>>::type, Op, Args...> { 142 using type = std::true_type; 143 }; 144 145 template <template <class...> class Op, class... Args> 146 struct is_detected : is_detected_impl<void, Op, Args...>::type {}; 147 148 template <class Enabler, class To, template <class...> class Op, class... Args> 149 struct is_detected_convertible_impl { 150 using type = std::false_type; 151 }; 152 153 template <class To, template <class...> class Op, class... Args> 154 struct is_detected_convertible_impl< 155 typename std::enable_if<std::is_convertible<Op<Args...>, To>::value>::type, 156 To, Op, Args...> { 157 using type = std::true_type; 158 }; 159 160 template <class To, template <class...> class Op, class... Args> 161 struct is_detected_convertible 162 : is_detected_convertible_impl<void, To, Op, Args...>::type {}; 163 164 template <typename T> 165 using IsCopyAssignableImpl = 166 decltype(std::declval<T&>() = std::declval<const T&>()); 167 168 template <typename T> 169 using IsMoveAssignableImpl = decltype(std::declval<T&>() = std::declval<T&&>()); 170 171 } // namespace type_traits_internal 172 173 // MSVC 19.20 has a regression that causes our workarounds to fail, but their 174 // std forms now appear to be compliant. 175 #if defined(_MSC_VER) && !defined(__clang__) && (_MSC_VER >= 1920) 176 177 template <typename T> 178 using is_copy_assignable = std::is_copy_assignable<T>; 179 180 template <typename T> 181 using is_move_assignable = std::is_move_assignable<T>; 182 183 #else 184 185 template <typename T> 186 struct is_copy_assignable : type_traits_internal::is_detected< 187 type_traits_internal::IsCopyAssignableImpl, T> { 188 }; 189 190 template <typename T> 191 struct is_move_assignable : type_traits_internal::is_detected< 192 type_traits_internal::IsMoveAssignableImpl, T> { 193 }; 194 195 #endif 196 197 // void_t() 198 // 199 // Ignores the type of any its arguments and returns `void`. In general, this 200 // metafunction allows you to create a general case that maps to `void` while 201 // allowing specializations that map to specific types. 202 // 203 // This metafunction is designed to be a drop-in replacement for the C++17 204 // `std::void_t` metafunction. 205 // 206 // NOTE: `absl::void_t` does not use the standard-specified implementation so 207 // that it can remain compatible with gcc < 5.1. This can introduce slightly 208 // different behavior, such as when ordering partial specializations. 209 template <typename... Ts> 210 using void_t = typename type_traits_internal::VoidTImpl<Ts...>::type; 211 212 // conjunction 213 // 214 // Performs a compile-time logical AND operation on the passed types (which 215 // must have `::value` members convertible to `bool`. Short-circuits if it 216 // encounters any `false` members (and does not compare the `::value` members 217 // of any remaining arguments). 218 // 219 // This metafunction is designed to be a drop-in replacement for the C++17 220 // `std::conjunction` metafunction. 221 template <typename... Ts> 222 struct conjunction; 223 224 template <typename T, typename... Ts> 225 struct conjunction<T, Ts...> 226 : std::conditional<T::value, conjunction<Ts...>, T>::type {}; 227 228 template <typename T> 229 struct conjunction<T> : T {}; 230 231 template <> 232 struct conjunction<> : std::true_type {}; 233 234 // disjunction 235 // 236 // Performs a compile-time logical OR operation on the passed types (which 237 // must have `::value` members convertible to `bool`. Short-circuits if it 238 // encounters any `true` members (and does not compare the `::value` members 239 // of any remaining arguments). 240 // 241 // This metafunction is designed to be a drop-in replacement for the C++17 242 // `std::disjunction` metafunction. 