1// Implementation of std::function -*- 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/std_function.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_STD_FUNCTION_H
31#define _GLIBCXX_STD_FUNCTION_H 1
32
33#pragma GCC system_header
34
35#if __cplusplus < 201103L
36# include <bits/c++0x_warning.h>
37#else
38
39#if __cpp_rtti
40# include <typeinfo>
41#endif
42#include <bits/stl_function.h>
43#include <bits/invoke.h>
44#include <bits/refwrap.h>
45#include <bits/functexcept.h>
46
47namespace std _GLIBCXX_VISIBILITY(default)
48{
49_GLIBCXX_BEGIN_NAMESPACE_VERSION
50
51 /**
52 * @brief Exception class thrown when class template function's
53 * operator() is called with an empty target.
54 * @ingroup exceptions
55 */
56 class bad_function_call : public std::exception
57 {
58 public:
59 virtual ~bad_function_call() noexcept;
60
61 const char* what() const noexcept;
62 };
63
64 /**
65 * Trait identifying "location-invariant" types, meaning that the
66 * address of the object (or any of its members) will not escape.
67 * Trivially copyable types are location-invariant and users can
68 * specialize this trait for other types.
69 */
70 template<typename _Tp>
71 struct __is_location_invariant
72 : is_trivially_copyable<_Tp>::type
73 { };
74
75 class _Undefined_class;
76
77 union _Nocopy_types
78 {
79 void* _M_object;
80 const void* _M_const_object;
81 void (*_M_function_pointer)();
82 void (_Undefined_class::*_M_member_pointer)();
83 };
84
85 union [[gnu::may_alias]] _Any_data
86 {
87 void* _M_access() { return &_M_pod_data[0]; }
88 const void* _M_access() const { return &_M_pod_data[0]; }
89
90 template<typename _Tp>
91 _Tp&
92 _M_access()
93 { return *static_cast<_Tp*>(_M_access()); }
94
95 template<typename _Tp>
96 const _Tp&
97 _M_access() const
98 { return *static_cast<const _Tp*>(_M_access()); }
99
100 _Nocopy_types _M_unused;
101 char _M_pod_data[sizeof(_Nocopy_types)];
102 };
103
104 enum _Manager_operation
105 {
106 __get_type_info,
107 __get_functor_ptr,
108 __clone_functor,
109 __destroy_functor
110 };
111
112 // Simple type wrapper that helps avoid annoying const problems
113 // when casting between void pointers and pointers-to-pointers.
114 template<typename _Tp>
115 struct _Simple_type_wrapper
116 {
117 _Simple_type_wrapper(_Tp __value) : __value(__value) { }
118
119 _Tp __value;
120 };
121
122 template<typename _Tp>
123 struct __is_location_invariant<_Simple_type_wrapper<_Tp> >
124 : __is_location_invariant<_Tp>
125 { };
126
127 template<typename _Signature>
128 class function;
129
130 /// Base class of all polymorphic function object wrappers.
131 class _Function_base
132 {
133 public:
134 static const size_t _M_max_size = sizeof(_Nocopy_types);
135 static const size_t _M_max_align = __alignof__(_Nocopy_types);
136
137 template<typename _Functor>
138 class _Base_manager
139 {
140 protected:
141 static const bool __stored_locally =
142 (__is_location_invariant<_Functor>::value
143 && sizeof(_Functor) <= _M_max_size
144 && __alignof__(_Functor) <= _M_max_align
145 && (_M_max_align % __alignof__(_Functor) == 0));
146
147 typedef integral_constant<bool, __stored_locally> _Local_storage;
148
149 // Retrieve a pointer to the function object
150 static _Functor*
151 _M_get_pointer(const _Any_data& __source)
152 {
153 if _GLIBCXX17_CONSTEXPR (__stored_locally)
154 {
155 const _Functor& __f = __source._M_access<_Functor>();
156 return const_cast<_Functor*>(std::__addressof(__f));
157 }
158 else // have stored a pointer
159 return __source._M_access<_Functor*>();
160 }
161
162 // Clone a location-invariant function object that fits within
163 // an _Any_data structure.
