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 /**
363 * @brief Primary class template for std::function.
364 * @ingroup functors
365 *
366 * Polymorphic function wrapper.
367 */
368 template<typename _Res, typename... _ArgTypes>
369 class function<_Res(_ArgTypes...)>
370 : public _Maybe_unary_or_binary_function<_Res, _ArgTypes...>,
371 private _Function_base
372 {
373 template<typename _Func,
374 typename _Res2 = __invoke_result<_Func&, _ArgTypes...>>
375 struct _Callable
376 : __is_invocable_impl<_Res2, _Res>::type
377 { };
378
379 // Used so the return type convertibility checks aren't done when
380 // performing overload resolution for copy construction/assignment.
381 template<typename _Tp>
382 struct _Callable<function, _Tp> : false_type { };
383
384 template<typename _Cond, typename _Tp>
385 using _Requires = typename enable_if<_Cond::value, _Tp>::type;
386
387 public:
388 typedef _Res result_type;
389
390 // [3.7.2.1] construct/copy/destroy
391
392 /**
393 * @brief Default construct creates an empty function call wrapper.
394 * @post @c !(bool)*this
395 */
396 function() noexcept
397 : _Function_base() { }
398
399 /**
400 * @brief Creates an empty function call wrapper.
401 * @post @c !(bool)*this
402 */
403 function(nullptr_t) noexcept
404 : _Function_base() { }
405
406 /**
407 * @brief %Function copy constructor.
408 * @param __x A %function object with identical call signature.
409 * @post @c bool(*this) == bool(__x)
410 *
411 * The newly-created %function contains a copy of the target of @a
412 * __x (if it has one).
413 */
414 function(const function& __x);
415
416 /**
417 * @brief %Function move constructor.
418 * @param __x A %function object rvalue with identical call signature.
419 *
420 * The newly-created %function contains the target of @a __x
421 * (if it has one).
422 */
423 function(function&& __x) noexcept : _Function_base()
424 {
425 __x.swap(*this);
426 }
427
428 /**
429 * @brief Builds a %function that targets a copy of the incoming
430 * function object.
431 * @param __f A %function object that is callable with parameters of
432 * type @c T1, @c T2, ..., @c TN and returns a value convertible
433 * to @c Res.
434 *
435 * The newly-created %function object will target a copy of
436 * @a __f. If @a __f is @c reference_wrapper<F>, then this function
437 * object will contain a reference to the function object @c
438 * __f.get(). If @a __f is a NULL function pointer or NULL
439 * pointer-to-member, the newly-created object will be empty.
440 *
441 * If @a __f is a non-NULL function pointer or an object of type @c
442 * reference_wrapper<F>, this function will not throw.
443 */
444 template<typename _Functor,
445 typename = _Requires<__not_<is_same<_Functor, function>>, void>,
446 typename = _Requires<_Callable<_Functor>, void>>
447 function(_Functor);
448
449 /**
450 * @brief %Function assignment operator.
451 * @param __x A %function with identical call signature.
452 * @post @c (bool)*this == (bool)x
453 * @returns @c *this
454 *
455 * The target of @a __x is copied to @c *this. If @a __x has no
456 * target, then @c *this will be empty.
457 *
458 * If @a __x targets a function pointer or a reference to a function
459 * object, then this operation will not throw an %exception.
460 */
461 function&
462 operator=(const function& __x)
463 {
464 function(__x).swap(*this);
465 return *this;
466 }
467
468 /**
469 * @brief %Function move-assignment operator.
470 * @param __x A %function rvalue with identical call signature.
471 * @returns @c *this
472 *
473 * The target of @a __x is moved to @c *this. If @a __x has no
474 * target, then @c *this will be empty.
475 *
476 * If @a __x targets a function pointer or a reference to a function
477 * object, then this operation will not throw an %exception.
478 */
479 function&
480 operator=(function&& __x) noexcept
481 {
482 function(std::move(__x)).swap(*this);
483 return *this;
484 }
485
486 /**
487 * @brief %Function assignment to zero.
488 * @post @c !(bool)*this
489 * @returns @c *this
490 *
491 * The target of @c *this is deallocated, leaving it empty.
492 */
493 function&
494 operator=(nullptr_t) noexcept
495 {
496 if (_M_manager)
497 {
498 _M_manager(_M_functor, _M_functor, __destroy_functor);
499 _M_manager = nullptr;
500 _M_invoker = nullptr;
501 }
502 return *this;
503 }
504
505 /**
506 * @brief %Function assignment to a new target.
507 * @param __f A %function object that is callable with parameters of
508 * type @c T1, @c T2, ..., @c TN and returns a value convertible
509 * to @c Res.
510 * @return @c *this
511 *
512 * This %function object wrapper will target a copy of @a
513 * __f. If @a __f is @c reference_wrapper<F>, then this function
514 * object will contain a reference to the function object @c
515 * __f.get(). If @a __f is a NULL function pointer or NULL
516 * pointer-to-member, @c this object will be empty.