243 template <typename... Ts> 244 struct disjunction; 245 246 template <typename T, typename... Ts> 247 struct disjunction<T, Ts...> : 248 std::conditional<T::value, T, disjunction<Ts...>>::type {}; 249 250 template <typename T> 251 struct disjunction<T> : T {}; 252 253 template <> 254 struct disjunction<> : std::false_type {}; 255 256 // negation 257 // 258 // Performs a compile-time logical NOT operation on the passed type (which 259 // must have `::value` members convertible to `bool`. 260 // 261 // This metafunction is designed to be a drop-in replacement for the C++17 262 // `std::negation` metafunction. 263 template <typename T> 264 struct negation : std::integral_constant<bool, !T::value> {}; 265 266 // is_function() 267 // 268 // Determines whether the passed type `T` is a function type. 269 // 270 // This metafunction is designed to be a drop-in replacement for the C++11 271 // `std::is_function()` metafunction for platforms that have incomplete C++11 272 // support (such as libstdc++ 4.x). 273 // 274 // This metafunction works because appending `const` to a type does nothing to 275 // function types and reference types (and forms a const-qualified type 276 // otherwise). 277 template <typename T> 278 struct is_function 279 : std::integral_constant< 280 bool, !(std::is_reference<T>::value || 281 std::is_const<typename std::add_const<T>::type>::value)> {}; 282 283 // is_trivially_destructible() 284 // 285 // Determines whether the passed type `T` is trivially destructible. 286 // 287 // This metafunction is designed to be a drop-in replacement for the C++11 288 // `std::is_trivially_destructible()` metafunction for platforms that have 289 // incomplete C++11 support (such as libstdc++ 4.x). On any platforms that do 290 // fully support C++11, we check whether this yields the same result as the std 291 // implementation. 292 // 293 // NOTE: the extensions (__has_trivial_xxx) are implemented in gcc (version >= 294 // 4.3) and clang. Since we are supporting libstdc++ > 4.7, they should always 295 // be present. These extensions are documented at 296 // https://gcc.gnu.org/onlinedocs/gcc/Type-Traits.html#Type-Traits. 297 template <typename T> 298 struct is_trivially_destructible 299 : std::integral_constant<bool, __has_trivial_destructor(T) && 300 std::is_destructible<T>::value> { 301 #ifdef ABSL_HAVE_STD_IS_TRIVIALLY_DESTRUCTIBLE 302 private: 303 static constexpr bool compliant = std::is_trivially_destructible<T>::value == 304 is_trivially_destructible::value; 305 static_assert(compliant || std::is_trivially_destructible<T>::value, 306 "Not compliant with std::is_trivially_destructible; " 307 "Standard: false, Implementation: true"); 308 static_assert(compliant || !std::is_trivially_destructible<T>::value, 309 "Not compliant with std::is_trivially_destructible; " 310 "Standard: true, Implementation: false"); 311 #endif // ABSL_HAVE_STD_IS_TRIVIALLY_DESTRUCTIBLE 312 }; 313 314 // is_trivially_default_constructible() 315 // 316 // Determines whether the passed type `T` is trivially default constructible. 317 // 318 // This metafunction is designed to be a drop-in replacement for the C++11 319 // `std::is_trivially_default_constructible()` metafunction for platforms that 320 // have incomplete C++11 support (such as libstdc++ 4.x). On any platforms that 321 // do fully support C++11, we check whether this yields the same result as the 322 // std implementation. 323 // 324 // NOTE: according to the C++ standard, Section: 20.15.4.3 [meta.unary.prop] 325 // "The predicate condition for a template specialization is_constructible<T, 326 // Args...