1	// shared_ptr and weak_ptr implementation details -*- C++ -*-
2
3	// Copyright (C) 2007-2021 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	// GCC Note: Based on files from version 1.32.0 of the Boost library.
26
27	// shared_count.hpp
28	// Copyright (c) 2001, 2002, 2003 Peter Dimov and Multi Media Ltd.
29
30	// shared_ptr.hpp
31	// Copyright (C) 1998, 1999 Greg Colvin and Beman Dawes.
32	// Copyright (C) 2001, 2002, 2003 Peter Dimov
33
34	// weak_ptr.hpp
35	// Copyright (C) 2001, 2002, 2003 Peter Dimov
36
37	// enable_shared_from_this.hpp
38	// Copyright (C) 2002 Peter Dimov
39
40	// Distributed under the Boost Software License, Version 1.0. (See
41	// accompanying file LICENSE_1_0.txt or copy at
42	// http://www.boost.org/LICENSE_1_0.txt)
43
44	/** @file bits/shared_ptr_base.h
45	* This is an internal header file, included by other library headers.
46	* Do not attempt to use it directly. @headername{memory}
47	*/
48
49	#ifndef _SHARED_PTR_BASE_H
50	#define _SHARED_PTR_BASE_H 1
51
52	#include <typeinfo>
53	#include <bits/allocated_ptr.h>
54	#include <bits/allocator.h>
55	#include <bits/exception_defines.h>
56	#include <bits/functional_hash.h>
57	#include <bits/refwrap.h>
58	#include <bits/stl_function.h> // std::less
59	#include <bits/unique_ptr.h>
60	#include <ext/aligned_buffer.h>
61	#include <ext/atomicity.h>
62	#include <ext/concurrence.h>
63	#if __cplusplus > 201703L
64	# include <compare>
65	#endif
66
67	namespace std _GLIBCXX_VISIBILITY(default)
68	{
69	_GLIBCXX_BEGIN_NAMESPACE_VERSION
70
71	#if _GLIBCXX_USE_DEPRECATED
72	#pragma GCC diagnostic push
73	#pragma GCC diagnostic ignored "-Wdeprecated-declarations"
74	template<typename> class auto_ptr;
75	#pragma GCC diagnostic pop
76	#endif
77
78	/**
79	* @brief Exception possibly thrown by @c shared_ptr.
80	* @ingroup exceptions
81	*/
82	class bad_weak_ptr : public std::exception
83	{
84	public:
85	virtual char const* what() const noexcept;
86
87	virtual ~bad_weak_ptr() noexcept;
88	};
89
90	// Substitute for bad_weak_ptr object in the case of -fno-exceptions.
91	inline void
92	__throw_bad_weak_ptr()
93	{ _GLIBCXX_THROW_OR_ABORT(bad_weak_ptr()); }
94
95	using __gnu_cxx::_Lock_policy;
96	using __gnu_cxx::__default_lock_policy;
97	using __gnu_cxx::_S_single;
98	using __gnu_cxx::_S_mutex;
99	using __gnu_cxx::_S_atomic;
100
101	// Empty helper class except when the template argument is _S_mutex.
102	template<_Lock_policy _Lp>
103	class _Mutex_base
104	{
105	protected:
106	// The atomic policy uses fully-fenced builtins, single doesn't care.
107	enum { _S_need_barriers = 0 };
108	};
109
110	template<>
111	class _Mutex_base<_S_mutex>
112	: public __gnu_cxx::__mutex
113	{
114	protected:
115	// This policy is used when atomic builtins are not available.
116	// The replacement atomic operations might not have the necessary
117	// memory barriers.
118	enum { _S_need_barriers = 1 };
119	};
120
121	template<_Lock_policy _Lp = __default_lock_policy>
122	class _Sp_counted_base
123	: public _Mutex_base<_Lp>
124	{
125	public:
126	_Sp_counted_base() noexcept
127	: _M_use_count(1), _M_weak_count(1) { }
128
129	virtual
130	~_Sp_counted_base() noexcept
131	{ }
132
133	// Called when _M_use_count drops to zero, to release the resources
134	// managed by *this.
135	virtual void
136	_M_dispose() noexcept = 0;
137
138	// Called when _M_weak_count drops to zero.
139	virtual void
140	_M_destroy() noexcept
141	{ delete this; }
142
143	virtual void*
144	_M_get_deleter(const std::type_info&) noexcept = 0;
145
146	void
147	_M_add_ref_copy()
148	{ __gnu_cxx::__atomic_add_dispatch(mem: &_M_use_count, val: 1); }
149
150	void
151	_M_add_ref_lock()
152	{
153	if (!_M_add_ref_lock_nothrow())
154	__throw_bad_weak_ptr();
155	}
156
157	bool
158	_M_add_ref_lock_nothrow() noexcept;
159
160	void
161	_M_release() noexcept
162	{
163	// Be race-detector-friendly. For more info see bits/c++config.
164	_GLIBCXX_SYNCHRONIZATION_HAPPENS_BEFORE(&_M_use_count);
165	if (__gnu_cxx::__exchange_and_add_dispatch(mem: &_M_use_count, val: -1) == 1)
166	{
167	_GLIBCXX_SYNCHRONIZATION_HAPPENS_AFTER(&_M_use_count);
168	_M_dispose();
169	// There must be a memory barrier between dispose() and destroy()
170	// to ensure that the effects of dispose() are observed in the
171	// thread that runs destroy().
172	// See http://gcc.gnu.org/ml/libstdc++/2005-11/msg00136.html
173	if (_Mutex_base<_Lp>::_S_need_barriers)
174	{
175	__atomic_thread_fence (__ATOMIC_ACQ_REL);
176	}
177
178	// Be race-detector-friendly. For more info see bits/c++config.
179	_GLIBCXX_SYNCHRONIZATION_HAPPENS_BEFORE(&_M_weak_count);
180	if (__gnu_cxx::__exchange_and_add_dispatch(mem: &_M_weak_count,
181	val: -1) == 1)
182	{
183	_GLIBCXX_SYNCHRONIZATION_HAPPENS_AFTER(&_M_weak_count);
184	_M_destroy();
185	}
186	}
187	}
188
189	void
190	_M_weak_add_ref() noexcept
191	{ __gnu_cxx::__atomic_add_dispatch(mem: &_M_weak_count, val: 1); }
192
193	void
194	_M_weak_release() noexcept
195	{
196	// Be race-detector-friendly. For more info see bits/c++config.
197	_GLIBCXX_SYNCHRONIZATION_HAPPENS_BEFORE(&_M_weak_count);
198	if (__gnu_cxx::__exchange_and_add_dispatch(mem: &_M_weak_count, val: -1) == 1)
199	{
200	_GLIBCXX_SYNCHRONIZATION_HAPPENS_AFTER(&_M_weak_count);
201	if (_Mutex_base<_Lp>::_S_need_barriers)
202	{
203	// See _M_release(),
204	// destroy() must observe results of dispose()
205	__atomic_thread_fence (__ATOMIC_ACQ_REL);
206	}
207	_M_destroy();
208	}
209	}
210
211	long
212	_M_get_use_count() const noexcept
213	{
214	// No memory barrier is used here so there is no synchronization
215	// with other threads.
216	return __atomic_load_n(&_M_use_count, __ATOMIC_RELAXED);
217	}
218
219	private:
220	_Sp_counted_base(_Sp_counted_base const&) = delete;
221	_Sp_counted_base& operator=(_Sp_counted_base const&) = delete;
222
223	_Atomic_word _M_use_count; // #shared
224	_Atomic_word _M_weak_count; // #weak + (#shared != 0)
225	};
226
227	template<>
228	inline bool
229	_Sp_counted_base<_S_single>::
230	_M_add_ref_lock_nothrow() noexcept
231	{
232	if (_M_use_count == 0)
233	return false;
234	++_M_use_count;
235	return true;
236	}
237
238	template<>
239	inline bool
240	_Sp_counted_base<_S_mutex>::
241	_M_add_ref_lock_nothrow() noexcept
242	{
243	__gnu_cxx::__scoped_lock sentry(*this);
244	if (__gnu_cxx::__exchange_and_add_dispatch(mem: &_M_use_count, val: 1) == 0)
245	{
246	_M_use_count = 0;
247	return false;
248	}
249	return true;
250	}
251
252	template<>
253	inline bool
254	_Sp_counted_base<_S_atomic>::
255	_M_add_ref_lock_nothrow() noexcept
256	{
257	// Perform lock-free add-if-not-zero operation.
258	_Atomic_word __count = _M_get_use_count();
259	do
260	{
261	if (__count == 0)
262	return false;
263	// Replace the current counter value with the old value + 1, as
264	// long as it's not changed meanwhile.