517 *
518 * If @a __f is a non-NULL function pointer or an object of type @c
519 * reference_wrapper<F>, this function will not throw.
520 */
521 template<typename _Functor>
522 _Requires<_Callable<typename decay<_Functor>::type>, function&>
523 operator=(_Functor&& __f)
524 {
525 function(std::forward<_Functor>(__f)).swap(*this);
526 return *this;
527 }
528
529 /// @overload
530 template<typename _Functor>
531 function&
532 operator=(reference_wrapper<_Functor> __f) noexcept
533 {
534 function(__f).swap(*this);
535 return *this;
536 }
537
538 // [3.7.2.2] function modifiers
539
540 /**
541 * @brief Swap the targets of two %function objects.
542 * @param __x A %function with identical call signature.
543 *
544 * Swap the targets of @c this function object and @a __f. This
545 * function will not throw an %exception.
546 */
547 void swap(function& __x) noexcept
548 {
549 std::swap(_M_functor, __x._M_functor);
550 std::swap(_M_manager, __x._M_manager);
551 std::swap(_M_invoker, __x._M_invoker);
552 }
553
554 // [3.7.2.3] function capacity
555
556 /**
557 * @brief Determine if the %function wrapper has a target.
558 *
559 * @return @c true when this %function object contains a target,
560 * or @c false when it is empty.
561 *
562 * This function will not throw an %exception.
563 */
564 explicit operator bool() const noexcept
565 { return !_M_empty(); }
566
567 // [3.7.2.4] function invocation
568
569 /**
570 * @brief Invokes the function targeted by @c *this.
571 * @returns the result of the target.
572 * @throws bad_function_call when @c !(bool)*this
573 *
574 * The function call operator invokes the target function object
575 * stored by @c this.
576 */
577 _Res operator()(_ArgTypes... __args) const;
578
579#if __cpp_rtti
580 // [3.7.2.5] function target access
581 /**
582 * @brief Determine the type of the target of this function object
583 * wrapper.
584 *
585 * @returns the type identifier of the target function object, or
586 * @c typeid(void) if @c !(bool)*this.
587 *
588 * This function will not throw an %exception.
589 */
590 const type_info& target_type() const noexcept;
591
592 /**
593 * @brief Access the stored target function object.
594 *
595 * @return Returns a pointer to the stored target function object,
596 * if @c typeid(_Functor).equals(target_type()); otherwise, a NULL
597 * pointer.
598 *
599 * This function does not throw exceptions.
600 *
601 * @{
602 */
603 template<typename _Functor> _Functor* target() noexcept;
604
605 template<typename _Functor> const _Functor* target() const noexcept;
606 /// @}
607#endif
608
609 private:
610 using _Invoker_type = _Res (*)(const _Any_data&, _ArgTypes&&...);
611 _Invoker_type _M_invoker;
612 };
613
614#if __cpp_deduction_guides >= 201606
615 template<typename>
616 struct __function_guide_helper
617 { };
618
619 template<typename _Res, typename _Tp, bool _Nx, typename... _Args>
620 struct __function_guide_helper<
621 _Res (_Tp::*) (_Args...) noexcept(_Nx)
622 >
623 { using type = _Res(_Args...); };
624
625 template<typename _Res, typename _Tp, bool _Nx, typename... _Args>
626 struct __function_guide_helper<
627 _Res (_Tp::*) (_Args...) & noexcept(_Nx)
628 >
629 { using type = _Res(_Args...); };
630
631 template<typename _Res, typename _Tp, bool _Nx, typename... _Args>
632 struct __function_guide_helper<
633 _Res (_Tp::*) (_Args...) const noexcept(_Nx)
634 >
635 { using type = _Res(_Args...); };
636
637 template<typename _Res, typename _Tp, bool _Nx, typename... _Args>
638 struct __function_guide_helper<
639 _Res (_Tp::*) (_Args...) const & noexcept(_Nx)
640 >
641 { using type = _Res(_Args...); };
642
643 template<typename _Res, typename... _ArgTypes>
644 function(_Res(*)(_ArgTypes...)) -> function<_Res(_ArgTypes...)>;
645
646 template<typename _Functor, typename _Signature = typename
647 __function_guide_helper<decltype(&_Functor::operator())>::type>
648 function(_Functor) -> function<_Signature>;
649#endif
650
651 // Out-of-line member definitions.
