libstdc++
any
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1 // <any> -*- C++ -*-
2 
3 // Copyright (C) 2014-2021 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/any
26  * This is a Standard C++ Library header.
27  */
28 
29 #ifndef _GLIBCXX_ANY
30 #define _GLIBCXX_ANY 1
31 
32 #pragma GCC system_header
33 
34 #if __cplusplus >= 201703L
35 
36 #include <typeinfo>
37 #include <new>
38 #include <utility>
39 #include <type_traits>
40 
41 namespace std _GLIBCXX_VISIBILITY(default)
42 {
43 _GLIBCXX_BEGIN_NAMESPACE_VERSION
44 
45  /**
46  * @addtogroup utilities
47  * @{
48  */
49 
50  /**
51  * @brief Exception class thrown by a failed @c any_cast
52  * @ingroup exceptions
53  */
54  class bad_any_cast : public bad_cast
55  {
56  public:
57  virtual const char* what() const noexcept { return "bad any_cast"; }
58  };
59 
60  [[gnu::noreturn]] inline void __throw_bad_any_cast()
61  {
62 #if __cpp_exceptions
63  throw bad_any_cast{};
64 #else
65  __builtin_abort();
66 #endif
67  }
68 
69 #define __cpp_lib_any 201606L
70 
71  /**
72  * @brief A type-safe container of any type.
73  *
74  * An @c any object's state is either empty or it stores a contained object
75  * of CopyConstructible type.
76  */
77  class any
78  {
79  // Holds either pointer to a heap object or the contained object itself.
80  union _Storage
81  {
82  constexpr _Storage() : _M_ptr{nullptr} {}
83 
84  // Prevent trivial copies of this type, buffer might hold a non-POD.
85  _Storage(const _Storage&) = delete;
86  _Storage& operator=(const _Storage&) = delete;
87 
88  void* _M_ptr;
89  aligned_storage<sizeof(_M_ptr), alignof(void*)>::type _M_buffer;
90  };
91 
92  template<typename _Tp, typename _Safe = is_nothrow_move_constructible<_Tp>,
93  bool _Fits = (sizeof(_Tp) <= sizeof(_Storage))
94  && (alignof(_Tp) <= alignof(_Storage))>
95  using _Internal = std::integral_constant<bool, _Safe::value && _Fits>;
96 
97  template<typename _Tp>
98  struct _Manager_internal; // uses small-object optimization
99 
100  template<typename _Tp>
101  struct _Manager_external; // creates contained object on the heap
102 
103  template<typename _Tp>
104  using _Manager = conditional_t<_Internal<_Tp>::value,
105  _Manager_internal<_Tp>,
106  _Manager_external<_Tp>>;
107 
108  template<typename _Tp, typename _VTp = decay_t<_Tp>>
109  using _Decay_if_not_any = enable_if_t<!is_same_v<_VTp, any>, _VTp>;
110 
111  /// Emplace with an object created from @p __args as the contained object.
112  template <typename _Tp, typename... _Args,
113  typename _Mgr = _Manager<_Tp>>
114  void __do_emplace(_Args&&... __args)
115  {
116  reset();
117  _Mgr::_S_create(_M_storage, std::forward<_Args>(__args)...);
118  _M_manager = &_Mgr::_S_manage;
119  }
120 
121  /// Emplace with an object created from @p __il and @p __args as
122  /// the contained object.
123  template <typename _Tp, typename _Up, typename... _Args,
124  typename _Mgr = _Manager<_Tp>>
125  void __do_emplace(initializer_list<_Up> __il, _Args&&... __args)
126  {
127  reset();
128  _Mgr::_S_create(_M_storage, __il, std::forward<_Args>(__args)...);
129  _M_manager = &_Mgr::_S_manage;
130  }
131 
132  template <typename _Res, typename _Tp, typename... _Args>
133  using __any_constructible
135  is_constructible<_Tp, _Args...>>::value,
136  _Res>;
137 
138  template <typename _Tp, typename... _Args>
139  using __any_constructible_t
140  = typename __any_constructible<bool, _Tp, _Args...>::type;
141 
142  template<typename _VTp, typename... _Args>
143  using __emplace_t
144  = typename __any_constructible<_VTp&, _VTp, _Args...>::type;
145 
146  public:
147  // construct/destruct
148 
149  /// Default constructor, creates an empty object.