265	}
266	while (!__atomic_compare_exchange_n(&_M_use_count, &__count, __count + 1,
267	true, __ATOMIC_ACQ_REL,
268	__ATOMIC_RELAXED));
269	return true;
270	}
271
272	template<>
273	inline void
274	_Sp_counted_base<_S_single>::_M_add_ref_copy()
275	{ ++_M_use_count; }
276
277	template<>
278	inline void
279	_Sp_counted_base<_S_single>::_M_release() noexcept
280	{
281	if (--_M_use_count == 0)
282	{
283	_M_dispose();
284	if (--_M_weak_count == 0)
285	_M_destroy();
286	}
287	}
288
289	template<>
290	inline void
291	_Sp_counted_base<_S_single>::_M_weak_add_ref() noexcept
292	{ ++_M_weak_count; }
293
294	template<>
295	inline void
296	_Sp_counted_base<_S_single>::_M_weak_release() noexcept
297	{
298	if (--_M_weak_count == 0)
299	_M_destroy();
300	}
301
302	template<>
303	inline long
304	_Sp_counted_base<_S_single>::_M_get_use_count() const noexcept
305	{ return _M_use_count; }
306
307
308	// Forward declarations.
309	template<typename _Tp, _Lock_policy _Lp = __default_lock_policy>
310	class __shared_ptr;
311
312	template<typename _Tp, _Lock_policy _Lp = __default_lock_policy>
313	class __weak_ptr;
314
315	template<typename _Tp, _Lock_policy _Lp = __default_lock_policy>
316	class __enable_shared_from_this;
317
318	template<typename _Tp>
319	class shared_ptr;
320
321	template<typename _Tp>
322	class weak_ptr;
323
324	template<typename _Tp>
325	struct owner_less;
326
327	template<typename _Tp>
328	class enable_shared_from_this;
329
330	template<_Lock_policy _Lp = __default_lock_policy>
331	class __weak_count;
332
333	template<_Lock_policy _Lp = __default_lock_policy>
334	class __shared_count;
335
336
337	// Counted ptr with no deleter or allocator support
338	template<typename _Ptr, _Lock_policy _Lp>
339	class _Sp_counted_ptr final : public _Sp_counted_base<_Lp>
340	{
341	public:
342	explicit
343	_Sp_counted_ptr(_Ptr __p) noexcept
344	: _M_ptr(__p) { }
345
346	virtual void
347	_M_dispose() noexcept
348	{ delete _M_ptr; }
349
350	virtual void
351	_M_destroy() noexcept
352	{ delete this; }
353
354	virtual void*
355	_M_get_deleter(const std::type_info&) noexcept
356	{ return nullptr; }
357
358	_Sp_counted_ptr(const _Sp_counted_ptr&) = delete;
359	_Sp_counted_ptr& operator=(const _Sp_counted_ptr&) = delete;
360
361	private:
362	_Ptr _M_ptr;
363	};
364
365	template<>
366	inline void
367	_Sp_counted_ptr<nullptr_t, _S_single>::_M_dispose() noexcept { }
368
369	template<>
370	inline void
371	_Sp_counted_ptr<nullptr_t, _S_mutex>::_M_dispose() noexcept { }
372
373	template<>
374	inline void
375	_Sp_counted_ptr<nullptr_t, _S_atomic>::_M_dispose() noexcept { }
376
377	template<int _Nm, typename _Tp,
378	bool __use_ebo = !__is_final(_Tp) && __is_empty(_Tp)>
379	struct _Sp_ebo_helper;
380
381	/// Specialization using EBO.
382	template<int _Nm, typename _Tp>
383	struct _Sp_ebo_helper<_Nm, _Tp, true> : private _Tp
384	{
385	explicit _Sp_ebo_helper(const _Tp& __tp) : _Tp(__tp) { }
386	explicit _Sp_ebo_helper(_Tp&& __tp) : _Tp(std::move(__tp)) { }
387
388	static _Tp&
389	_S_get(_Sp_ebo_helper& __eboh) { return static_cast<_Tp&>(__eboh); }
390	};
391
392	/// Specialization not using EBO.
393	template<int _Nm, typename _Tp>
394	struct _Sp_ebo_helper<_Nm, _Tp, false>
395	{
396	explicit _Sp_ebo_helper(const _Tp& __tp) : _M_tp(__tp) { }
397	explicit _Sp_ebo_helper(_Tp&& __tp) : _M_tp(std::move(__tp)) { }
398
399	static _Tp&
400	_S_get(_Sp_ebo_helper& __eboh)
401	{ return __eboh._M_tp; }
402
403	private:
404	_Tp _M_tp;
405	};
406
407	// Support for custom deleter and/or allocator
408	template<typename _Ptr, typename _Deleter, typename _Alloc, _Lock_policy _Lp>
409	class _Sp_counted_deleter final : public _Sp_counted_base<_Lp>
410	{
411	class _Impl : _Sp_ebo_helper<0, _Deleter>, _Sp_ebo_helper<1, _Alloc>
412	{
413	typedef _Sp_ebo_helper<0, _Deleter> _Del_base;
414	typedef _Sp_ebo_helper<1, _Alloc> _Alloc_base;
415
416	public:
417	_Impl(_Ptr __p, _Deleter __d, const _Alloc& __a) noexcept
418	: _Del_base(std::move(__d)), _Alloc_base(__a), _M_ptr(__p)
419	{ }
420
421	_Deleter& _M_del() noexcept { return _Del_base::_S_get(*this); }
422	_Alloc& _M_alloc() noexcept { return _Alloc_base::_S_get(*this); }
423
424	_Ptr _M_ptr;
425	};
426
427	public:
428	using __allocator_type = __alloc_rebind<_Alloc, _Sp_counted_deleter>;
429
430	// __d(__p) must not throw.
431	_Sp_counted_deleter(_Ptr __p, _Deleter __d) noexcept
432	: _M_impl(__p, std::move(__d), _Alloc()) { }
433
434	// __d(__p) must not throw.
435	_Sp_counted_deleter(_Ptr __p, _Deleter __d, const _Alloc& __a) noexcept
436	: _M_impl(__p, std::move(__d), __a) { }
437
438	~_Sp_counted_deleter() noexcept { }
439
440	virtual void
441	_M_dispose() noexcept
442	{ _M_impl._M_del()(_M_impl._M_ptr); }
443
444	virtual void
445	_M_destroy() noexcept
446	{
447	__allocator_type __a(_M_impl._M_alloc());
448	__allocated_ptr<__allocator_type> __guard_ptr{ __a, this };
449	this->~_Sp_counted_deleter();
450	}
451
452	virtual void*
453	_M_get_deleter(const type_info& __ti [[__gnu__::__unused__]]) noexcept
454	{
455	#if __cpp_rtti
456	// _GLIBCXX_RESOLVE_LIB_DEFECTS
457	// 2400. shared_ptr's get_deleter() should use addressof()
458	return __ti == typeid(_Deleter)
459	? std::__addressof(_M_impl._M_del())
460	: nullptr;
461	#else
462	return nullptr;
463	#endif
464	}
465
466	private:
467	_Impl _M_impl;
468	};
469
470	// helpers for make_shared / allocate_shared
471
472	struct _Sp_make_shared_tag
473	{
474	private:
475	template<typename _Tp, typename _Alloc, _Lock_policy _Lp>
476	friend class _Sp_counted_ptr_inplace;
477
478	static const type_info&
479	_S_ti() noexcept _GLIBCXX_VISIBILITY(default)
480	{
481	alignas(type_info) static constexpr char __tag[sizeof(type_info)] = { };
482	return reinterpret_cast<const type_info&>(__tag);
483	}
484
485	static bool _S_eq(const type_info&) noexcept;
486	};
487
488	template<typename _Alloc>
489	struct _Sp_alloc_shared_tag
490	{
491	const _Alloc& _M_a;
492	};
493
494	template<typename _Tp, typename _Alloc, _Lock_policy _Lp>
495	class _Sp_counted_ptr_inplace final : public _Sp_counted_base<_Lp>
496	{
497	class _Impl : _Sp_ebo_helper<0, _Alloc>
498	{
499	typedef _Sp_ebo_helper<0, _Alloc> _A_base;
500
501	public:
502	explicit _Impl(_Alloc __a) noexcept : _A_base(__a) { }
503
504	_Alloc& _M_alloc() noexcept { return _A_base::_S_get(*this); }
505
506	__gnu_cxx::__aligned_buffer<_Tp> _M_storage;
507	};
508
509	public:
510	using __allocator_type = __alloc_rebind<_Alloc, _Sp_counted_ptr_inplace>;
511
512	// Alloc parameter is not a reference so doesn't alias anything in __args
513	template<typename... _Args>
514	_Sp_counted_ptr_inplace(_Alloc __a, _Args&&... __args)
515	: _M_impl(__a)
516	{
517	// _GLIBCXX_RESOLVE_LIB_DEFECTS
518	// 2070. allocate_shared should use allocator_traits<A>::construct
519	allocator_traits<_Alloc>::construct(__a, _M_ptr(),
520	std::forward<_Args>(__args)...); // might throw
521	}
522
523	~_Sp_counted_ptr_inplace() noexcept { }
524
525	virtual void
526	_M_dispose() noexcept
527	{
528	allocator_traits<_Alloc>::destroy(_M_impl._M_alloc(), _M_ptr());
529	}
530
531	// Override because the allocator needs to know the dynamic type
532	virtual void
533	_M_destroy() noexcept
534	{
535	__allocator_type __a(_M_impl._M_alloc());
536	__allocated_ptr<__allocator_type> __guard_ptr{ __a, this };
537	this->~_Sp_counted_ptr_inplace();
538	}
539
540	private:
541	friend class __shared_count<_Lp>; // To be able to call _M_ptr().