164 static void
165 _M_clone(_Any_data& __dest, const _Any_data& __source, true_type)
166 {
167 ::new (__dest._M_access()) _Functor(__source._M_access<_Functor>());
168 }
169
170 // Clone a function object that is not location-invariant or
171 // that cannot fit into an _Any_data structure.
172 static void
173 _M_clone(_Any_data& __dest, const _Any_data& __source, false_type)
174 {
175 __dest._M_access<_Functor*>() =
176 new _Functor(*__source._M_access<const _Functor*>());
177 }
178
179 // Destroying a location-invariant object may still require
180 // destruction.
181 static void
182 _M_destroy(_Any_data& __victim, true_type)
183 {
184 __victim._M_access<_Functor>().~_Functor();
185 }
186
187 // Destroying an object located on the heap.
188 static void
189 _M_destroy(_Any_data& __victim, false_type)
190 {
191 delete __victim._M_access<_Functor*>();
192 }
193
194 public:
195 static bool
196 _M_manager(_Any_data& __dest, const _Any_data& __source,
197 _Manager_operation __op)
198 {
199 switch (__op)
200 {
201#if __cpp_rtti
202 case __get_type_info:
203 __dest._M_access<const type_info*>() = &typeid(_Functor);
204 break;
205#endif
206 case __get_functor_ptr:
207 __dest._M_access<_Functor*>() = _M_get_pointer(__source);
208 break;
209
210 case __clone_functor:
211 _M_clone(__dest, __source, _Local_storage());
212 break;
213
214 case __destroy_functor:
215 _M_destroy(__dest, _Local_storage());
216 break;
217 }
218 return false;
219 }
220
221 static void
222 _M_init_functor(_Any_data& __functor, _Functor&& __f)
223 { _M_init_functor(__functor, std::move(__f), _Local_storage()); }
224
225 template<typename _Signature>
226 static bool
227 _M_not_empty_function(const function<_Signature>& __f)
228 { return static_cast<bool>(__f); }
229
230 template<typename _Tp>
231 static bool
232 _M_not_empty_function(_Tp* __fp)
233 { return __fp != nullptr; }
234
235 template<typename _Class, typename _Tp>
236 static bool
237 _M_not_empty_function(_Tp _Class::* __mp)
238 { return __mp != nullptr; }
239
240 template<typename _Tp>
241 static bool
242 _M_not_empty_function(const _Tp&)
243 { return true; }
244
245 private:
246 static void
247 _M_init_functor(_Any_data& __functor, _Functor&& __f, true_type)
248 { ::new (__functor._M_access()) _Functor(std::move(__f)); }
249
250 static void
251 _M_init_functor(_Any_data& __functor, _Functor&& __f, false_type)
252 { __functor._M_access<_Functor*>() = new _Functor(std::move(__f)); }
253 };
254
255 _Function_base() : _M_manager(nullptr) { }
256
257 ~_Function_base()
258 {
259 if (_M_manager)
260 _M_manager(_M_functor, _M_functor, __destroy_functor);
261 }
262
263 bool _M_empty() const { return !_M_manager; }
264
265 typedef bool (*_Manager_type)(_Any_data&, const _Any_data&,
266 _Manager_operation);
267
268 _Any_data _M_functor;
269 _Manager_type _M_manager;
270 };
271
272 template<typename _Signature, typename _Functor>
273 class _Function_handler;
274
275 template<typename _Res, typename _Functor, typename... _ArgTypes>
276 class _Function_handler<_Res(_ArgTypes...), _Functor>
277 : public _Function_base::_Base_manager<_Functor>
278 {
279 typedef _Function_base::_Base_manager<_Functor> _Base;
280
281 public:
282 static _Res
283 _M_invoke(const _Any_data& __functor, _ArgTypes&&... __args)
284 {
285 return (*_Base::_M_get_pointer(__functor))(
286 std::forward<_ArgTypes>(__args)...);
287 }
288 };
289
290 template<typename _Functor, typename... _ArgTypes>
291 class _Function_handler<void(_ArgTypes...), _Functor>
292 : public _Function_base::_Base_manager<_Functor>
293 {
294 typedef _Function_base::_Base_manager<_Functor> _Base;
295
296 public:
297 static void
298 _M_invoke(const _Any_data& __functor, _ArgTypes&&... __args)
