【轉載】C++ 智慧指標（shared_ptr/weak_ptr）原始碼分析
阿新 • • 發佈：2018-10-31
發現一篇對C++11智慧指標分析很透徹的文章，特轉載備忘！
以下轉載自：https://blog.csdn.net/ithiker/article/details/51532484?utm_source=blogxgwz1
C++11目前已經引入了unique_ptr, shared_ptr, weak_ptr等智慧指標以及相關的模板類enable_shared_from_this等。shared_ptr實現了C++中的RAII機制，它不僅僅具有一般指標(build-in/raw)的特性，更重要的是它可以自動管理使用者在堆上建立的物件的生命週期，讓使用者不用負責記憶體回收，避免記憶體洩漏。一般的智慧指標都定義為一個模板類，它的型別由被管理的物件型別初始化，內部包含了指向該物件的指標以及指向輔助生命週期管理的管理物件的指標。
C++11中unique_ptr, shared_ptr, weak_ptr的特點如下：
- unique_ptr獨享被管理物件，同一時刻只能有一個unique_ptr擁有物件的所有權，當其被賦值時物件的所有權也發生轉移，當其被銷燬時被管理物件也自動被銷燬
- shared_ptr共享被管理物件，同一時刻可以有多個shared_ptr擁有物件的所有權，當最後一個shared_ptr物件銷燬時，被管理物件自動銷燬
- weak_ptr不擁有物件的所有權，但是它可以判斷物件是否存在和返回指向物件的shared_ptr型別指標；它的用途之一是解決了多個物件內部含有shared_ptr迴圈指向，導致物件無法釋放的問題
那麼C++中是怎麼實現這些特性的呢，我們可以在gcc的原始碼目錄（gcc-6.1.0\gcc-6.1.0\libstdc++-v3\include\tr1）中找到智慧指標的一種實現，通過分析其原始碼找到答案（其它boost::shared_ptr等的實現也是類似的）。gcc中相關智慧指標的實現原始碼如下：
// <tr1/shared_ptr.h> -*- C++ -*- // Copyright (C) 2007-2016 Free Software Foundation, Inc. // // This file is part of the GNU ISO C++ Library. This library is free// software; you can redistribute it and/or modify it under the // terms of the GNU General Public License as published by the // Free Software Foundation; either version 3, or (at your option) // any later version. // This library is distributed in the hope that it will be useful, // but WITHOUT ANY WARRANTY; without even the implied warranty of // MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the // GNU General Public License for more details. // Under Section 7 of GPL version 3, you are granted additional // permissions described in the GCC Runtime Library Exception, version // 3.1, as published by the Free Software Foundation. // You should have received a copy of the GNU General Public License and // a copy of the GCC Runtime Library Exception along with this program; // see the files COPYING3 and COPYING.RUNTIME respectively. If not, see // <http://www.gnu.org/licenses/>. // shared_count.hpp // Copyright (c) 2001, 2002, 2003 Peter Dimov and Multi Media Ltd. // shared_ptr.hpp // Copyright (C) 1998, 1999 Greg Colvin and Beman Dawes. // Copyright (C) 2001, 2002, 2003 Peter Dimov // weak_ptr.hpp // Copyright (C) 2001, 2002, 2003 Peter Dimov // enable_shared_from_this.hpp // Copyright (C) 2002 Peter Dimov // Distributed under the Boost Software License, Version 1.0. (See // accompanying file LICENSE_1_0.txt or copy at // http://www.boost.org/LICENSE_1_0.txt) // GCC Note: based on version 1.32.0 of the Boost library. /** @file tr1/shared_ptr.h * This is an internal header file, included by other library headers. * Do not attempt to use it directly. @headername{tr1/memory} */ #ifndef _TR1_SHARED_PTR_H #define _TR1_SHARED_PTR_H 1 namespace std _GLIBCXX_VISIBILITY(default) { namespace tr1 { _GLIBCXX_BEGIN_NAMESPACE_VERSION /** * @brief Exception possibly thrown by @c shared_ptr. * @ingroup exceptions */ class bad_weak_ptr : public std::exception { public: virtual char const* what() const throw() { return "tr1::bad_weak_ptr"; } }; // Substitute for bad_weak_ptr object in the case of -fno-exceptions. inline void __throw_bad_weak_ptr() { _GLIBCXX_THROW_OR_ABORT(bad_weak_ptr()); } using __gnu_cxx::_Lock_policy; using __gnu_cxx::__default_lock_policy; using __gnu_cxx::_S_single; using __gnu_cxx::_S_mutex; using __gnu_cxx::_S_atomic; // Empty helper class except when the template argument is _S_mutex. template<_Lock_policy _Lp> class _Mutex_base { protected: // The atomic policy uses fully-fenced builtins, single doesn't care. enum { _S_need_barriers = 0 }; }; template<> class _Mutex_base<_S_mutex> : public __gnu_cxx::__mutex { protected: // This policy is used when atomic builtins are not available. // The replacement atomic operations might not have the necessary // memory barriers. enum { _S_need_barriers = 1 }; }; template<_Lock_policy _Lp = __default_lock_policy> class _Sp_counted_base : public _Mutex_base<_Lp> { public: _Sp_counted_base() : _M_use_count(1), _M_weak_count(1) { } virtual ~_Sp_counted_base() // nothrow { } // Called when _M_use_count drops to zero, to release the resources // managed by *this. virtual void _M_dispose() = 0; // nothrow // Called when _M_weak_count drops to zero. virtual void _M_destroy() // nothrow { delete this; } virtual void* _M_get_deleter(const std::type_info&) = 0; void _M_add_ref_copy() { __gnu_cxx::__atomic_add_dispatch(&_M_use_count, 1); } void _M_add_ref_lock(); void _M_release() // nothrow { // Be race-detector-friendly. For more info see bits/c++config. _GLIBCXX_SYNCHRONIZATION_HAPPENS_BEFORE(&_M_use_count); if (__gnu_cxx::__exchange_and_add_dispatch(&_M_use_count, -1) == 1) { _GLIBCXX_SYNCHRONIZATION_HAPPENS_AFTER(&_M_use_count); _M_dispose(); // There must be a memory barrier between dispose() and destroy() // to ensure that the effects of dispose() are observed in the // thread that runs destroy(). // See http://gcc.gnu.org/ml/libstdc++/2005-11/msg00136.html if (_Mutex_base<_Lp>::_S_need_barriers) { __atomic_thread_fence (__ATOMIC_ACQ_REL); } // Be race-detector-friendly. For more info see bits/c++config. _GLIBCXX_SYNCHRONIZATION_HAPPENS_BEFORE(&_M_weak_count); if (__gnu_cxx::__exchange_and_add_dispatch(&_M_weak_count, -1) == 1) { _GLIBCXX_SYNCHRONIZATION_HAPPENS_AFTER(&_M_weak_count); _M_destroy(); } } } void _M_weak_add_ref() // nothrow { __gnu_cxx::__atomic_add_dispatch(&_M_weak_count, 1); } void _M_weak_release() // nothrow { // Be race-detector-friendly. For more info see bits/c++config. _GLIBCXX_SYNCHRONIZATION_HAPPENS_BEFORE(&_M_weak_count); if (__gnu_cxx::__exchange_and_add_dispatch(&_M_weak_count, -1) == 1) { _GLIBCXX_SYNCHRONIZATION_HAPPENS_AFTER(&_M_weak_count); if (_Mutex_base<_Lp>::_S_need_barriers) { // See _M_release(), // destroy() must observe results of dispose() __atomic_thread_fence (__ATOMIC_ACQ_REL); } _M_destroy(); } } long _M_get_use_count() const // nothrow { // No memory barrier is used here so there is no synchronization // with other threads. return const_cast<const volatile _Atomic_word&>(_M_use_count); } private: _Sp_counted_base(_Sp_counted_base const&); _Sp_counted_base& operator=(_Sp_counted_base const&); _Atomic_word _M_use_count; // #shared _Atomic_word _M_weak_count; // #weak + (#shared != 0) }; template<> inline void _Sp_counted_base<_S_single>:: _M_add_ref_lock() { if (__gnu_cxx::__exchange_and_add_dispatch(&_M_use_count, 