652 template<typename _Res, typename... _ArgTypes>
653 function<_Res(_ArgTypes...)>::
654 function(const function& __x)
655 : _Function_base()
656 {
657 if (static_cast<bool>(__x))
658 {
659 __x._M_manager(_M_functor, __x._M_functor, __clone_functor);
660 _M_invoker = __x._M_invoker;
661 _M_manager = __x._M_manager;
662 }
663 }
664
665 template<typename _Res, typename... _ArgTypes>
666 template<typename _Functor, typename, typename>
667 function<_Res(_ArgTypes...)>::
668 function(_Functor __f)
669 : _Function_base()
670 {
671 typedef _Function_handler<_Res(_ArgTypes...), _Functor> _My_handler;
672
673 if (_My_handler::_M_not_empty_function(__f))
674 {
675 _My_handler::_M_init_functor(_M_functor, std::move(__f));
676 _M_invoker = &_My_handler::_M_invoke;
677 _M_manager = &_My_handler::_M_manager;
678 }
679 }
680
681 template<typename _Res, typename... _ArgTypes>
682 _Res
683 function<_Res(_ArgTypes...)>::
684 operator()(_ArgTypes... __args) const
685 {
686 if (_M_empty())
687 __throw_bad_function_call();
688 return _M_invoker(_M_functor, std::forward<_ArgTypes>(__args)...);
689 }
690
691#if __cpp_rtti
692 template<typename _Res, typename... _ArgTypes>
693 const type_info&
694 function<_Res(_ArgTypes...)>::
695 target_type() const noexcept
696 {
697 if (_M_manager)
698 {
699 _Any_data __typeinfo_result;
700 _M_manager(__typeinfo_result, _M_functor, __get_type_info);
701 return *__typeinfo_result._M_access<const type_info*>();
702 }
703 else
704 return typeid(void);
705 }
706
707 template<typename _Res, typename... _ArgTypes>
708 template<typename _Functor>
709 _Functor*
710 function<_Res(_ArgTypes...)>::
711 target() noexcept
712 {
713 const function* __const_this = this;
714 const _Functor* __func = __const_this->template target<_Functor>();
715 return const_cast<_Functor*>(__func);
716 }
717
718 template<typename _Res, typename... _ArgTypes>
719 template<typename _Functor>
720 const _Functor*
721 function<_Res(_ArgTypes...)>::
722 target() const noexcept
723 {
724 if (typeid(_Functor) == target_type() && _M_manager)
725 {
726 _Any_data __ptr;
727 _M_manager(__ptr, _M_functor, __get_functor_ptr);
728 return __ptr._M_access<const _Functor*>();
729 }
730 else
731 return nullptr;
732 }
733#endif
734
735 // [20.7.15.2.6] null pointer comparisons
736
737 /**
738 * @brief Compares a polymorphic function object wrapper against 0
739 * (the NULL pointer).
740 * @returns @c true if the wrapper has no target, @c false otherwise
741 *
742 * This function will not throw an %exception.
743 */
744 template<typename _Res, typename... _Args>
745 inline bool
746 operator==(const function<_Res(_Args...)>& __f, nullptr_t) noexcept
747 { return !static_cast<bool>(__f); }
748
749 /// @overload
750 template<typename _Res, typename... _Args>
751 inline bool
752 operator==(nullptr_t, const function<_Res(_Args...)>& __f) noexcept
753 { return !static_cast<bool>(__f); }
754
755 /**
756 * @brief Compares a polymorphic function object wrapper against 0
757 * (the NULL pointer).
758 * @returns @c false if the wrapper has no target, @c true otherwise
759 *
760 * This function will not throw an %exception.
761 */
762 template<typename _Res, typename... _Args>
763 inline bool
764 operator!=(const function<_Res(_Args...)>& __f, nullptr_t) noexcept
765 { return static_cast<bool>(__f); }
766
767 /// @overload
768 template<typename _Res, typename... _Args>
769 inline bool
770 operator!=(nullptr_t, const function<_Res(_Args...)>& __f) noexcept
771 { return static_cast<bool>(__f); }
772
773
774 // [20.7.15.2.7] specialized algorithms
775
776 /**
777 * @brief Swap the targets of two polymorphic function object wrappers.
778 *
779 * This function will not throw an %exception.
780 */
781 // _GLIBCXX_RESOLVE_LIB_DEFECTS
782 // 2062. Effect contradictions w/o no-throw guarantee of std::function swaps
783 template<typename _Res, typename... _Args>
784 inline void
785 swap(function<_Res(_Args...)>& __x, function<_Res(_Args...)>& __y) noexcept
786 { __x.swap(__y); }
787
788#if __cplusplus >= 201703L
789 namespace __detail::__variant
790 {
791 template<typename> struct _Never_valueless_alt; // see <variant>
792
793 // Provide the strong exception-safety guarantee when emplacing a
794 // function into a variant.
795 template<typename _Signature>
796 struct _Never_valueless_alt<std::function<_Signature>>
797 : std::true_type
798 { };
799 } // namespace __detail::__variant
800#endif // C++17
801
802_GLIBCXX_END_NAMESPACE_VERSION
803} // namespace std
804
805#endif // C++11
806#endif // _GLIBCXX_STD_FUNCTION_H
807