299 {
300 (*_Base::_M_get_pointer(__functor))(
301 std::forward<_ArgTypes>(__args)...);
302 }
303 };
304
305 template<typename _Class, typename _Member, typename _Res,
306 typename... _ArgTypes>
307 class _Function_handler<_Res(_ArgTypes...), _Member _Class::*>
308 : public _Function_handler<void(_ArgTypes...), _Member _Class::*>
309 {
310 typedef _Function_handler<void(_ArgTypes...), _Member _Class::*>
311 _Base;
312
313 public:
314 static _Res
315 _M_invoke(const _Any_data& __functor, _ArgTypes&&... __args)
316 {
317 return std::__invoke(_Base::_M_get_pointer(__functor)->__value,
318 std::forward<_ArgTypes>(__args)...);
319 }
320 };
321
322 template<typename _Class, typename _Member, typename... _ArgTypes>
323 class _Function_handler<void(_ArgTypes...), _Member _Class::*>
324 : public _Function_base::_Base_manager<
325 _Simple_type_wrapper< _Member _Class::* > >
326 {
327 typedef _Member _Class::* _Functor;
328 typedef _Simple_type_wrapper<_Functor> _Wrapper;
329 typedef _Function_base::_Base_manager<_Wrapper> _Base;
330
331 public:
332 static bool
333 _M_manager(_Any_data& __dest, const _Any_data& __source,
334 _Manager_operation __op)
335 {
336 switch (__op)
337 {
338#if __cpp_rtti
339 case __get_type_info:
340 __dest._M_access<const type_info*>() = &typeid(_Functor);
341 break;
342#endif
343 case __get_functor_ptr:
344 __dest._M_access<_Functor*>() =
345 &_Base::_M_get_pointer(__source)->__value;
346 break;
347
348 default:
349 _Base::_M_manager(__dest, __source, __op);
350 }
351 return false;
352 }
353
354 static void
355 _M_invoke(const _Any_data& __functor, _ArgTypes&&... __args)
356 {
357 std::__invoke(_Base::_M_get_pointer(__functor)->__value,
358 std::forward<_ArgTypes>(__args)...);
359 }
360 };
361
362 template<typename _From, typename _To>
363 using __check_func_return_type
364 = __or_<is_void<_To>, is_same<_From, _To>, is_convertible<_From, _To>>;
365
366 /**
367 * @brief Primary class template for std::function.
368 * @ingroup functors
369 *
370 * Polymorphic function wrapper.
371 */
372 template<typename _Res, typename... _ArgTypes>
373 class function<_Res(_ArgTypes...)>
374 : public _Maybe_unary_or_binary_function<_Res, _ArgTypes...>,
375 private _Function_base
376 {
377 template<typename _Func,
378 typename _Res2 = typename result_of<_Func&(_ArgTypes...)>::type>
379 struct _Callable : __check_func_return_type<_Res2, _Res> { };
380
381 // Used so the return type convertibility checks aren't done when
382 // performing overload resolution for copy construction/assignment.
383 template<typename _Tp>
384 struct _Callable<function, _Tp> : false_type { };
385
386 template<typename _Cond, typename _Tp>
387 using _Requires = typename enable_if<_Cond::value, _Tp>::type;
388
389 public:
390 typedef _Res result_type;
391
392 // [3.7.2.1] construct/copy/destroy
393
394 /**
395 * @brief Default construct creates an empty function call wrapper.
396 * @post @c !(bool)*this
397 */
398 function() noexcept
399 : _Function_base() { }
400
401 /**
402 * @brief Creates an empty function call wrapper.
403 * @post @c !(bool)*this
404 */
405 function(nullptr_t) noexcept
406 : _Function_base() { }
407
408 /**
409 * @brief %Function copy constructor.
410 * @param __x A %function object with identical call signature.
411 * @post @c bool(*this) == bool(__x)
412 *
413 * The newly-created %function contains a copy of the target of @a
414 * __x (if it has one).
415 */
416 function(const function& __x);
417
418 /**
419 * @brief %Function move constructor.
420 * @param __x A %function object rvalue with identical call signature.
421 *
422 * The newly-created %function contains the target of @a __x
423 * (if it has one).