150  constexpr any() noexcept : _M_manager(nullptr) { }
151 
152  /// Copy constructor, copies the state of @p __other
153  any(const any& __other)
154  {
155  if (!__other.has_value())
156  _M_manager = nullptr;
157  else
158  {
159  _Arg __arg;
160  __arg._M_any = this;
161  __other._M_manager(_Op_clone, &__other, &__arg);
162  }
163  }
164 
165  /**
166  * @brief Move constructor, transfer the state from @p __other
167  *
168  * @post @c !__other.has_value() (this postcondition is a GNU extension)
169  */
170  any(any&& __other) noexcept
171  {
172  if (!__other.has_value())
173  _M_manager = nullptr;
174  else
175  {
176  _Arg __arg;
177  __arg._M_any = this;
178  __other._M_manager(_Op_xfer, &__other, &__arg);
179  }
180  }
181 
182  /// Construct with a copy of @p __value as the contained object.
183  template <typename _Tp, typename _VTp = _Decay_if_not_any<_Tp>,
184  typename _Mgr = _Manager<_VTp>,
185  enable_if_t<is_copy_constructible<_VTp>::value
186  && !__is_in_place_type<_VTp>::value, bool> = true>
187  any(_Tp&& __value)
188  : _M_manager(&_Mgr::_S_manage)
189  {
190  _Mgr::_S_create(_M_storage, std::forward<_Tp>(__value));
191  }
192 
193  /// Construct with an object created from @p __args as the contained object.
194  template <typename _Tp, typename... _Args, typename _VTp = decay_t<_Tp>,
195  typename _Mgr = _Manager<_VTp>,
196  __any_constructible_t<_VTp, _Args&&...> = false>
197  explicit
198  any(in_place_type_t<_Tp>, _Args&&... __args)
199  : _M_manager(&_Mgr::_S_manage)
200  {
201  _Mgr::_S_create(_M_storage, std::forward<_Args>(__args)...);
202  }
203 
204  /// Construct with an object created from @p __il and @p __args as
205  /// the contained object.
206  template <typename _Tp, typename _Up, typename... _Args,
207  typename _VTp = decay_t<_Tp>, typename _Mgr = _Manager<_VTp>,
208  __any_constructible_t<_VTp, initializer_list<_Up>&,
209  _Args&&...> = false>
210  explicit
211  any(in_place_type_t<_Tp>, initializer_list<_Up> __il, _Args&&... __args)
212  : _M_manager(&_Mgr::_S_manage)
213  {
214  _Mgr::_S_create(_M_storage, __il, std::forward<_Args>(__args)...);
215  }
216 
217  /// Destructor, calls @c reset()
218  ~any() { reset(); }
219 
220  // assignments
221 
222  /// Copy the state of another object.
223  any&
224  operator=(const any& __rhs)
225  {
226  *this = any(__rhs);
227  return *this;
228  }
229 
230  /**
231  * @brief Move assignment operator
232  *
233  * @post @c !__rhs.has_value() (not guaranteed for other implementations)
234  */
235  any&
236  operator=(any&& __rhs) noexcept
237  {
238  if (!__rhs.has_value())
239  reset();
240  else if (this != &__rhs)
241  {
242  reset();
243  _Arg __arg;
244  __arg._M_any = this;
245  __rhs._M_manager(_Op_xfer, &__rhs, &__arg);
246  }
247  return *this;
248  }
249 
250  /// Store a copy of @p __rhs as the contained object.
251  template<typename _Tp>
253  operator=(_Tp&& __rhs)
254  {
255  *this = any(std::forward<_Tp>(__rhs));
256  return *this;
257  }
258 
259  /// Emplace with an object created from @p __args as the contained object.
260  template <typename _Tp, typename... _Args>
261  __emplace_t<decay_t<_Tp>, _Args...>
262  emplace(_Args&&... __args)
263  {
264  using _VTp = decay_t<_Tp>;
265  __do_emplace<_VTp>(std::forward<_Args>(__args)...);
266  return *any::_Manager<_VTp>::_S_access(_M_storage);
267  }
268 
269  /// Emplace with an object created from @p __il and @p __args as
270  /// the contained object.