> shall be satisfied if and only if the following variable 327 // definition would be well-formed for some invented variable t: 328 // 329 // T t(declval<Args>()...); 330 // 331 // is_trivially_constructible<T, Args...> additionally requires that the 332 // variable definition does not call any operation that is not trivial. 333 // For the purposes of this check, the call to std::declval is considered 334 // trivial." 335 // 336 // Notes from https://en.cppreference.com/w/cpp/types/is_constructible: 337 // In many implementations, is_nothrow_constructible also checks if the 338 // destructor throws because it is effectively noexcept(T(arg)). Same 339 // applies to is_trivially_constructible, which, in these implementations, also 340 // requires that the destructor is trivial. 341 // GCC bug 51452: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=51452 342 // LWG issue 2116: http://cplusplus.github.io/LWG/lwg-active.html#2116. 343 // 344 // "T obj();" need to be well-formed and not call any nontrivial operation. 345 // Nontrivially destructible types will cause the expression to be nontrivial. 346 template <typename T> 347 struct is_trivially_default_constructible 348 : std::integral_constant<bool, __has_trivial_constructor(T) && 349 std::is_default_constructible<T>::value && 350 is_trivially_destructible<T>::value> { 351 #if defined(ABSL_HAVE_STD_IS_TRIVIALLY_CONSTRUCTIBLE) && \ 352 !defined( \ 353 ABSL_META_INTERNAL_STD_CONSTRUCTION_TRAITS_DONT_CHECK_DESTRUCTION) 354 private: 355 static constexpr bool compliant = 356 std::is_trivially_default_constructible<T>::value == 357 is_trivially_default_constructible::value; 358 static_assert(compliant || std::is_trivially_default_constructible<T>::value, 359 "Not compliant with std::is_trivially_default_constructible; " 360 "Standard: false, Implementation: true"); 361 static_assert(compliant || !std::is_trivially_default_constructible<T>::value, 362 "Not compliant with std::is_trivially_default_constructible; " 363 "Standard: true, Implementation: false"); 364 #endif // ABSL_HAVE_STD_IS_TRIVIALLY_CONSTRUCTIBLE 365 }; 366 367 // is_trivially_move_constructible() 368 // 369 // Determines whether the passed type `T` is trivially move constructible. 370 // 371 // This metafunction is designed to be a drop-in replacement for the C++11 372 // `std::is_trivially_move_constructible()` metafunction for platforms that have 373 // incomplete C++11 support (such as libstdc++ 4.x). On any platforms that do 374 // fully support C++11, we check whether this yields the same result as the std 375 // implementation. 376 // 377 // NOTE: `T obj(declval<T>());` needs to be well-formed and not call any 378 // nontrivial operation. Nontrivially destructible types will cause the 379 // expression to be nontrivial. 380 template <typename T> 381 struct is_trivially_move_constructible 382 : std::conditional< 383 std::is_object<T>::value && !std::is_array<T>::value, 384 type_traits_internal::IsTriviallyMoveConstructibleObject<T>, 385 std::is_reference<T>>::type::type { 386 #if defined(ABSL_HAVE_STD_IS_TRIVIALLY_CONSTRUCTIBLE) && \ 387 !defined( \ 388 ABSL_META_INTERNAL_STD_CONSTRUCTION_TRAITS_DONT_CHECK_DESTRUCTION) 389 private: 390 static constexpr bool compliant = 391 std::is_trivially_move_constructible<T>::value == 392 is_trivially_move_constructible::value; 393 static_assert(compliant || std::is_trivially_move_constructible<T>::value, 394 "Not compliant with std::is_trivially_move_constructible; " 395 "Standard: false, Implementation: true"); 396 static_assert(compliant || !std::is_trivially_move_constructible<T>::value, 397 "Not compliant with std::is_trivially_move_constructible; " 398 "Standard: true, Implementation: false"); 399 #endif // ABSL_HAVE_STD_IS_TRIVIALLY_CONSTRUCTIBLE 400 }; 401 402 // is_trivially_copy_constructible() 403 // 404 // Determines whether the passed type `T` is trivially copy constructible. 