542
543	// No longer used, but code compiled against old libstdc++ headers
544	// might still call it from __shared_ptr ctor to get the pointer out.
545	virtual void*
546	_M_get_deleter(const std::type_info& __ti) noexcept override
547	{
548	auto __ptr = const_cast<typename remove_cv<_Tp>::type*>(_M_ptr());
549	// Check for the fake type_info first, so we don't try to access it
550	// as a real type_info object. Otherwise, check if it's the real
551	// type_info for this class. With RTTI enabled we can check directly,
552	// or call a library function to do it.
553	if (&__ti == &_Sp_make_shared_tag::_S_ti()
554	||
555	#if __cpp_rtti
556	__ti == typeid(_Sp_make_shared_tag)
557	#else
558	_Sp_make_shared_tag::_S_eq(__ti)
559	#endif
560	)
561	return __ptr;
562	return nullptr;
563	}
564
565	_Tp* _M_ptr() noexcept { return _M_impl._M_storage._M_ptr(); }
566
567	_Impl _M_impl;
568	};
569
570	// The default deleter for shared_ptr<T[]> and shared_ptr<T[N]>.
571	struct __sp_array_delete
572	{
573	template<typename _Yp>
574	void operator()(_Yp* __p) const { delete[] __p; }
575	};
576
577	template<_Lock_policy _Lp>
578	class __shared_count
579	{
580	template<typename _Tp>
581	struct __not_alloc_shared_tag { using type = void; };
582
583	template<typename _Tp>
584	struct __not_alloc_shared_tag<_Sp_alloc_shared_tag<_Tp>> { };
585
586	public:
587	constexpr __shared_count() noexcept : _M_pi(0)
588	{ }
589
590	template<typename _Ptr>
591	explicit
592	__shared_count(_Ptr __p) : _M_pi(0)
593	{
594	__try
595	{
596	_M_pi = new _Sp_counted_ptr<_Ptr, _Lp>(__p);
597	}
598	__catch(...)
599	{
600	delete __p;
601	__throw_exception_again;
602	}
603	}
604
605	template<typename _Ptr>
606	__shared_count(_Ptr __p, /* is_array = */ false_type)
607	: __shared_count(__p)
608	{ }
609
610	template<typename _Ptr>
611	__shared_count(_Ptr __p, /* is_array = */ true_type)
612	: __shared_count(__p, __sp_array_delete{}, allocator<void>())
613	{ }
614
615	template<typename _Ptr, typename _Deleter,
616	typename = typename __not_alloc_shared_tag<_Deleter>::type>
617	__shared_count(_Ptr __p, _Deleter __d)
618	: __shared_count(__p, std::move(__d), allocator<void>())
619	{ }
620
621	template<typename _Ptr, typename _Deleter, typename _Alloc,
622	typename = typename __not_alloc_shared_tag<_Deleter>::type>
623	__shared_count(_Ptr __p, _Deleter __d, _Alloc __a) : _M_pi(0)
624	{
625	typedef _Sp_counted_deleter<_Ptr, _Deleter, _Alloc, _Lp> _Sp_cd_type;
626	__try
627	{
628	typename _Sp_cd_type::__allocator_type __a2(__a);
629	auto __guard = std::__allocate_guarded(__a2);
630	_Sp_cd_type* __mem = __guard.get();
631	::new (__mem) _Sp_cd_type(__p, std::move(__d), std::move(__a));
632	_M_pi = __mem;
633	__guard = nullptr;
634	}
635	__catch(...)
636	{
637	__d(__p); // Call _Deleter on __p.
638	__throw_exception_again;
639	}
640	}
641
642	template<typename _Tp, typename _Alloc, typename... _Args>
643	__shared_count(_Tp*& __p, _Sp_alloc_shared_tag<_Alloc> __a,
644	_Args&&... __args)
645	{
646	typedef _Sp_counted_ptr_inplace<_Tp, _Alloc, _Lp> _Sp_cp_type;
647	typename _Sp_cp_type::__allocator_type __a2(__a._M_a);
648	auto __guard = std::__allocate_guarded(__a2);
649	_Sp_cp_type* __mem = __guard.get();
650	auto __pi = ::new (__mem)
651	_Sp_cp_type(__a._M_a, std::forward<_Args>(__args)...);
652	__guard = nullptr;
653	_M_pi = __pi;
654	__p = __pi->_M_ptr();
655	}
656
657	#if _GLIBCXX_USE_DEPRECATED
658	#pragma GCC diagnostic push
659	#pragma GCC diagnostic ignored "-Wdeprecated-declarations"
660	// Special case for auto_ptr<_Tp> to provide the strong guarantee.
661	template<typename _Tp>
662	explicit
663	__shared_count(std::auto_ptr<_Tp>&& __r);
664	#pragma GCC diagnostic pop
665	#endif
666
667	// Special case for unique_ptr<_Tp,_Del> to provide the strong guarantee.
668	template<typename _Tp, typename _Del>
669	explicit
670	__shared_count(std::unique_ptr<_Tp, _Del>&& __r) : _M_pi(0)
671	{
672	// _GLIBCXX_RESOLVE_LIB_DEFECTS
673	// 2415. Inconsistency between unique_ptr and shared_ptr
674	if (__r.get() == nullptr)
675	return;
676
677	using _Ptr = typename unique_ptr<_Tp, _Del>::pointer;
678	using _Del2 = typename conditional<is_reference<_Del>::value,
679	reference_wrapper<typename remove_reference<_Del>::type>,
680	_Del>::type;
681	using _Sp_cd_type
682	= _Sp_counted_deleter<_Ptr, _Del2, allocator<void>, _Lp>;
683	using _Alloc = allocator<_Sp_cd_type>;
684	using _Alloc_traits = allocator_traits<_Alloc>;
685	_Alloc __a;
686	_Sp_cd_type* __mem = _Alloc_traits::allocate(__a, 1);
687	// _GLIBCXX_RESOLVE_LIB_DEFECTS
688	// 3548. shared_ptr construction from unique_ptr should move
689	// (not copy) the deleter
690	_Alloc_traits::construct(__a, __mem, __r.release(),
691	std::forward<_Del>(__r.get_deleter()));
692	_M_pi = __mem;
693	}
694
695	// Throw bad_weak_ptr when __r._M_get_use_count() == 0.
696	explicit __shared_count(const __weak_count<_Lp>& __r);
697
698	// Does not throw if __r._M_get_use_count() == 0, caller must check.
699	explicit
700	__shared_count(const __weak_count<_Lp>& __r, std::nothrow_t) noexcept;
701
702	~__shared_count() noexcept
703	{
704	if (_M_pi != nullptr)
705	_M_pi->_M_release();
706	}
707
708	__shared_count(const __shared_count& __r) noexcept
709	: _M_pi(__r._M_pi)
710	{
711	if (_M_pi != nullptr)
712	_M_pi->_M_add_ref_copy();
713	}
714
715	__shared_count&
716	operator=(const __shared_count& __r) noexcept
717	{
718	_Sp_counted_base<_Lp>* __tmp = __r._M_pi;
719	if (__tmp != _M_pi)
720	{
721	if (__tmp != nullptr)
722	__tmp->_M_add_ref_copy();
723	if (_M_pi != nullptr)
724	_M_pi->_M_release();
725	_M_pi = __tmp;
726	}
727	return *this;
728	}
729
730	void
731	_M_swap(__shared_count& __r) noexcept
732	{
733	_Sp_counted_base<_Lp>* __tmp = __r._M_pi;
734	__r._M_pi = _M_pi;
735	_M_pi = __tmp;
736	}
737
738	long
739	_M_get_use_count() const noexcept
740	{ return _M_pi ? _M_pi->_M_get_use_count() : 0; }
741
742	bool
743	_M_unique() const noexcept
744	{ return this->_M_get_use_count() == 1; }
745
746	void*
747	_M_get_deleter(const std::type_info& __ti) const noexcept
748	{ return _M_pi ? _M_pi->_M_get_deleter(__ti) : nullptr; }
749
750	bool
751	_M_less(const __shared_count& __rhs) const noexcept
752	{ return std::less<_Sp_counted_base<_Lp>*>()(this->_M_pi, __rhs._M_pi); }
753
754	bool
755	_M_less(const __weak_count<_Lp>& __rhs) const noexcept
756	{ return std::less<_Sp_counted_base<_Lp>*>()(this->_M_pi, __rhs._M_pi); }
757
758	// Friend function injected into enclosing namespace and found by ADL
759	friend inline bool
760	operator==(const __shared_count& __a, const __shared_count& __b) noexcept
761	{ return __a._M_pi == __b._M_pi; }
762
763	private:
764	friend class __weak_count<_Lp>;
765