424 */
425 function(function&& __x) noexcept : _Function_base()
426 {
427 __x.swap(*this);
428 }
429
430 /**
431 * @brief Builds a %function that targets a copy of the incoming
432 * function object.
433 * @param __f A %function object that is callable with parameters of
434 * type @c T1, @c T2, ..., @c TN and returns a value convertible
435 * to @c Res.
436 *
437 * The newly-created %function object will target a copy of
438 * @a __f. If @a __f is @c reference_wrapper<F>, then this function
439 * object will contain a reference to the function object @c
440 * __f.get(). If @a __f is a NULL function pointer or NULL
441 * pointer-to-member, the newly-created object will be empty.
442 *
443 * If @a __f is a non-NULL function pointer or an object of type @c
444 * reference_wrapper<F>, this function will not throw.
445 */
446 template<typename _Functor,
447 typename = _Requires<__not_<is_same<_Functor, function>>, void>,
448 typename = _Requires<_Callable<_Functor>, void>>
449 function(_Functor);
450
451 /**
452 * @brief %Function assignment operator.
453 * @param __x A %function with identical call signature.
454 * @post @c (bool)*this == (bool)x
455 * @returns @c *this
456 *
457 * The target of @a __x is copied to @c *this. If @a __x has no
458 * target, then @c *this will be empty.
459 *
460 * If @a __x targets a function pointer or a reference to a function
461 * object, then this operation will not throw an %exception.
462 */
463 function&
464 operator=(const function& __x)
465 {
466 function(__x).swap(*this);
467 return *this;
468 }
469
470 /**
471 * @brief %Function move-assignment operator.
472 * @param __x A %function rvalue with identical call signature.
473 * @returns @c *this
474 *
475 * The target of @a __x is moved to @c *this. If @a __x has no
476 * target, then @c *this will be empty.
477 *
478 * If @a __x targets a function pointer or a reference to a function
479 * object, then this operation will not throw an %exception.
480 */
481 function&
482 operator=(function&& __x) noexcept
483 {
484 function(std::move(__x)).swap(*this);
485 return *this;
486 }
487
488 /**
489 * @brief %Function assignment to zero.
490 * @post @c !(bool)*this
491 * @returns @c *this
492 *
493 * The target of @c *this is deallocated, leaving it empty.
494 */
495 function&
496 operator=(nullptr_t) noexcept
497 {
498 if (_M_manager)
499 {
500 _M_manager(_M_functor, _M_functor, __destroy_functor);
501 _M_manager = nullptr;
502 _M_invoker = nullptr;
503 }
504 return *this;
505 }
506
507 /**
508 * @brief %Function assignment to a new target.
509 * @param __f A %function object that is callable with parameters of
510 * type @c T1, @c T2, ..., @c TN and returns a value convertible
511 * to @c Res.
512 * @return @c *this
513 *
514 * This %function object wrapper will target a copy of @a
515 * __f. If @a __f is @c reference_wrapper<F>, then this function
516 * object will contain a reference to the function object @c
517 * __f.get(). If @a __f is a NULL function pointer or NULL
518 * pointer-to-member, @c this object will be empty.
519 *
520 * If @a __f is a non-NULL function pointer or an object of type @c
521 * reference_wrapper<F>, this function will not throw.
522 */
523 template<typename _Functor>
524 _Requires<_Callable<typename decay<_Functor>::type>, function&>
525 operator=(_Functor&& __f)
526 {
527 function(std::forward<_Functor>(__f)).swap(*this);
528 return *this;
529 }
530
531 /// @overload
532 template<typename _Functor>
533 function&
534 operator=(reference_wrapper<_Functor> __f) noexcept
535 {
536 function(__f).swap(*this);
537 return *this;
538 }
539
540 // [3.7.2.2] function modifiers
541
542 /**
543 * @brief Swap the targets of two %function objects.
544 * @param __x A %function with identical call signature.
545 *
546 * Swap the targets of @c this function object and @a __f. This
547 * function will not throw an %exception.
548 */
549 void swap(function& __x) noexcept
550 {
551 std::swap(_M_functor, __x._M_functor);
552 std::swap(_M_manager, __x._M_manager);
553 std::swap(_M_invoker, __x._M_invoker);
554 }
555
556 // [3.7.2.3] function capacity
557
558 /**
559 * @brief Determine if the %function wrapper has a target.