405 // 406 // This metafunction is designed to be a drop-in replacement for the C++11 407 // `std::is_trivially_copy_constructible()` metafunction for platforms that have 408 // incomplete C++11 support (such as libstdc++ 4.x). On any platforms that do 409 // fully support C++11, we check whether this yields the same result as the std 410 // implementation. 411 // 412 // NOTE: `T obj(declval<const T&>());` needs to be well-formed and not call any 413 // nontrivial operation. Nontrivially destructible types will cause the 414 // expression to be nontrivial. 415 template <typename T> 416 struct is_trivially_copy_constructible 417 : std::conditional< 418 std::is_object<T>::value && !std::is_array<T>::value, 419 type_traits_internal::IsTriviallyCopyConstructibleObject<T>, 420 std::is_lvalue_reference<T>>::type::type { 421 #if defined(ABSL_HAVE_STD_IS_TRIVIALLY_CONSTRUCTIBLE) && \ 422 !defined( \ 423 ABSL_META_INTERNAL_STD_CONSTRUCTION_TRAITS_DONT_CHECK_DESTRUCTION) 424 private: 425 static constexpr bool compliant = 426 std::is_trivially_copy_constructible<T>::value == 427 is_trivially_copy_constructible::value; 428 static_assert(compliant || std::is_trivially_copy_constructible<T>::value, 429 "Not compliant with std::is_trivially_copy_constructible; " 430 "Standard: false, Implementation: true"); 431 static_assert(compliant || !std::is_trivially_copy_constructible<T>::value, 432 "Not compliant with std::is_trivially_copy_constructible; " 433 "Standard: true, Implementation: false"); 434 #endif // ABSL_HAVE_STD_IS_TRIVIALLY_CONSTRUCTIBLE 435 }; 436 437 // is_trivially_move_assignable() 438 // 439 // Determines whether the passed type `T` is trivially move assignable. 440 // 441 // This metafunction is designed to be a drop-in replacement for the C++11 442 // `std::is_trivially_move_assignable()` metafunction for platforms that have 443 // incomplete C++11 support (such as libstdc++ 4.x). On any platforms that do 444 // fully support C++11, we check whether this yields the same result as the std 445 // implementation. 446 // 447 // NOTE: `is_assignable<T, U>::value` is `true` if the expression 448 // `declval<T>() = declval<U>()` is well-formed when treated as an unevaluated 449 // operand. `is_trivially_assignable<T, U>` requires the assignment to call no 450 // operation that is not trivial. `is_trivially_copy_assignable<T>` is simply 451 // `is_trivially_assignable<T&, T>`. 452 template <typename T> 453 struct is_trivially_move_assignable 454 : std::conditional< 455 std::is_object<T>::value && !std::is_array<T>::value && 456 std::is_move_assignable<T>::value, 457 std::is_move_assignable<type_traits_internal::SingleMemberUnion<T>>, 458 type_traits_internal::IsTriviallyMoveAssignableReference<T>>::type:: 459 type { 460 #ifdef ABSL_HAVE_STD_IS_TRIVIALLY_ASSIGNABLE 461 private: 462 static constexpr bool compliant = 463 std::is_trivially_move_assignable<T>::value == 464 is_trivially_move_assignable::value; 465 static_assert(compliant || std::is_trivially_move_assignable<T>::value, 466 "Not compliant with std::is_trivially_move_assignable; " 467 "Standard: false, Implementation: true"); 468 static_assert(compliant || !std::is_trivially_move_assignable<T>::value, 469 "Not compliant with std::is_trivially_move_assignable; " 470 "Standard: true, Implementation: false"); 471 #endif // ABSL_HAVE_STD_IS_TRIVIALLY_ASSIGNABLE 472 }; 473 474 // is_trivially_copy_assignable() 475 // 476 // Determines whether the passed type `T` is trivially copy assignable. 