271  template <typename _Tp, typename _Up, typename... _Args>
272  __emplace_t<decay_t<_Tp>, initializer_list<_Up>&, _Args&&...>
273  emplace(initializer_list<_Up> __il, _Args&&... __args)
274  {
275  using _VTp = decay_t<_Tp>;
276  __do_emplace<_VTp, _Up>(__il, std::forward<_Args>(__args)...);
277  return *any::_Manager<_VTp>::_S_access(_M_storage);
278  }
279 
280  // modifiers
281 
282  /// If not empty, destroy the contained object.
283  void reset() noexcept
284  {
285  if (has_value())
286  {
287  _M_manager(_Op_destroy, this, nullptr);
288  _M_manager = nullptr;
289  }
290  }
291 
292  /// Exchange state with another object.
293  void swap(any& __rhs) noexcept
294  {
295  if (!has_value() && !__rhs.has_value())
296  return;
297 
298  if (has_value() && __rhs.has_value())
299  {
300  if (this == &__rhs)
301  return;
302 
303  any __tmp;
304  _Arg __arg;
305  __arg._M_any = &__tmp;
306  __rhs._M_manager(_Op_xfer, &__rhs, &__arg);
307  __arg._M_any = &__rhs;
308  _M_manager(_Op_xfer, this, &__arg);
309  __arg._M_any = this;
310  __tmp._M_manager(_Op_xfer, &__tmp, &__arg);
311  }
312  else
313  {
314  any* __empty = !has_value() ? this : &__rhs;
315  any* __full = !has_value() ? &__rhs : this;
316  _Arg __arg;
317  __arg._M_any = __empty;
318  __full->_M_manager(_Op_xfer, __full, &__arg);
319  }
320  }
321 
322  // observers
323 
324  /// Reports whether there is a contained object or not.
325  bool has_value() const noexcept { return _M_manager != nullptr; }
326 
327 #if __cpp_rtti
328  /// The @c typeid of the contained object, or @c typeid(void) if empty.
329  const type_info& type() const noexcept
330  {
331  if (!has_value())
332  return typeid(void);
333  _Arg __arg;
334  _M_manager(_Op_get_type_info, this, &__arg);
335  return *__arg._M_typeinfo;
336  }
337 #endif
338 
339  template<typename _Tp>
340  static constexpr bool __is_valid_cast()
341  { return __or_<is_reference<_Tp>, is_copy_constructible<_Tp>>::value; }
342 
343  private:
344  enum _Op {
345  _Op_access, _Op_get_type_info, _Op_clone, _Op_destroy, _Op_xfer
346  };
347 
348  union _Arg
349  {
350  void* _M_obj;
351  const std::type_info* _M_typeinfo;
352  any* _M_any;
353  };
354 
355  void (*_M_manager)(_Op, const any*, _Arg*);
356  _Storage _M_storage;
357 
358  template<typename _Tp>
359  friend void* __any_caster(const any* __any);
360 
361  // Manage in-place contained object.
362  template<typename _Tp>
363  struct _Manager_internal
364  {
365  static void
366  _S_manage(_Op __which, const any* __anyp, _Arg* __arg);
367 
368  template<typename _Up>
369  static void
370  _S_create(_Storage& __storage, _Up&& __value)
371  {
372  void* __addr = &__storage._M_buffer;
373  ::new (__addr) _Tp(std::forward<_Up>(__value));
374  }
375 
376  template<typename... _Args>
377  static void
378  _S_create(_Storage& __storage, _Args&&... __args)
379  {
380  void* __addr = &__storage._M_buffer;
381  ::new (__addr) _Tp(std::forward<_Args>(__args)...);
382  }
383 
384  static _Tp*
385  _S_access(const _Storage& __storage)
386  {
387  // The contained object is in __storage._M_buffer
388  const void* __addr = &__storage._M_buffer;
389  return static_cast<_Tp*>(const_cast<void*>(__addr));
390  }
391  };
392 
393  // Manage external contained object.