560 *
561 * @return @c true when this %function object contains a target,
562 * or @c false when it is empty.
563 *
564 * This function will not throw an %exception.
565 */
566 explicit operator bool() const noexcept
567 { return !_M_empty(); }
568
569 // [3.7.2.4] function invocation
570
571 /**
572 * @brief Invokes the function targeted by @c *this.
573 * @returns the result of the target.
574 * @throws bad_function_call when @c !(bool)*this
575 *
576 * The function call operator invokes the target function object
577 * stored by @c this.
578 */
579 _Res operator()(_ArgTypes... __args) const;
580
581#if __cpp_rtti
582 // [3.7.2.5] function target access
583 /**
584 * @brief Determine the type of the target of this function object
585 * wrapper.
586 *
587 * @returns the type identifier of the target function object, or
588 * @c typeid(void) if @c !(bool)*this.
589 *
590 * This function will not throw an %exception.
591 */
592 const type_info& target_type() const noexcept;
593
594 /**
595 * @brief Access the stored target function object.
596 *
597 * @return Returns a pointer to the stored target function object,
598 * if @c typeid(_Functor).equals(target_type()); otherwise, a NULL
599 * pointer.
600 *
601 * This function does not throw exceptions.
602 *
603 * @{
604 */
605 template<typename _Functor> _Functor* target() noexcept;
606
607 template<typename _Functor> const _Functor* target() const noexcept;
608 // @}
609#endif
610
611 private:
612 using _Invoker_type = _Res (*)(const _Any_data&, _ArgTypes&&...);
613 _Invoker_type _M_invoker;
614 };
615
616#if __cpp_deduction_guides >= 201606
617 template<typename>
618 struct __function_guide_helper
619 { };
620
621 template<typename _Res, typename _Tp, bool _Nx, typename... _Args>
622 struct __function_guide_helper<
623 _Res (_Tp::*) (_Args...) noexcept(_Nx)
624 >
625 { using type = _Res(_Args...); };
626
627 template<typename _Res, typename _Tp, bool _Nx, typename... _Args>
628 struct __function_guide_helper<
629 _Res (_Tp::*) (_Args...) & noexcept(_Nx)
630 >
631 { using type = _Res(_Args...); };
632
633 template<typename _Res, typename _Tp, bool _Nx, typename... _Args>
634 struct __function_guide_helper<
635 _Res (_Tp::*) (_Args...) const noexcept(_Nx)
636 >
637 { using type = _Res(_Args...); };
638
639 template<typename _Res, typename _Tp, bool _Nx, typename... _Args>
640 struct __function_guide_helper<
641 _Res (_Tp::*) (_Args...) const & noexcept(_Nx)
642 >
643 { using type = _Res(_Args...); };
644
645 template<typename _Res, typename... _ArgTypes>
646 function(_Res(*)(_ArgTypes...)) -> function<_Res(_ArgTypes...)>;
647
648 template<typename _Functor, typename _Signature = typename
649 __function_guide_helper<decltype(&_Functor::operator())>::type>
650 function(_Functor) -> function<_Signature>;
651#endif
652
653 // Out-of-line member definitions.