477 // 478 // This metafunction is designed to be a drop-in replacement for the C++11 479 // `std::is_trivially_copy_assignable()` metafunction for platforms that have 480 // incomplete C++11 support (such as libstdc++ 4.x). On any platforms that do 481 // fully support C++11, we check whether this yields the same result as the std 482 // implementation. 483 // 484 // NOTE: `is_assignable<T, U>::value` is `true` if the expression 485 // `declval<T>() = declval<U>()` is well-formed when treated as an unevaluated 486 // operand. `is_trivially_assignable<T, U>` requires the assignment to call no 487 // operation that is not trivial. `is_trivially_copy_assignable<T>` is simply 488 // `is_trivially_assignable<T&, const T&>`. 489 template <typename T> 490 struct is_trivially_copy_assignable 491 : std::integral_constant< 492 bool, __has_trivial_assign(typename std::remove_reference<T>::type) && 493 absl::is_copy_assignable<T>::value> { 494 #ifdef ABSL_HAVE_STD_IS_TRIVIALLY_ASSIGNABLE 495 private: 496 static constexpr bool compliant = 497 std::is_trivially_copy_assignable<T>::value == 498 is_trivially_copy_assignable::value; 499 static_assert(compliant || std::is_trivially_copy_assignable<T>::value, 500 "Not compliant with std::is_trivially_copy_assignable; " 501 "Standard: false, Implementation: true"); 502 static_assert(compliant || !std::is_trivially_copy_assignable<T>::value, 503 "Not compliant with std::is_trivially_copy_assignable; " 504 "Standard: true, Implementation: false"); 505 #endif // ABSL_HAVE_STD_IS_TRIVIALLY_ASSIGNABLE 506 }; 507 508 namespace type_traits_internal { 509 // is_trivially_copyable() 510 // 511 // Determines whether the passed type `T` is trivially copyable. 512 // 513 // This metafunction is designed to be a drop-in replacement for the C++11 514 // `std::is_trivially_copyable()` metafunction for platforms that have 515 // incomplete C++11 support (such as libstdc++ 4.x). We use the C++17 definition 516 // of TriviallyCopyable. 517 // 518 // NOTE: `is_trivially_copyable<T>::value` is `true` if all of T's copy/move 519 // constructors/assignment operators are trivial or deleted, T has at least 520 // one non-deleted copy/move constructor/assignment operator, and T is trivially 521 // destructible. Arrays of trivially copyable types are trivially copyable. 522 // 523 // We expose this metafunction only for internal use within absl. 524 template <typename T> 525 class is_trivially_copyable_impl { 526 using ExtentsRemoved = typename std::remove_all_extents<T>::type; 527 static constexpr bool kIsCopyOrMoveConstructible = 528 std::is_copy_constructible<ExtentsRemoved>::value || 529 std::is_move_constructible<ExtentsRemoved>::value; 530 static constexpr bool kIsCopyOrMoveAssignable = 531 absl::is_copy_assignable<ExtentsRemoved>::value || 532 absl::is_move_assignable<ExtentsRemoved>::value; 533 534 public: 535 static constexpr bool kValue = 536 (__has_trivial_copy(ExtentsRemoved) || !kIsCopyOrMoveConstructible) && 537 (__has_trivial_assign(ExtentsRemoved) || !kIsCopyOrMoveAssignable) && 538 (kIsCopyOrMoveConstructible || kIsCopyOrMoveAssignable) && 539 is_trivially_destructible<ExtentsRemoved>::value && 540 // We need to check for this explicitly because otherwise we'll say 541 // references are trivial copyable when compiled by MSVC. 542 !std::is_reference<ExtentsRemoved>::value; 543 }; 544 545 template <typename T> 546 struct is_trivially_copyable 547 : std::integral_constant< 548 bool, type_traits_internal::is_trivially_copyable_impl<T>::kValue> {}; 549 } // namespace type_traits_internal 550 551 // ----------------------------------------------------------------------------- 