766	_Sp_counted_base<_Lp>* _M_pi;
767	};
768
769
770	template<_Lock_policy _Lp>
771	class __weak_count
772	{
773	public:
774	constexpr __weak_count() noexcept : _M_pi(nullptr)
775	{ }
776
777	__weak_count(const __shared_count<_Lp>& __r) noexcept
778	: _M_pi(__r._M_pi)
779	{
780	if (_M_pi != nullptr)
781	_M_pi->_M_weak_add_ref();
782	}
783
784	__weak_count(const __weak_count& __r) noexcept
785	: _M_pi(__r._M_pi)
786	{
787	if (_M_pi != nullptr)
788	_M_pi->_M_weak_add_ref();
789	}
790
791	__weak_count(__weak_count&& __r) noexcept
792	: _M_pi(__r._M_pi)
793	{ __r._M_pi = nullptr; }
794
795	~__weak_count() noexcept
796	{
797	if (_M_pi != nullptr)
798	_M_pi->_M_weak_release();
799	}
800
801	__weak_count&
802	operator=(const __shared_count<_Lp>& __r) noexcept
803	{
804	_Sp_counted_base<_Lp>* __tmp = __r._M_pi;
805	if (__tmp != nullptr)
806	__tmp->_M_weak_add_ref();
807	if (_M_pi != nullptr)
808	_M_pi->_M_weak_release();
809	_M_pi = __tmp;
810	return *this;
811	}
812
813	__weak_count&
814	operator=(const __weak_count& __r) noexcept
815	{
816	_Sp_counted_base<_Lp>* __tmp = __r._M_pi;
817	if (__tmp != nullptr)
818	__tmp->_M_weak_add_ref();
819	if (_M_pi != nullptr)
820	_M_pi->_M_weak_release();
821	_M_pi = __tmp;
822	return *this;
823	}
824
825	__weak_count&
826	operator=(__weak_count&& __r) noexcept
827	{
828	if (_M_pi != nullptr)
829	_M_pi->_M_weak_release();
830	_M_pi = __r._M_pi;
831	__r._M_pi = nullptr;
832	return *this;
833	}
834
835	void
836	_M_swap(__weak_count& __r) noexcept
837	{
838	_Sp_counted_base<_Lp>* __tmp = __r._M_pi;
839	__r._M_pi = _M_pi;
840	_M_pi = __tmp;
841	}
842
843	long
844	_M_get_use_count() const noexcept
845	{ return _M_pi != nullptr ? _M_pi->_M_get_use_count() : 0; }
846
847	bool
848	_M_less(const __weak_count& __rhs) const noexcept
849	{ return std::less<_Sp_counted_base<_Lp>*>()(this->_M_pi, __rhs._M_pi); }
850
851	bool
852	_M_less(const __shared_count<_Lp>& __rhs) const noexcept
853	{ return std::less<_Sp_counted_base<_Lp>*>()(this->_M_pi, __rhs._M_pi); }
854
855	// Friend function injected into enclosing namespace and found by ADL
856	friend inline bool
857	operator==(const __weak_count& __a, const __weak_count& __b) noexcept
858	{ return __a._M_pi == __b._M_pi; }
859
860	private:
861	friend class __shared_count<_Lp>;
862
863	_Sp_counted_base<_Lp>* _M_pi;
864	};
865
866	// Now that __weak_count is defined we can define this constructor:
867	template<_Lock_policy _Lp>
868	inline
869	__shared_count<_Lp>::__shared_count(const __weak_count<_Lp>& __r)
870	: _M_pi(__r._M_pi)
871	{
872	if (_M_pi == nullptr || !_M_pi->_M_add_ref_lock_nothrow())
873	__throw_bad_weak_ptr();
874	}
875
876	// Now that __weak_count is defined we can define this constructor:
877	template<_Lock_policy _Lp>
878	inline
879	__shared_count<_Lp>::
880	__shared_count(const __weak_count<_Lp>& __r, std::nothrow_t) noexcept
881	: _M_pi(__r._M_pi)
882	{
883	if (_M_pi && !_M_pi->_M_add_ref_lock_nothrow())
884	_M_pi = nullptr;
885	}
886
887	#define __cpp_lib_shared_ptr_arrays 201611L
888
889	// Helper traits for shared_ptr of array:
890
891	// A pointer type Y* is said to be compatible with a pointer type T* when
892	// either Y* is convertible to T* or Y is U[N] and T is U cv [].
893	template<typename _Yp_ptr, typename _Tp_ptr>
894	struct __sp_compatible_with
895	: false_type
896	{ };
897
898	template<typename _Yp, typename _Tp>
899	struct __sp_compatible_with<_Yp*, _Tp*>
900	: is_convertible<_Yp*, _Tp*>::type
901	{ };
902
903	template<typename _Up, size_t _Nm>
904	struct __sp_compatible_with<_Up(*)[_Nm], _Up(*)[]>
905	: true_type
906	{ };
907
908	template<typename _Up, size_t _Nm>
909	struct __sp_compatible_with<_Up(*)[_Nm], const _Up(*)[]>
910	: true_type
911	{ };
912
913	template<typename _Up, size_t _Nm>
914	struct __sp_compatible_with<_Up(*)[_Nm], volatile _Up(*)[]>
915	: true_type
916	{ };
917
918	template<typename _Up, size_t _Nm>
919	struct __sp_compatible_with<_Up(*)[_Nm], const volatile _Up(*)[]>
920	: true_type
921	{ };
922
923	// Test conversion from Y(*)[N] to U(*)[N] without forming invalid type Y[N].
924	template<typename _Up, size_t _Nm, typename _Yp, typename = void>
925	struct __sp_is_constructible_arrN
926	: false_type
927	{ };
928
929	template<typename _Up, size_t _Nm, typename _Yp>
930	struct __sp_is_constructible_arrN<_Up, _Nm, _Yp, __void_t<_Yp[_Nm]>>
931	: is_convertible<_Yp(*)[_Nm], _Up(*)[_Nm]>::type
932	{ };
933
934	// Test conversion from Y(*)[] to U(*)[] without forming invalid type Y[].
935	template<typename _Up, typename _Yp, typename = void>
936	struct __sp_is_constructible_arr
937	: false_type
938	{ };
939
940	template<typename _Up, typename _Yp>
941	struct __sp_is_constructible_arr<_Up, _Yp, __void_t<_Yp[]>>
942	: is_convertible<_Yp(*)[], _Up(*)[]>::type
943	{ };
944
945	// Trait to check if shared_ptr<T> can be constructed from Y*.
946	template<typename _Tp, typename _Yp>
947	struct __sp_is_constructible;
948
949	// When T is U[N], Y(*)[N] shall be convertible to T*;
950	template<typename _Up, size_t _Nm, typename _Yp>
951	struct __sp_is_constructible<_Up[_Nm], _Yp>
952	: __sp_is_constructible_arrN<_Up, _Nm, _Yp>::type
953	{ };
954
955	// when T is U[], Y(*)[] shall be convertible to T*;
956	template<typename _Up, typename _Yp>
957	struct __sp_is_constructible<_Up[], _Yp>
958	: __sp_is_constructible_arr<_Up, _Yp>::type
959	{ };
960
961	// otherwise, Y* shall be convertible to T*.
962	template<typename _Tp, typename _Yp>
963	struct __sp_is_constructible
964	: is_convertible<_Yp*, _Tp*>::type
965	{ };
966
967
968	// Define operator* and operator-> for shared_ptr<T>.
969	template<typename _Tp, _Lock_policy _Lp,
970	bool = is_array<_Tp>::value, bool = is_void<_Tp>::value>
971	class __shared_ptr_access
972	{
973	public:
974	using element_type = _Tp;
975
976	element_type&
977	operator*() const noexcept
978	{
979	__glibcxx_assert(_M_get() != nullptr);
980	return *_M_get();
981	}
982
983	element_type*
984	operator->() const noexcept
985	{
986	_GLIBCXX_DEBUG_PEDASSERT(_M_get() != nullptr);
987	return _M_get();
988	}
989
990	private:
991	element_type*
992	_M_get() const noexcept
993	{ return static_cast<const __shared_ptr<_Tp, _Lp>*>(this)->get(); }
994	};
995
996	// Define operator-> for shared_ptr<cv void>.
997	template<typename _Tp, _Lock_policy _Lp>
998	class __shared_ptr_access<_Tp, _Lp, false, true>
999	{
1000	public:
1001	using element_type = _Tp;
1002
1003	element_type*
1004	operator->() const noexcept
1005	{
1006	auto __ptr = static_cast<const __shared_ptr<_Tp, _Lp>*>(this)->get();
1007	_GLIBCXX_DEBUG_PEDASSERT(__ptr != nullptr);
1008	return __ptr;
1009	}
1010	};
1011
1012	// Define operator[] for shared_ptr<T[]> and shared_ptr<T[N]>.