654 template<typename _Res, typename... _ArgTypes>
655 function<_Res(_ArgTypes...)>::
656 function(const function& __x)
657 : _Function_base()
658 {
659 if (static_cast<bool>(__x))
660 {
661 __x._M_manager(_M_functor, __x._M_functor, __clone_functor);
662 _M_invoker = __x._M_invoker;
663 _M_manager = __x._M_manager;
664 }
665 }
666
667 template<typename _Res, typename... _ArgTypes>
668 template<typename _Functor, typename, typename>
669 function<_Res(_ArgTypes...)>::
670 function(_Functor __f)
671 : _Function_base()
672 {
673 typedef _Function_handler<_Res(_ArgTypes...), _Functor> _My_handler;
674
675 if (_My_handler::_M_not_empty_function(__f))
676 {
677 _My_handler::_M_init_functor(_M_functor, std::move(__f));
678 _M_invoker = &_My_handler::_M_invoke;
679 _M_manager = &_My_handler::_M_manager;
680 }
681 }
682
683 template<typename _Res, typename... _ArgTypes>
684 _Res
685 function<_Res(_ArgTypes...)>::
686 operator()(_ArgTypes... __args) const
687 {
688 if (_M_empty())
689 __throw_bad_function_call();
690 return _M_invoker(_M_functor, std::forward<_ArgTypes>(__args)...);
691 }
692
693#if __cpp_rtti
694 template<typename _Res, typename... _ArgTypes>
695 const type_info&
696 function<_Res(_ArgTypes...)>::
697 target_type() const noexcept
698 {
699 if (_M_manager)
700 {
701 _Any_data __typeinfo_result;
702 _M_manager(__typeinfo_result, _M_functor, __get_type_info);
703 return *__typeinfo_result._M_access<const type_info*>();
704 }
705 else
706 return typeid(void);
707 }
708
709 template<typename _Res, typename... _ArgTypes>
710 template<typename _Functor>
711 _Functor*
712 function<_Res(_ArgTypes...)>::
713 target() noexcept
714 {
715 const function* __const_this = this;
716 const _Functor* __func = __const_this->template target<_Functor>();
717 return const_cast<_Functor*>(__func);
718 }
719
720 template<typename _Res, typename... _ArgTypes>
721 template<typename _Functor>
722 const _Functor*
723 function<_Res(_ArgTypes...)>::
724 target() const noexcept
725 {
726 if (typeid(_Functor) == target_type() && _M_manager)
727 {
728 _Any_data __ptr;
729 _M_manager(__ptr, _M_functor, __get_functor_ptr);
730 return __ptr._M_access<const _Functor*>();
731 }
732 else
733 return nullptr;
734 }
735#endif
736
737 // [20.7.15.2.6] null pointer comparisons
738
739 /**
740 * @brief Compares a polymorphic function object wrapper against 0
741 * (the NULL pointer).
742 * @returns @c true if the wrapper has no target, @c false otherwise
743 *
744 * This function will not throw an %exception.
745 */
746 template<typename _Res, typename... _Args>
747 inline bool
748 operator==(const function<_Res(_Args...)>& __f, nullptr_t) noexcept
749 { return !static_cast<bool>(__f); }
750
751 /// @overload
752 template<typename _Res, typename... _Args>
753 inline bool
754 operator==(nullptr_t, const function<_Res(_Args...)>& __f) noexcept
755 { return !static_cast<bool>(__f); }
756
757 /**
758 * @brief Compares a polymorphic function object wrapper against 0
759 * (the NULL pointer).
760 * @returns @c false if the wrapper has no target, @c true otherwise
761 *
762 * This function will not throw an %exception.
763 */
764 template<typename _Res, typename... _Args>
765 inline bool
766 operator!=(const function<_Res(_Args...)>& __f, nullptr_t) noexcept
767 { return static_cast<bool>(__f); }
768
769 /// @overload
770 template<typename _Res, typename... _Args>
771 inline bool
772 operator!=(nullptr_t, const function<_Res(_Args...)>& __f) noexcept
773 { return static_cast<bool>(__f); }
774
775
776 // [20.7.15.2.7] specialized algorithms
777
778 /**
779 * @brief Swap the targets of two polymorphic function object wrappers.
780 *
781 * This function will not throw an %exception.
782 */
783 // _GLIBCXX_RESOLVE_LIB_DEFECTS
784 // 2062. Effect contradictions w/o no-throw guarantee of std::function swaps
785 template<typename _Res, typename... _Args>
786 inline void
787 swap(function<_Res(_Args...)>& __x, function<_Res(_Args...)>& __y) noexcept
788 { __x.swap(__y); }
789
790#if __cplusplus >= 201703L
791 namespace __detail::__variant
792 {
793 template<typename> struct _Never_valueless_alt; // see <variant>
794
795 // Provide the strong exception-safety guarantee when emplacing a
796 // function into a variant.
797 template<typename _Signature>
798 struct _Never_valueless_alt<std::function<_Signature>>
799 : std::true_type
800 { };
801 } // namespace __detail::__variant
802#endif // C++17
803
804_GLIBCXX_END_NAMESPACE_VERSION
805} // namespace std
806
807#endif // C++11
808#endif // _GLIBCXX_STD_FUNCTION_H
809