552 // C++14 "_t" trait aliases 553 // ----------------------------------------------------------------------------- 554 555 template <typename T> 556 using remove_cv_t = typename std::remove_cv<T>::type; 557 558 template <typename T> 559 using remove_const_t = typename std::remove_const<T>::type; 560 561 template <typename T> 562 using remove_volatile_t = typename std::remove_volatile<T>::type; 563 564 template <typename T> 565 using add_cv_t = typename std::add_cv<T>::type; 566 567 template <typename T> 568 using add_const_t = typename std::add_const<T>::type; 569 570 template <typename T> 571 using add_volatile_t = typename std::add_volatile<T>::type; 572 573 template <typename T> 574 using remove_reference_t = typename std::remove_reference<T>::type; 575 576 template <typename T> 577 using add_lvalue_reference_t = typename std::add_lvalue_reference<T>::type; 578 579 template <typename T> 580 using add_rvalue_reference_t = typename std::add_rvalue_reference<T>::type; 581 582 template <typename T> 583 using remove_pointer_t = typename std::remove_pointer<T>::type; 584 585 template <typename T> 586 using add_pointer_t = typename std::add_pointer<T>::type; 587 588 template <typename T> 589 using make_signed_t = typename std::make_signed<T>::type; 590 591 template <typename T> 592 using make_unsigned_t = typename std::make_unsigned<T>::type; 593 594 template <typename T> 595 using remove_extent_t = typename std::remove_extent<T>::type; 596 597 template <typename T> 598 using remove_all_extents_t = typename std::remove_all_extents<T>::type; 599 600 template <size_t Len, size_t Align = type_traits_internal:: 601 default_alignment_of_aligned_storage<Len>::value> 602 using aligned_storage_t = typename std::aligned_storage<Len, Align>::type; 603 604 template <typename T> 605 using decay_t = typename std::decay<T>::type; 606 607 template <bool B, typename T = void> 608 using enable_if_t = typename std::enable_if<B, T>::type; 609 610 template <bool B, typename T, typename F> 611 using conditional_t = typename std::conditional<B, T, F>::type; 612 613 template <typename... T> 614 using common_type_t = typename std::common_type<T...>::type; 615 616 template <typename T> 617 using underlying_type_t = typename std::underlying_type<T>::type; 618 619 template <typename T> 620 using result_of_t = typename std::result_of<T>::type; 621 622 namespace type_traits_internal { 623 // In MSVC we can't probe std::hash or stdext::hash because it triggers a 624 // static_assert instead of failing substitution. Libc++ prior to 4.0 625 // also used a static_assert. 626 // 627 #if defined(_MSC_VER) || (defined(_LIBCPP_VERSION) && \ 628 _LIBCPP_VERSION < 4000 && _LIBCPP_STD_VER > 11) 629 #define ABSL_META_INTERNAL_STD_HASH_SFINAE_FRIENDLY_ 0 630 #else 631 #define ABSL_META_INTERNAL_STD_HASH_SFINAE_FRIENDLY_ 1 632 #endif 633 634 #if !ABSL_META_INTERNAL_STD_HASH_SFINAE_FRIENDLY_ 635 template <typename Key, typename = size_t> 636 struct IsHashable : std::true_type {}; 637 #else // ABSL_META_INTERNAL_STD_HASH_SFINAE_FRIENDLY_ 638 template <typename Key, typename = void> 639 struct IsHashable : std::false_type {}; 640 641 template <typename Key> 642 struct IsHashable< 643 Key, 644 absl::enable_if_t<std::is_convertible< 645 decltype(std::declval<std::hash<Key>&>()(std::declval<Key const&>())), 646 std::size_t>::value>> : std::true_type {}; 647 #endif // !ABSL_META_INTERNAL_STD_HASH_SFINAE_FRIENDLY_ 648 649 struct AssertHashEnabledHelper { 650 private: 651 static void Sink(...) {} 652 struct NAT {}; 653 654 template <class Key> 655 static auto GetReturnType(int) 656 -> decltype(std::declval<std::hash<Key>>()(std::declval<Key const&>())); 