1013	template<typename _Tp, _Lock_policy _Lp>
1014	class __shared_ptr_access<_Tp, _Lp, true, false>
1015	{
1016	public:
1017	using element_type = typename remove_extent<_Tp>::type;
1018
1019	#if __cplusplus <= 201402L
1020	[[__deprecated__("shared_ptr<T[]>::operator* is absent from C++17")]]
1021	element_type&
1022	operator*() const noexcept
1023	{
1024	__glibcxx_assert(_M_get() != nullptr);
1025	return *_M_get();
1026	}
1027
1028	[[__deprecated__("shared_ptr<T[]>::operator-> is absent from C++17")]]
1029	element_type*
1030	operator->() const noexcept
1031	{
1032	_GLIBCXX_DEBUG_PEDASSERT(_M_get() != nullptr);
1033	return _M_get();
1034	}
1035	#endif
1036
1037	element_type&
1038	operator[](ptrdiff_t __i) const
1039	{
1040	__glibcxx_assert(_M_get() != nullptr);
1041	__glibcxx_assert(!extent<_Tp>::value || __i < extent<_Tp>::value);
1042	return _M_get()[__i];
1043	}
1044
1045	private:
1046	element_type*
1047	_M_get() const noexcept
1048	{ return static_cast<const __shared_ptr<_Tp, _Lp>*>(this)->get(); }
1049	};
1050
1051	template<typename _Tp, _Lock_policy _Lp>
1052	class __shared_ptr
1053	: public __shared_ptr_access<_Tp, _Lp>
1054	{
1055	public:
1056	using element_type = typename remove_extent<_Tp>::type;
1057
1058	private:
1059	// Constraint for taking ownership of a pointer of type _Yp*:
1060	template<typename _Yp>
1061	using _SafeConv
1062	= typename enable_if<__sp_is_constructible<_Tp, _Yp>::value>::type;
1063
1064	// Constraint for construction from shared_ptr and weak_ptr:
1065	template<typename _Yp, typename _Res = void>
1066	using _Compatible = typename
1067	enable_if<__sp_compatible_with<_Yp*, _Tp*>::value, _Res>::type;
1068
1069	// Constraint for assignment from shared_ptr and weak_ptr:
1070	template<typename _Yp>
1071	using _Assignable = _Compatible<_Yp, __shared_ptr&>;
1072
1073	// Constraint for construction from unique_ptr:
1074	template<typename _Yp, typename _Del, typename _Res = void,
1075	typename _Ptr = typename unique_ptr<_Yp, _Del>::pointer>
1076	using _UniqCompatible = __enable_if_t<__and_<
1077	__sp_compatible_with<_Yp*, _Tp*>,
1078	is_convertible<_Ptr, element_type*>,
1079	is_move_constructible<_Del>
1080	>::value, _Res>;
1081
1082	// Constraint for assignment from unique_ptr:
1083	template<typename _Yp, typename _Del>
1084	using _UniqAssignable = _UniqCompatible<_Yp, _Del, __shared_ptr&>;
1085
1086	public:
1087
1088	#if __cplusplus > 201402L
1089	using weak_type = __weak_ptr<_Tp, _Lp>;
1090	#endif
1091
1092	constexpr __shared_ptr() noexcept
1093	: _M_ptr(0), _M_refcount()
1094	{ }
1095
1096	template<typename _Yp, typename = _SafeConv<_Yp>>
1097	explicit
1098	__shared_ptr(_Yp* __p)
1099	: _M_ptr(__p), _M_refcount(__p, typename is_array<_Tp>::type())
1100	{
1101	static_assert( !is_void<_Yp>::value, "incomplete type" );
1102	static_assert( sizeof(_Yp) > 0, "incomplete type" );
1103	_M_enable_shared_from_this_with(__p);
1104	}
1105
1106	template<typename _Yp, typename _Deleter, typename = _SafeConv<_Yp>>
1107	__shared_ptr(_Yp* __p, _Deleter __d)
1108	: _M_ptr(__p), _M_refcount(__p, std::move(__d))
1109	{
1110	static_assert(__is_invocable<_Deleter&, _Yp*&>::value,
1111	"deleter expression d(p) is well-formed");
1112	_M_enable_shared_from_this_with(__p);
1113	}
1114
1115	template<typename _Yp, typename _Deleter, typename _Alloc,
1116	typename = _SafeConv<_Yp>>
1117	__shared_ptr(_Yp* __p, _Deleter __d, _Alloc __a)
1118	: _M_ptr(__p), _M_refcount(__p, std::move(__d), std::move(__a))
1119	{
1120	static_assert(__is_invocable<_Deleter&, _Yp*&>::value,
1121	"deleter expression d(p) is well-formed");
1122	_M_enable_shared_from_this_with(__p);
1123	}
1124
1125	template<typename _Deleter>
1126	__shared_ptr(nullptr_t __p, _Deleter __d)
1127	: _M_ptr(0), _M_refcount(__p, std::move(__d))
1128	{ }
1129
1130	template<typename _Deleter, typename _Alloc>
1131	__shared_ptr(nullptr_t __p, _Deleter __d, _Alloc __a)
1132	: _M_ptr(0), _M_refcount(__p, std::move(__d), std::move(__a))
1133	{ }
1134
1135	// Aliasing constructor
1136	template<typename _Yp>
1137	__shared_ptr(const __shared_ptr<_Yp, _Lp>& __r,
1138	element_type* __p) noexcept
1139	: _M_ptr(__p), _M_refcount(__r._M_refcount) // never throws
1140	{ }
1141
1142	// Aliasing constructor
1143	template<typename _Yp>
1144	__shared_ptr(__shared_ptr<_Yp, _Lp>&& __r,
1145	element_type* __p) noexcept
1146	: _M_ptr(__p), _M_refcount()
1147	{
1148	_M_refcount._M_swap(__r._M_refcount);
1149	__r._M_ptr = nullptr;
1150	}
1151
1152	__shared_ptr(const __shared_ptr&) noexcept = default;
1153	__shared_ptr& operator=(const __shared_ptr&) noexcept = default;
1154	~__shared_ptr() = default;
1155
1156	template<typename _Yp, typename = _Compatible<_Yp>>
1157	__shared_ptr(const __shared_ptr<_Yp, _Lp>& __r) noexcept
1158	: _M_ptr(__r._M_ptr), _M_refcount(__r._M_refcount)
1159	{ }
1160
1161	__shared_ptr(__shared_ptr&& __r) noexcept
1162	: _M_ptr(__r._M_ptr), _M_refcount()
1163	{
1164	_M_refcount._M_swap(__r._M_refcount);
1165	__r._M_ptr = nullptr;
1166	}
1167
1168	template<typename _Yp, typename = _Compatible<_Yp>>
1169	__shared_ptr(__shared_ptr<_Yp, _Lp>&& __r) noexcept
1170	: _M_ptr(__r._M_ptr), _M_refcount()
1171	{
1172	_M_refcount._M_swap(__r._M_refcount);
1173	__r._M_ptr = nullptr;
1174	}
1175
1176	template<typename _Yp, typename = _Compatible<_Yp>>
1177	explicit __shared_ptr(const __weak_ptr<_Yp, _Lp>& __r)
1178	: _M_refcount(__r._M_refcount) // may throw
1179	{
1180	// It is now safe to copy __r._M_ptr, as
1181	// _M_refcount(__r._M_refcount) did not throw.
1182	_M_ptr = __r._M_ptr;
1183	}
1184
1185	// If an exception is thrown this constructor has no effect.
1186	template<typename _Yp, typename _Del,
1187	typename = _UniqCompatible<_Yp, _Del>>
1188	__shared_ptr(unique_ptr<_Yp, _Del>&& __r)
1189	: _M_ptr(__r.get()), _M_refcount()
1190	{
1191	auto __raw = __to_address(__r.get());
1192	_M_refcount = __shared_count<_Lp>(std::move(__r));
1193	_M_enable_shared_from_this_with(__raw);
1194	}
1195
1196	#if __cplusplus <= 201402L && _GLIBCXX_USE_DEPRECATED
1197	protected:
1198	// If an exception is thrown this constructor has no effect.
1199	template<typename _Tp1, typename _Del,
1200	typename enable_if<__and_<
1201	__not_<is_array<_Tp>>, is_array<_Tp1>,
1202	is_convertible<typename unique_ptr<_Tp1, _Del>::pointer, _Tp*>
1203	>::value, bool>::type = true>
1204	__shared_ptr(unique_ptr<_Tp1, _Del>&& __r, __sp_array_delete)
1205	: _M_ptr(__r.get()), _M_refcount()
1206	{
1207	auto __raw = __to_address(__r.get());
1208	_M_refcount = __shared_count<_Lp>(std::move(__r));
1209	_M_enable_shared_from_this_with(__raw);
1210	}
1211	public:
1212	#endif
1213
1214	#if _GLIBCXX_USE_DEPRECATED
1215	#pragma GCC diagnostic push
1216	#pragma GCC diagnostic ignored "-Wdeprecated-declarations"
1217	// Postcondition: use_count() == 1 and __r.get() == 0
1218	template<typename _Yp, typename = _Compatible<_Yp>>
1219	__shared_ptr(auto_ptr<_Yp>&& __r);
1220	#pragma GCC diagnostic pop
1221	#endif
1222
1223	constexpr __shared_ptr(nullptr_t) noexcept : __shared_ptr() { }
1224
1225	template<typename _Yp>
1226	_Assignable<_Yp>
1227	operator=(const __shared_ptr<_Yp, _Lp>& __r) noexcept
1228	{
1229	_M_ptr = __r._M_ptr;
1230	_M_refcount = __r._M_refcount; // __shared_count::op= doesn't throw
1231	return *this;
1232	}
1233
1234	#if _GLIBCXX_USE_DEPRECATED
1235	#pragma GCC diagnostic push
1236	#pragma GCC diagnostic ignored "-Wdeprecated-declarations"
1237	template<typename _Yp>
1238	_Assignable<_Yp>
1239	operator=(auto_ptr<_Yp>&& __r)
1240	{
1241	__shared_ptr(std::move(__r)).swap(*this);
1242	return *this;
1243	}
1244	#pragma GCC diagnostic pop
1245	#endif
1246
1247	__shared_ptr&
1248	operator=(__shared_ptr&& __r) noexcept
1249	{
1250	__shared_ptr(std::move(__r)).swap(*this);
1251	return *this;
1252	}
1253
1254	template<class _Yp>
1255	_Assignable<_Yp>
1256	operator=(__shared_ptr<_Yp, _Lp>&& __r) noexcept
1257	{
1258	__shared_ptr(std::move(__r)).swap(*this);
1259	return *this;
1260	}
1261
1262	template<typename _Yp, typename _Del>
1263	_UniqAssignable<_Yp, _Del>
1264	operator=(unique_ptr<_Yp, _Del>&& __r)
1265	{
1266	__shared_ptr(std::move(__r)).swap(*this);
1267	return *this;
1268	}
1269
1270	void
1271	reset() noexcept
1272	{ __shared_ptr().swap(*this); }
1273
1274	template<typename _Yp>
1275	_SafeConv<_Yp>
1276	reset(_Yp* __p) // _Yp must be complete.