394  template<typename _Tp>
395  struct _Manager_external
396  {
397  static void
398  _S_manage(_Op __which, const any* __anyp, _Arg* __arg);
399 
400  template<typename _Up>
401  static void
402  _S_create(_Storage& __storage, _Up&& __value)
403  {
404  __storage._M_ptr = new _Tp(std::forward<_Up>(__value));
405  }
406  template<typename... _Args>
407  static void
408  _S_create(_Storage& __storage, _Args&&... __args)
409  {
410  __storage._M_ptr = new _Tp(std::forward<_Args>(__args)...);
411  }
412  static _Tp*
413  _S_access(const _Storage& __storage)
414  {
415  // The contained object is in *__storage._M_ptr
416  return static_cast<_Tp*>(__storage._M_ptr);
417  }
418  };
419  };
420 
421  /// Exchange the states of two @c any objects.
422  inline void swap(any& __x, any& __y) noexcept { __x.swap(__y); }
423 
424  /// Create an any holding a @c _Tp constructed from @c __args.
425  template <typename _Tp, typename... _Args>
426  any make_any(_Args&&... __args)
427  {
428  return any(in_place_type<_Tp>, std::forward<_Args>(__args)...);
429  }
430 
431  /// Create an any holding a @c _Tp constructed from @c __il and @c __args.
432  template <typename _Tp, typename _Up, typename... _Args>
433  any make_any(initializer_list<_Up> __il, _Args&&... __args)
434  {
435  return any(in_place_type<_Tp>, __il, std::forward<_Args>(__args)...);
436  }
437 
438  /**
439  * @brief Access the contained object.
440  *
441  * @tparam _ValueType A const-reference or CopyConstructible type.
442  * @param __any The object to access.
443  * @return The contained object.
444  * @throw bad_any_cast If <code>
445  * __any.type() != typeid(remove_reference_t<_ValueType>)
446  * </code>
447  */
448  template<typename _ValueType>
449  inline _ValueType any_cast(const any& __any)
450  {
451  using _Up = __remove_cvref_t<_ValueType>;
452  static_assert(any::__is_valid_cast<_ValueType>(),
453  "Template argument must be a reference or CopyConstructible type");
454  static_assert(is_constructible_v<_ValueType, const _Up&>,
455  "Template argument must be constructible from a const value.");
456  auto __p = any_cast<_Up>(&__any);
457  if (__p)
458  return static_cast<_ValueType>(*__p);
459  __throw_bad_any_cast();
460  }
461 
462  /**
463  * @brief Access the contained object.
464  *
465  * @tparam _ValueType A reference or CopyConstructible type.
466  * @param __any The object to access.
467  * @return The contained object.
468  * @throw bad_any_cast If <code>
469  * __any.type() != typeid(remove_reference_t<_ValueType>)
470  * </code>
471  *
472  * @{
473  */
474  template<typename _ValueType>
475  inline _ValueType any_cast(any& __any)
476  {
477  using _Up = __remove_cvref_t<_ValueType>;
478  static_assert(any::__is_valid_cast<_ValueType>(),
479  "Template argument must be a reference or CopyConstructible type");
480  static_assert(is_constructible_v<_ValueType, _Up&>,
481  "Template argument must be constructible from an lvalue.");
482  auto __p = any_cast<_Up>(&__any);
483  if (__p)
484  return static_cast<_ValueType>(*__p);
485  __throw_bad_any_cast();
486  }
487 
488  template<typename _ValueType>
489  inline _ValueType any_cast(any&& __any)
490  {
491  using _Up = __remove_cvref_t<_ValueType>;
492  static_assert(any::__is_valid_cast<_ValueType>(),
493  "Template argument must be a reference or CopyConstructible type");
494  static_assert(is_constructible_v<_ValueType, _Up>,
495  "Template argument must be constructible from an rvalue.");
496  auto __p = any_cast<_Up>(&__any);
497  if (__p)
498  return static_cast<_ValueType>(std::move(*__p));
499  __throw_bad_any_cast();
500  }
501  /// @}
502 
503  /// @cond undocumented
504  template<typename _Tp>
505  void* __any_caster(const any* __any)
506  {
507  // any_cast<T> returns non-null if __any->type() == typeid(T) and
508  // typeid(T) ignores cv-qualifiers so remove them:
509  using _Up = remove_cv_t<_Tp>;
510  // The contained value has a decayed type, so if decay_t<U> is not U,
511  // then it's not possible to have a contained value of type U:
512  if constexpr (!is_same_v<decay_t<_Up>, _Up>)
513  return nullptr;
514  // Only copy constructible types can be used for contained values:
515  else if constexpr (!is_copy_constructible_v<_Up>)
516  return nullptr;
517  // First try comparing function addresses, which works without RTTI
518  else if (__any->_M_manager == &any::_Manager<_Up>::_S_manage
519 #if __cpp_rtti
520  || __any->type() == typeid(_Tp)
521 #endif
522  )
523  {
524  return any::_Manager<_Up>::_S_access(__any->_M_storage);
525  }
526  return nullptr;
527  }
528  /// @endcond
529 
530  /**
531  * @brief Access the contained object.