657 template <class Key> 658 static NAT GetReturnType(...); 659 660 template <class Key> 661 static std::nullptr_t DoIt() { 662 static_assert(IsHashable<Key>::value, 663 "std::hash<Key> does not provide a call operator"); 664 static_assert( 665 std::is_default_constructible<std::hash<Key>>::value, 666 "std::hash<Key> must be default constructible when it is enabled"); 667 static_assert( 668 std::is_copy_constructible<std::hash<Key>>::value, 669 "std::hash<Key> must be copy constructible when it is enabled"); 670 static_assert(absl::is_copy_assignable<std::hash<Key>>::value, 671 "std::hash<Key> must be copy assignable when it is enabled"); 672 // is_destructible is unchecked as it's implied by each of the 673 // is_constructible checks. 674 using ReturnType = decltype(GetReturnType<Key>(0)); 675 static_assert(std::is_same<ReturnType, NAT>::value || 676 std::is_same<ReturnType, size_t>::value, 677 "std::hash<Key> must return size_t"); 678 return nullptr; 679 } 680 681 template <class... Ts> 682 friend void AssertHashEnabled(); 683 }; 684 685 template <class... Ts> 686 inline void AssertHashEnabled() { 687 using Helper = AssertHashEnabledHelper; 688 Helper::Sink(Helper::DoIt<Ts>()...); 689 } 690 691 } // namespace type_traits_internal 692 693 // An internal namespace that is required to implement the C++17 swap traits. 694 // It is not further nested in type_traits_internal to avoid long symbol names. 695 namespace swap_internal { 696 697 // Necessary for the traits. 698 using std::swap; 699 700 // This declaration prevents global `swap` and `absl::swap` overloads from being 701 // considered unless ADL picks them up. 702 void swap(); 703 704 template <class T> 705 using IsSwappableImpl = decltype(swap(std::declval<T&>(), std::declval<T&>())); 706 707 // NOTE: This dance with the default template parameter is for MSVC. 708 template <class T, 709 class IsNoexcept = std::integral_constant< 710 bool, noexcept(swap(std::declval<T&>(), std::declval<T&>()))>> 711 using IsNothrowSwappableImpl = typename std::enable_if<IsNoexcept::value>::type; 712 713 // IsSwappable 714 // 715 // Determines whether the standard swap idiom is a valid expression for 716 // arguments of type `T`. 717 template <class T> 718 struct IsSwappable 719 : absl::type_traits_internal::is_detected<IsSwappableImpl, T> {}; 720 721 // IsNothrowSwappable 722 // 723 // Determines whether the standard swap idiom is a valid expression for 724 // arguments of type `T` and is noexcept. 725 template <class T> 726 struct IsNothrowSwappable 727 : absl::type_traits_internal::is_detected<IsNothrowSwappableImpl, T> {}; 728 729 // Swap() 730 // 731 // Performs the swap idiom from a namespace where valid candidates may only be 732 // found in `std` or via ADL. 733 template <class T, absl::enable_if_t<IsSwappable<T>::value, int> = 0> 734 void Swap(T& lhs, T& rhs) noexcept(IsNothrowSwappable<T>::value) { 735 swap(lhs, rhs); 736 } 737 738 // StdSwapIsUnconstrained 739 // 740 // Some standard library implementations are broken in that they do not 741 // constrain `std::swap`. This will effectively tell us if we are dealing with 742 // one of those implementations. 743 using StdSwapIsUnconstrained = IsSwappable<void()>; 744 745 } // namespace swap_internal 746 747 namespace type_traits_internal { 748 749 // Make the swap-related traits/function accessible from this namespace. 750 using swap_internal::IsNothrowSwappable; 751 using swap_internal::IsSwappable; 752 using swap_internal::Swap; 753 using swap_internal::StdSwapIsUnconstrained; 754 755 } // namespace type_traits_internal 756 ABSL_NAMESPACE_END 757 } // namespace absl 758 759 #endif // ABSL_META_TYPE_TRAITS_H_ 760