1277	{
1278	// Catch self-reset errors.
1279	__glibcxx_assert(__p == nullptr || __p != _M_ptr);
1280	__shared_ptr(__p).swap(*this);
1281	}
1282
1283	template<typename _Yp, typename _Deleter>
1284	_SafeConv<_Yp>
1285	reset(_Yp* __p, _Deleter __d)
1286	{ __shared_ptr(__p, std::move(__d)).swap(*this); }
1287
1288	template<typename _Yp, typename _Deleter, typename _Alloc>
1289	_SafeConv<_Yp>
1290	reset(_Yp* __p, _Deleter __d, _Alloc __a)
1291	{ __shared_ptr(__p, std::move(__d), std::move(__a)).swap(*this); }
1292
1293	/// Return the stored pointer.
1294	element_type*
1295	get() const noexcept
1296	{ return _M_ptr; }
1297
1298	/// Return true if the stored pointer is not null.
1299	explicit operator bool() const noexcept
1300	{ return _M_ptr != nullptr; }
1301
1302	/// Return true if use_count() == 1.
1303	bool
1304	unique() const noexcept
1305	{ return _M_refcount._M_unique(); }
1306
1307	/// If *this owns a pointer, return the number of owners, otherwise zero.
1308	long
1309	use_count() const noexcept
1310	{ return _M_refcount._M_get_use_count(); }
1311
1312	/// Exchange both the owned pointer and the stored pointer.
1313	void
1314	swap(__shared_ptr<_Tp, _Lp>& __other) noexcept
1315	{
1316	std::swap(_M_ptr, __other._M_ptr);
1317	_M_refcount._M_swap(__other._M_refcount);
1318	}
1319
1320	/** @brief Define an ordering based on ownership.
1321	*
1322	* This function defines a strict weak ordering between two shared_ptr
1323	* or weak_ptr objects, such that one object is less than the other
1324	* unless they share ownership of the same pointer, or are both empty.
1325	* @{
1326	*/
1327	template<typename _Tp1>
1328	bool
1329	owner_before(__shared_ptr<_Tp1, _Lp> const& __rhs) const noexcept
1330	{ return _M_refcount._M_less(__rhs._M_refcount); }
1331
1332	template<typename _Tp1>
1333	bool
1334	owner_before(__weak_ptr<_Tp1, _Lp> const& __rhs) const noexcept
1335	{ return _M_refcount._M_less(__rhs._M_refcount); }
1336	/// @}
1337
1338	protected:
1339	// This constructor is non-standard, it is used by allocate_shared.
1340	template<typename _Alloc, typename... _Args>
1341	__shared_ptr(_Sp_alloc_shared_tag<_Alloc> __tag, _Args&&... __args)
1342	: _M_ptr(), _M_refcount(_M_ptr, __tag, std::forward<_Args>(__args)...)
1343	{ _M_enable_shared_from_this_with(_M_ptr); }
1344
1345	template<typename _Tp1, _Lock_policy _Lp1, typename _Alloc,
1346	typename... _Args>
1347	friend __shared_ptr<_Tp1, _Lp1>
1348	__allocate_shared(const _Alloc& __a, _Args&&... __args);
1349
1350	// This constructor is used by __weak_ptr::lock() and
1351	// shared_ptr::shared_ptr(const weak_ptr&, std::nothrow_t).
1352	__shared_ptr(const __weak_ptr<_Tp, _Lp>& __r, std::nothrow_t) noexcept
1353	: _M_refcount(__r._M_refcount, std::nothrow)
1354	{
1355	_M_ptr = _M_refcount._M_get_use_count() ? __r._M_ptr : nullptr;
1356	}
1357
1358	friend class __weak_ptr<_Tp, _Lp>;
1359
1360	private:
1361
1362	template<typename _Yp>
1363	using __esft_base_t = decltype(__enable_shared_from_this_base(
1364	std::declval<const __shared_count<_Lp>&>(),
1365	std::declval<_Yp*>()));
1366
1367	// Detect an accessible and unambiguous enable_shared_from_this base.
1368	template<typename _Yp, typename = void>
1369	struct __has_esft_base
1370	: false_type { };
1371
1372	template<typename _Yp>
1373	struct __has_esft_base<_Yp, __void_t<__esft_base_t<_Yp>>>
1374	: __not_<is_array<_Tp>> { }; // No enable shared_from_this for arrays
1375
1376	template<typename _Yp, typename _Yp2 = typename remove_cv<_Yp>::type>
1377	typename enable_if<__has_esft_base<_Yp2>::value>::type
1378	_M_enable_shared_from_this_with(_Yp* __p) noexcept
1379	{
1380	if (auto __base = __enable_shared_from_this_base(_M_refcount, __p))
1381	__base->_M_weak_assign(const_cast<_Yp2*>(__p), _M_refcount);
1382	}
1383
1384	template<typename _Yp, typename _Yp2 = typename remove_cv<_Yp>::type>
1385	typename enable_if<!__has_esft_base<_Yp2>::value>::type
1386	_M_enable_shared_from_this_with(_Yp*) noexcept
1387	{ }
1388
1389	void*
1390	_M_get_deleter(const std::type_info& __ti) const noexcept
1391	{ return _M_refcount._M_get_deleter(__ti); }
1392
1393	template<typename _Tp1, _Lock_policy _Lp1> friend class __shared_ptr;
1394	template<typename _Tp1, _Lock_policy _Lp1> friend class __weak_ptr;
1395
1396	template<typename _Del, typename _Tp1, _Lock_policy _Lp1>
1397	friend _Del* get_deleter(const __shared_ptr<_Tp1, _Lp1>&) noexcept;
1398
1399	template<typename _Del, typename _Tp1>
1400	friend _Del* get_deleter(const shared_ptr<_Tp1>&) noexcept;
1401
1402	element_type* _M_ptr; // Contained pointer.
1403	__shared_count<_Lp> _M_refcount; // Reference counter.