532  *
533  * @tparam _ValueType The type of the contained object.
534  * @param __any A pointer to the object to access.
535  * @return The address of the contained object if <code>
536  * __any != nullptr && __any.type() == typeid(_ValueType)
537  * </code>, otherwise a null pointer.
538  *
539  * @{
540  */
541  template<typename _ValueType>
542  inline const _ValueType* any_cast(const any* __any) noexcept
543  {
544  if constexpr (is_object_v<_ValueType>)
545  if (__any)
546  return static_cast<_ValueType*>(__any_caster<_ValueType>(__any));
547  return nullptr;
548  }
549 
550  template<typename _ValueType>
551  inline _ValueType* any_cast(any* __any) noexcept
552  {
553  if constexpr (is_object_v<_ValueType>)
554  if (__any)
555  return static_cast<_ValueType*>(__any_caster<_ValueType>(__any));
556  return nullptr;
557  }
558  /// @}
559 
560  template<typename _Tp>
561  void
562  any::_Manager_internal<_Tp>::
563  _S_manage(_Op __which, const any* __any, _Arg* __arg)
564  {
565  // The contained object is in _M_storage._M_buffer
566  auto __ptr = reinterpret_cast<const _Tp*>(&__any->_M_storage._M_buffer);
567  switch (__which)
568  {
569  case _Op_access:
570  __arg->_M_obj = const_cast<_Tp*>(__ptr);
571  break;
572  case _Op_get_type_info:
573 #if __cpp_rtti
574  __arg->_M_typeinfo = &typeid(_Tp);
575 #endif
576  break;
577  case _Op_clone:
578  ::new(&__arg->_M_any->_M_storage._M_buffer) _Tp(*__ptr);
579  __arg->_M_any->_M_manager = __any->_M_manager;
580  break;
581  case _Op_destroy:
582  __ptr->~_Tp();
583  break;
584  case _Op_xfer:
585  ::new(&__arg->_M_any->_M_storage._M_buffer) _Tp
586  (std::move(*const_cast<_Tp*>(__ptr)));
587  __ptr->~_Tp();
588  __arg->_M_any->_M_manager = __any->_M_manager;
589  const_cast<any*>(__any)->_M_manager = nullptr;
590  break;
591  }
592  }
593 
594  template<typename _Tp>
595  void
596  any::_Manager_external<_Tp>::
597  _S_manage(_Op __which, const any* __any, _Arg* __arg)
598  {
599  // The contained object is *_M_storage._M_ptr
600  auto __ptr = static_cast<const _Tp*>(__any->_M_storage._M_ptr);
601  switch (__which)
602  {
603  case _Op_access:
604  __arg->_M_obj = const_cast<_Tp*>(__ptr);
605  break;
606  case _Op_get_type_info:
607 #if __cpp_rtti
608  __arg->_M_typeinfo = &typeid(_Tp);
609 #endif
610  break;
611  case _Op_clone:
612  __arg->_M_any->_M_storage._M_ptr = new _Tp(*__ptr);
613  __arg->_M_any->_M_manager = __any->_M_manager;
614  break;
615  case _Op_destroy:
616  delete __ptr;
617  break;
618  case _Op_xfer:
619  __arg->_M_any->_M_storage._M_ptr = __any->_M_storage._M_ptr;
620  __arg->_M_any->_M_manager = __any->_M_manager;
621  const_cast<any*>(__any)->_M_manager = nullptr;
622  break;
623  }
624  }
625 
626  /// @}
627 
628  namespace __detail::__variant
629  {
630  template<typename> struct _Never_valueless_alt; // see <variant>
631 
632  // Provide the strong exception-safety guarantee when emplacing an
633  // any into a variant.
634  template<>
635  struct _Never_valueless_alt<std::any>
637  { };
638  } // namespace __detail::__variant
639 
640 _GLIBCXX_END_NAMESPACE_VERSION
641 } // namespace std
642 
643 #endif // C++17
644 #endif // _GLIBCXX_ANY
typename conditional< _Cond, _Iftrue, _Iffalse >::type conditional_t
Alias template for conditional.
Definition: type_traits:2589
typename decay< _Tp >::type decay_t
Alias template for decay.
Definition: type_traits:2581
typename enable_if< _Cond, _Tp >::type enable_if_t
Alias template for enable_if.
Definition: type_traits:2585
_ValueType any_cast(const any &__any)
Access the contained object.
Definition: any:449
constexpr std::remove_reference< _Tp >::type && move(_Tp &&__t) noexcept
Convert a value to an rvalue.
Definition: move.h:104
any make_any(_Args &&... __args)
Create an any holding a _Tp constructed from __args.
Definition: any:426
void swap(any &__x, any &__y) noexcept
Exchange the states of two any objects.
Definition: any:422
ISO C++ entities toplevel namespace is std.
initializer_list
Part of RTTI.
Definition: typeinfo:89
Thrown during incorrect typecasting.
Definition: typeinfo:191
Exception class thrown by a failed any_cast.
Definition: any:55
virtual const char * what() const noexcept
Definition: any:57
A type-safe container of any type.
Definition: any:78
enable_if_t< is_copy_constructible< _Decay_if_not_any< _Tp > >::value, any & > operator=(_Tp &&__rhs)
Store a copy of __rhs as the contained object.
Definition: any:253
__emplace_t< decay_t< _Tp >, _Args... > emplace(_Args &&... __args)
Emplace with an object created from __args as the contained object.
Definition: any:262
any & operator=(const any &__rhs)
Copy the state of another object.
Definition: any:224
__emplace_t< decay_t< _Tp >, initializer_list< _Up > &, _Args &&... > emplace(initializer_list< _Up > __il, _Args &&... __args)
Emplace with an object created from __il and __args as the contained object.
Definition: any:273
any(_Tp &&__value)
Construct with a copy of __value as the contained object.
Definition: any:187
any(const any &__other)
Copy constructor, copies the state of __other.
Definition: any:153
any(any &&__other) noexcept
Move constructor, transfer the state from __other.
Definition: any:170
any & operator=(any &&__rhs) noexcept
Move assignment operator.
Definition: any:236
~any()
Destructor, calls reset()
Definition: any:218
constexpr any() noexcept
Default constructor, creates an empty object.
Definition: any:150
const type_info & type() const noexcept
The typeid of the contained object, or typeid(void) if empty.
Definition: any:329
any(in_place_type_t< _Tp >, initializer_list< _Up > __il, _Args &&... __args)
Construct with an object created from __il and __args as the contained object.
Definition: any:211
void reset() noexcept
If not empty, destroy the contained object.
Definition: any:283
any(in_place_type_t< _Tp >, _Args &&... __args)
Construct with an object created from __args as the contained object.
Definition: any:198
void swap(any &__rhs) noexcept
Exchange state with another object.
Definition: any:293
bool has_value() const noexcept
Reports whether there is a contained object or not.
Definition: any:325
integral_constant
Definition: type_traits:66
is_constructible
Definition: type_traits:954
is_copy_constructible
Definition: type_traits:986
Alignment type.
Definition: type_traits:2085
Define a member typedef type only if a boolean constant is true.
Definition: type_traits:2200