1404	};
1405
1406
1407	// 20.7.2.2.7 shared_ptr comparisons
1408	template<typename _Tp1, typename _Tp2, _Lock_policy _Lp>
1409	inline bool
1410	operator==(const __shared_ptr<_Tp1, _Lp>& __a,
1411	const __shared_ptr<_Tp2, _Lp>& __b) noexcept
1412	{ return __a.get() == __b.get(); }
1413
1414	template<typename _Tp, _Lock_policy _Lp>
1415	inline bool
1416	operator==(const __shared_ptr<_Tp, _Lp>& __a, nullptr_t) noexcept
1417	{ return !__a; }
1418
1419	#ifdef __cpp_lib_three_way_comparison
1420	template<typename _Tp, typename _Up, _Lock_policy _Lp>
1421	inline strong_ordering
1422	operator<=>(const __shared_ptr<_Tp, _Lp>& __a,
1423	const __shared_ptr<_Up, _Lp>& __b) noexcept
1424	{ return compare_three_way()(__a.get(), __b.get()); }
1425
1426	template<typename _Tp, _Lock_policy _Lp>
1427	inline strong_ordering
1428	operator<=>(const __shared_ptr<_Tp, _Lp>& __a, nullptr_t) noexcept
1429	{
1430	using pointer = typename __shared_ptr<_Tp, _Lp>::element_type*;
1431	return compare_three_way()(__a.get(), static_cast<pointer>(nullptr));
1432	}
1433	#else
1434	template<typename _Tp, _Lock_policy _Lp>
1435	inline bool
1436	operator==(nullptr_t, const __shared_ptr<_Tp, _Lp>& __a) noexcept
1437	{ return !__a; }
1438
1439	template<typename _Tp1, typename _Tp2, _Lock_policy _Lp>
1440	inline bool
1441	operator!=(const __shared_ptr<_Tp1, _Lp>& __a,
1442	const __shared_ptr<_Tp2, _Lp>& __b) noexcept
1443	{ return __a.get() != __b.get(); }
1444
1445	template<typename _Tp, _Lock_policy _Lp>
1446	inline bool
1447	operator!=(const __shared_ptr<_Tp, _Lp>& __a, nullptr_t) noexcept
1448	{ return (bool)__a; }
1449
1450	template<typename _Tp, _Lock_policy _Lp>
1451	inline bool
1452	operator!=(nullptr_t, const __shared_ptr<_Tp, _Lp>& __a) noexcept
1453	{ return (bool)__a; }
1454
1455	template<typename _Tp, typename _Up, _Lock_policy _Lp>
1456	inline bool
1457	operator<(const __shared_ptr<_Tp, _Lp>& __a,
1458	const __shared_ptr<_Up, _Lp>& __b) noexcept
1459	{
1460	using _Tp_elt = typename __shared_ptr<_Tp, _Lp>::element_type;
1461	using _Up_elt = typename __shared_ptr<_Up, _Lp>::element_type;
1462	using _Vp = typename common_type<_Tp_elt*, _Up_elt*>::type;
1463	return less<_Vp>()(__a.get(), __b.get());
1464	}
1465
1466	template<typename _Tp, _Lock_policy _Lp>
1467	inline bool
1468	operator<(const __shared_ptr<_Tp, _Lp>& __a, nullptr_t) noexcept
1469	{
1470	using _Tp_elt = typename __shared_ptr<_Tp, _Lp>::element_type;
1471	return less<_Tp_elt*>()(__a.get(), nullptr);
1472	}
1473
1474	template<typename _Tp, _Lock_policy _Lp>
1475	inline bool
1476	operator<(nullptr_t, const __shared_ptr<_Tp, _Lp>& __a) noexcept
1477	{
1478	using _Tp_elt = typename __shared_ptr<_Tp, _Lp>::element_type;
1479	return less<_Tp_elt*>()(nullptr, __a.get());
1480	}
1481
1482	template<typename _Tp1, typename _Tp2, _Lock_policy _Lp>
1483	inline bool
1484	operator<=(const __shared_ptr<_Tp1, _Lp>& __a,
1485	const __shared_ptr<_Tp2, _Lp>& __b) noexcept
1486	{ return !(__b < __a); }
1487
1488	template<typename _Tp, _Lock_policy _Lp>
1489	inline bool
1490	operator<=(const __shared_ptr<_Tp, _Lp>& __a, nullptr_t) noexcept
1491	{ return !(nullptr < __a); }
1492
1493	template<typename _Tp, _Lock_policy _Lp>
1494	inline bool
1495	operator<=(nullptr_t, const __shared_ptr<_Tp, _Lp>& __a) noexcept
1496	{ return !(__a < nullptr); }
1497
1498	template<typename _Tp1, typename _Tp2, _Lock_policy _Lp>
1499	inline bool
1500	operator>(const __shared_ptr<_Tp1, _Lp>& __a,
1501	const __shared_ptr<_Tp2, _Lp>& __b) noexcept
1502	{ return (__b < __a); }
1503
1504	template<typename _Tp, _Lock_policy _Lp>
1505	inline bool
1506	operator>(const __shared_ptr<_Tp, _Lp>& __a, nullptr_t) noexcept
1507	{ return nullptr < __a; }
1508
1509	template<typename _Tp, _Lock_policy _Lp>
1510	inline bool
1511	operator>(nullptr_t, const __shared_ptr<_Tp, _Lp>& __a) noexcept
1512	{ return __a < nullptr; }
1513
1514	template<typename _Tp1, typename _Tp2, _Lock_policy _Lp>
1515	inline bool
1516	operator>=(const __shared_ptr<_Tp1, _Lp>& __a,
1517	const __shared_ptr<_Tp2, _Lp>& __b) noexcept
1518	{ return !(__a < __b); }
1519
1520	template<typename _Tp, _Lock_policy _Lp>
1521	inline bool
1522	operator>=(const __shared_ptr<_Tp, _Lp>& __a, nullptr_t) noexcept
1523	{ return !(__a < nullptr); }
1524
1525	template<typename _Tp, _Lock_policy _Lp>
1526	inline bool
1527	operator>=(nullptr_t, const __shared_ptr<_Tp, _Lp>& __a) noexcept
1528	{ return !(nullptr < __a); }
1529	#endif // three-way comparison
1530
1531	// 20.7.2.2.8 shared_ptr specialized algorithms.
1532	template<typename _Tp, _Lock_policy _Lp>
1533	inline void
1534	swap(__shared_ptr<_Tp, _Lp>& __a, __shared_ptr<_Tp, _Lp>& __b) noexcept
1535	{ __a.swap(__b); }
1536
1537	// 20.7.2.2.9 shared_ptr casts
1538
1539	// The seemingly equivalent code:
1540	// shared_ptr<_Tp, _Lp>(static_cast<_Tp*>(__r.get()))
1541	// will eventually result in undefined behaviour, attempting to
1542	// delete the same object twice.
1543	/// static_pointer_cast
1544	template<typename _Tp, typename _Tp1, _Lock_policy _Lp>
1545	inline __shared_ptr<_Tp, _Lp>
1546	static_pointer_cast(const __shared_ptr<_Tp1, _Lp>& __r) noexcept
1547	{
1548	using _Sp = __shared_ptr<_Tp, _Lp>;
1549	return _Sp(__r, static_cast<typename _Sp::element_type*>(__r.get()));
1550	}
1551
1552	// The seemingly equivalent code:
1553	// shared_ptr<_Tp, _Lp>(const_cast<_Tp*>(__r.get()))
1554	// will eventually result in undefined behaviour, attempting to
1555	// delete the same object twice.
1556	/// const_pointer_cast
1557	template<typename _Tp, typename _Tp1, _Lock_policy _Lp>
1558	inline __shared_ptr<_Tp, _Lp>
1559	const_pointer_cast(const __shared_ptr<_Tp1, _Lp>& __r) noexcept
1560	{
1561	using _Sp = __shared_ptr<_Tp, _Lp>;
1562	return _Sp(__r, const_cast<typename _Sp::element_type*>(__r.get()));
1563	}
1564
1565	// The seemingly equivalent code:
1566	// shared_ptr<_Tp, _Lp>(dynamic_cast<_Tp*>(__r.get()))
1567	// will eventually result in undefined behaviour, attempting to
1568	// delete the same object twice.
1569	/// dynamic_pointer_cast
1570	template<typename _Tp, typename _Tp1, _Lock_policy _Lp>
1571	inline __shared_ptr<_Tp, _Lp>
1572	dynamic_pointer_cast(const __shared_ptr<_Tp1, _Lp>& __r) noexcept
1573	{
1574	using _Sp = __shared_ptr<_Tp, _Lp>;
1575	if (auto* __p = dynamic_cast<typename _Sp::element_type*>(__r.get()))
1576	return _Sp(__r, __p);
1577	return _Sp();
1578	}
1579
1580	#if __cplusplus > 201402L
1581	template<typename _Tp, typename _Tp1, _Lock_policy _Lp>
1582	inline __shared_ptr<_Tp, _Lp>
1583	reinterpret_pointer_cast(const __shared_ptr<_Tp1, _Lp>& __r) noexcept
1584	{
1585	using _Sp = __shared_ptr<_Tp, _Lp>;
1586	return _Sp(__r, reinterpret_cast<typename _Sp::element_type*>(__r.get()));
1587	}
1588	#endif
1589
1590	template<typename _Tp, _Lock_policy _Lp>
1591	class __weak_ptr
1592	{
1593	template<typename _Yp, typename _Res = void>
1594	using _Compatible = typename
1595	enable_if<__sp_compatible_with<_Yp*, _Tp*>::value, _Res>::type;
1596
1597	// Constraint for assignment from shared_ptr and weak_ptr:
1598	template<typename _Yp>
1599	using _Assignable = _Compatible<_Yp, __weak_ptr&>;
1600
1601	public:
1602	using element_type = typename remove_extent<_Tp>::type;
1603
1604	constexpr __weak_ptr() noexcept
1605	: _M_ptr(nullptr), _M_refcount()
1606	{ }
1607
1608	__weak_ptr(const __weak_ptr&) noexcept = default;
1609
1610	~__weak_ptr() = default;
1611
1612	// The "obvious" converting constructor implementation:
1613	//
1614	// template<typename _Tp1>
1615	// __weak_ptr(const __weak_ptr<_Tp1, _Lp>& __r)
1616	// : _M_ptr(__r._M_ptr), _M_refcount(__r._M_refcount) // never throws
1617	// { }
1618	//
1619	// has a serious problem.
1620	//
1621	// __r._M_ptr may already have been invalidated. The _M_ptr(__r._M_ptr)
1622	// conversion may require access to *__r._M_ptr (virtual inheritance).
1623	//
1624	// It is not possible to avoid spurious access violations since
1625	// in multithreaded programs __r._M_ptr may be invalidated at any point.
1626	template<typename _Yp, typename = _Compatible<_Yp>>
1627	__weak_ptr(const __weak_ptr<_Yp, _Lp>& __r) noexcept
1628	: _M_refcount(__r._M_refcount)
1629	{ _M_ptr = __r.lock().get(); }
1630
1631	template<typename _Yp, typename = _Compatible<_Yp>>
1632	__weak_ptr(const __shared_ptr<_Yp, _Lp>& __r) noexcept
1633	: _M_ptr(__r._M_ptr), _M_refcount(__r._M_refcount)
1634	{ }
1635
1636	__weak_ptr(__weak_ptr&& __r) noexcept
1637	: _M_ptr(__r._M_ptr), _M_refcount(std::move(__r._M_refcount))
1638	{ __r._M_ptr = nullptr; }
1639
1640	template<typename _Yp, typename = _Compatible<_Yp>>
1641	__weak_ptr(__weak_ptr<_Yp, _Lp>&& __r) noexcept
1642	: _M_ptr(__r.lock().get()), _M_refcount(std::move(__r._M_refcount))
1643	{ __r._M_ptr = nullptr; }
1644
1645	__weak_ptr&
1646	operator=(const __weak_ptr& __r) noexcept = default;
1647
1648	template<typename _Yp>
1649	_Assignable<_Yp>
1650	operator=(const __weak_ptr<_Yp, _Lp>& __r) noexcept
1651	{
1652	_M_ptr = __r.lock().get();
1653	_M_refcount = __r._M_refcount;
1654	return *this;
1655	}
1656
1657	template<typename _Yp>
1658	_Assignable<_Yp>
1659	operator=(const __shared_ptr<_Yp, _Lp>& __r) noexcept
1660	{
1661	_M_ptr = __r._M_ptr;
1662	_M_refcount = __r._M_refcount;
1663	return *this;
1664	}
1665
1666	__weak_ptr&
1667	operator=(__weak_ptr&& __r) noexcept
1668	{
1669	__weak_ptr(std::move(__r)).swap(*this);
1670	return *this;
1671	}
1672
1673	template<typename _Yp>
1674	_Assignable<_Yp>
1675	operator=(__weak_ptr<_Yp, _Lp>&& __r) noexcept
1676	{
1677	_M_ptr = __r.lock().get();
1678	_M_refcount = std::move(__r._M_refcount);
1679	__r._M_ptr = nullptr;
1680	return *this;
1681	}
1682
1683	__shared_ptr<_Tp, _Lp>
1684	lock() const noexcept
1685	{ return __shared_ptr<element_type, _Lp>(*this, std::nothrow); }
1686
1687	long
1688	use_count() const noexcept
1689	{ return _M_refcount._M_get_use_count(); }
1690
1691	bool
1692	expired() const noexcept
1693	{ return _M_refcount._M_get_use_count() == 0; }
1694
1695	template<typename _Tp1>
1696	bool
1697	owner_before(const __shared_ptr<_Tp1, _Lp>& __rhs) const noexcept
1698	{ return _M_refcount._M_less(__rhs._M_refcount); }
1699
1700	template<typename _Tp1>
1701	bool
1702	owner_before(const __weak_ptr<_Tp1, _Lp>& __rhs) const noexcept
1703	{ return _M_refcount._M_less(__rhs._M_refcount); }
1704
1705	void
1706	reset() noexcept
1707	{ __weak_ptr().swap(*this); }
1708
1709	void
1710	swap(__weak_ptr& __s) noexcept
1711	{
1712	std::swap(_M_ptr, __s._M_ptr);
1713	_M_refcount._M_swap(__s._M_refcount);
1714	}
1715
1716	private:
1717	// Used by __enable_shared_from_this.
1718	void
1719	_M_assign(_Tp* __ptr, const __shared_count<_Lp>& __refcount) noexcept
1720	{
1721	if (use_count() == 0)
1722	{
1723	_M_ptr = __ptr;
1724	_M_refcount = __refcount;
1725	}
1726	}
1727
1728	template<typename _Tp1, _Lock_policy _Lp1> friend class __shared_ptr;
1729	template<typename _Tp1, _Lock_policy _Lp1> friend class __weak_ptr;
1730	friend class __enable_shared_from_this<_Tp, _Lp>;
1731	friend class enable_shared_from_this<_Tp>;
1732
1733	element_type* _M_ptr; // Contained pointer.
1734	__weak_count<_Lp> _M_refcount; // Reference counter.
1735	};
1736
1737	// 20.7.2.3.6 weak_ptr specialized algorithms.
1738	template<typename _Tp, _Lock_policy _Lp>
1739	inline void
1740	swap(__weak_ptr<_Tp, _Lp>& __a, __weak_ptr<_Tp, _Lp>& __b) noexcept
1741	{ __a.swap(__b); }
1742
1743	template<typename _Tp, typename _Tp1>
1744	struct _Sp_owner_less : public binary_function<_Tp, _Tp, bool>
1745	{
1746	bool
1747	operator()(const _Tp& __lhs, const _Tp& __rhs) const noexcept
1748	{ return __lhs.owner_before(__rhs); }
1749
1750	bool
1751	operator()(const _Tp& __lhs, const _Tp1& __rhs) const noexcept
1752	{ return __lhs.owner_before(__rhs); }
1753
1754	bool
1755	operator()(const _Tp1& __lhs, const _Tp& __rhs) const noexcept
1756	{ return __lhs.owner_before(__rhs); }
1757	};
1758
1759	template<>
1760	struct _Sp_owner_less<void, void>
1761	{
1762	template<typename _Tp, typename _Up>
1763	auto
1764	operator()(const _Tp& __lhs, const _Up& __rhs) const noexcept
1765	-> decltype(__lhs.owner_before(__rhs))
1766	{ return __lhs.owner_before(__rhs); }
1767
1768	using is_transparent = void;
1769	};
1770
1771	template<typename _Tp, _Lock_policy _Lp>
1772	struct owner_less<__shared_ptr<_Tp, _Lp>>
1773	: public _Sp_owner_less<__shared_ptr<_Tp, _Lp>, __weak_ptr<_Tp, _Lp>>
1774	{ };
1775
1776	template<typename _Tp, _Lock_policy _Lp>
1777	struct owner_less<__weak_ptr<_Tp, _Lp>>
1778	: public _Sp_owner_less<__weak_ptr<_Tp, _Lp>, __shared_ptr<_Tp, _Lp>>
1779	{ };
1780
1781
1782	template<typename _Tp, _Lock_policy _Lp>
1783	class __enable_shared_from_this
1784	{
1785	protected:
1786	constexpr __enable_shared_from_this() noexcept { }
1787
1788	__enable_shared_from_this(const __enable_shared_from_this&) noexcept { }
1789
1790	__enable_shared_from_this&
1791	operator=(const __enable_shared_from_this&) noexcept
1792	{ return *this; }
1793
1794	~__enable_shared_from_this() { }
1795
1796	public:
1797	__shared_ptr<_Tp, _Lp>
1798	shared_from_this()
1799	{ return __shared_ptr<_Tp, _Lp>(this->_M_weak_this); }
1800
1801	__shared_ptr<const _Tp, _Lp>
1802	shared_from_this() const
1803	{ return __shared_ptr<const _Tp, _Lp>(this->_M_weak_this); }
1804
1805	#if __cplusplus > 201402L || !defined(__STRICT_ANSI__) // c++1z or gnu++11
1806	__weak_ptr<_Tp, _Lp>
1807	weak_from_this() noexcept
1808	{ return this->_M_weak_this; }
1809
1810	__weak_ptr<const _Tp, _Lp>
1811	weak_from_this() const noexcept
1812	{ return this->_M_weak_this; }
1813	#endif
1814
1815	private:
1816	template<typename _Tp1>
1817	void
1818	_M_weak_assign(_Tp1* __p, const __shared_count<_Lp>& __n) const noexcept
1819	{ _M_weak_this._M_assign(__p, __n); }
1820
1821	friend const __enable_shared_from_this*
1822	__enable_shared_from_this_base(const __shared_count<_Lp>&,
1823	const __enable_shared_from_this* __p)
1824	{ return __p; }
1825
1826	template<typename, _Lock_policy>
1827	friend class __shared_ptr;
1828
1829	mutable __weak_ptr<_Tp, _Lp> _M_weak_this;
1830	};
1831
1832	template<typename _Tp, _Lock_policy _Lp = __default_lock_policy,
1833	typename _Alloc, typename... _Args>
1834	inline __shared_ptr<_Tp, _Lp>
1835	__allocate_shared(const _Alloc& __a, _Args&&... __args)
1836	{
1837	static_assert(!is_array<_Tp>::value, "make_shared<T[]> not supported");
1838
1839	return __shared_ptr<_Tp, _Lp>(_Sp_alloc_shared_tag<_Alloc>{__a},
1840	std::forward<_Args>(__args)...);
1841	}
1842
1843	template<typename _Tp, _Lock_policy _Lp = __default_lock_policy,
1844	typename... _Args>
1845	inline __shared_ptr<_Tp, _Lp>
1846	__make_shared(_Args&&... __args)
1847	{
1848	typedef typename std::remove_const<_Tp>::type _Tp_nc;
1849	return std::__allocate_shared<_Tp, _Lp>(std::allocator<_Tp_nc>(),
1850	std::forward<_Args>(__args)...);
1851	}
1852
1853	/// std::hash specialization for __shared_ptr.
1854	template<typename _Tp, _Lock_policy _Lp>
1855	struct hash<__shared_ptr<_Tp, _Lp>>
1856	: public __hash_base<size_t, __shared_ptr<_Tp, _Lp>>
1857	{
1858	size_t
1859	operator()(const __shared_ptr<_Tp, _Lp>& __s) const noexcept
1860	{
1861	return hash<typename __shared_ptr<_Tp, _Lp>::element_type*>()(
1862	__s.get());
1863	}
1864	};
1865
1866	_GLIBCXX_END_NAMESPACE_VERSION
1867	} // namespace
1868
1869	#endif // _SHARED_PTR_BASE_H
1870