1) == 0) { _M_use_count = 0; __throw_bad_weak_ptr(); } } template<> inline void _Sp_counted_base<_S_mutex>:: _M_add_ref_lock() { __gnu_cxx::__scoped_lock sentry(*this); if (__gnu_cxx::__exchange_and_add_dispatch(&_M_use_count, 1) == 0) { _M_use_count = 0; __throw_bad_weak_ptr(); } } template<> inline void _Sp_counted_base<_S_atomic>:: _M_add_ref_lock() { // Perform lock-free add-if-not-zero operation. _Atomic_word __count = _M_use_count; do { if (__count == 0) __throw_bad_weak_ptr(); // Replace the current counter value with the old value + 1, as // long as it's not changed meanwhile. } while (!__atomic_compare_exchange_n(&_M_use_count, &__count, __count + 1, true, __ATOMIC_ACQ_REL, __ATOMIC_RELAXED)); } template<typename _Ptr, typename _Deleter, _Lock_policy _Lp> class _Sp_counted_base_impl : public _Sp_counted_base<_Lp> { public: // Precondition: __d(__p) must not throw. _Sp_counted_base_impl(_Ptr __p, _Deleter __d) : _M_ptr(__p), _M_del(__d) { } virtual void _M_dispose() // nothrow { _M_del(_M_ptr); } virtual void* _M_get_deleter(const std::type_info& __ti) { #if __cpp_rtti return __ti == typeid(_Deleter) ? &_M_del : 0; #else return 0; #endif } private: _Sp_counted_base_impl(const _Sp_counted_base_impl&); _Sp_counted_base_impl& operator=(const _Sp_counted_base_impl&); _Ptr _M_ptr; // copy constructor must not throw _Deleter _M_del; // copy constructor must not throw }; template<_Lock_policy _Lp = __default_lock_policy> class __weak_count; template<typename _Tp> struct _Sp_deleter { typedef void result_type; typedef _Tp* argument_type; void operator()(_Tp* __p) const { delete __p; } }; template<_Lock_policy _Lp = __default_lock_policy> class __shared_count { public: __shared_count() : _M_pi(0) // nothrow { } template<typename _Ptr> __shared_count(_Ptr __p) : _M_pi(0) { __try { typedef typename std::tr1::remove_pointer<_Ptr>::type _Tp; _M_pi = new _Sp_counted_base_impl<_Ptr, _Sp_deleter<_Tp>, _Lp>( __p, _Sp_deleter<_Tp>()); } __catch(...) { delete __p; __throw_exception_again; } } template<typename _Ptr, typename _Deleter> __shared_count(_Ptr __p, _Deleter __d) : _M_pi(0) { __try { _M_pi = new _Sp_counted_base_impl<_Ptr, _Deleter, _Lp>(__p, __d); } __catch(...) { __d(__p); // Call _Deleter on __p. __throw_exception_again; } } // Special case for auto_ptr<_Tp> to provide the strong guarantee. template<typename _Tp> explicit __shared_count(std::auto_ptr<_Tp>& __r) : _M_pi(new _Sp_counted_base_impl<_Tp*, _Sp_deleter<_Tp>, _Lp >(__r.get(), _Sp_deleter<_Tp>())) { __r.release(); } // Throw bad_weak_ptr when __r._M_get_use_count() == 0. explicit __shared_count(const __weak_count<_Lp>& __r); ~__shared_count() // nothrow { if (_M_pi != 0) _M_pi->_M_release(); } __shared_count(const __shared_count& __r) : _M_pi(__r._M_pi) // nothrow { if (_M_pi != 0) _M_pi->_M_add_ref_copy(); } __shared_count& operator=(const __shared_count& __r) // nothrow { _Sp_counted_base<_Lp>* __tmp = __r._M_pi; if (__tmp != _M_pi) { if (__tmp != 0) __tmp->_M_add_ref_copy(); if (_M_pi != 0) _M_pi->_M_release(); _M_pi = __tmp; } return *this; } void _M_swap(__shared_count& __r) // nothrow { _Sp_counted_base<_Lp>* __tmp = __r._M_pi; __r._M_pi = _M_pi; _M_pi = __tmp; } long _M_get_use_count() const // nothrow { return _M_pi != 0 ? _M_pi->_M_get_use_count() : 0; } bool _M_unique() const // nothrow { return this->_M_get_use_count() == 1; } friend inline bool operator==(const __shared_count& __a, const __shared_count& __b) { return __a._M_pi == __b._M_pi; } friend inline bool operator<(const __shared_count& __a, const __shared_count& __b) { return std::less<_Sp_counted_base<_Lp>*>()(__a._M_pi, __b._M_pi); } void* _M_get_deleter(const std::type_info& __ti) const { return _M_pi ? _M_pi->_M_get_deleter(__ti) : 0; } private: friend class __weak_count<_Lp>; _Sp_counted_base<_Lp>* _M_pi; }; template<_Lock_policy _Lp> class __weak_count { public: __weak_count() : _M_pi(0) // nothrow { } __weak_count(const __shared_count<_Lp>& __r) : _M_pi(__r._M_pi) // nothrow { if (_M_pi != 0) _M_pi->_M_weak_add_ref(); } __weak_count(const __weak_count<_Lp>& __r) : _M_pi(__r._M_pi) // nothrow { if (_M_pi != 0) _M_pi->_M_weak_add_ref(); } ~__weak_count() // nothrow { if (_M_pi != 0) _M_pi->_M_weak_release(); } __weak_count<_Lp>& operator=(const __shared_count<_Lp>& __r) // nothrow { _Sp_counted_base<_Lp>* __tmp = __r._M_pi; if (__tmp != 0) __tmp->_M_weak_add_ref(); if (_M_pi != 0) _M_pi->_M_weak_release(); _M_pi = __tmp; return *this; } __weak_count<_Lp>& operator=(const __weak_count<_Lp>& __r) // nothrow { _Sp_counted_base<_Lp>* __tmp = __r._M_pi; if (__tmp != 0) __tmp->_M_weak_add_ref(); if (_M_pi != 0) _M_pi->_M_weak_release(); _M_pi = __tmp; return *this; } void _M_swap(__weak_count<_Lp>& __r) // nothrow { _Sp_counted_base<_Lp>* __tmp = __r._M_pi; __r._M_pi = _M_pi; _M_pi = __tmp; } long _M_get_use_count() const // nothrow { return _M_pi != 0 ? _M_pi->_M_get_use_count() : 0; } friend inline bool operator==(const __weak_count<_Lp>& __a, const __weak_count<_Lp>& __b) { return __a._M_pi == __b._M_pi; } friend inline bool operator<(const __weak_count<_Lp>& __a, const __weak_count<_Lp>& __b) { return std::less<_Sp_counted_base<_Lp>*>()(__a._M_pi, __b._M_pi); } private: friend class __shared_count<_Lp>; _Sp_counted_base<_Lp>* _M_pi; }; // now that __weak_count is defined we can define this constructor: template<_Lock_policy _Lp> inline __shared_count<_Lp>:: __shared_count(const __weak_count<_Lp>& __r) : _M_pi(__r._M_pi) { if (_M_pi != 0) _M_pi->_M_add_ref_lock(); else __throw_bad_weak_ptr(); } // Forward declarations. template<typename _Tp, _Lock_policy _Lp = __default_lock_policy> class __shared_ptr; template<typename _Tp, _Lock_policy _Lp = __default_lock_policy> class __weak_ptr; template<typename _Tp, _Lock_policy _Lp = __default_lock_policy> class __enable_shared_from_this; template<typename _Tp> class shared_ptr; template<typename _Tp> class weak_ptr; template<typename _Tp> class enable_shared_from_this; // Support for enable_shared_from_this. // Friend of __enable_shared_from_this. template<_Lock_policy _Lp, typename _Tp1, typename _Tp2> void __enable_shared_from_this_helper(const __shared_count<_Lp>&, const __enable_shared_from_this<_Tp1, _Lp>*, const _Tp2*); // Friend of enable_shared_from_this. template<typename _Tp1, typename _Tp2> void __enable_shared_from_this_helper(const __shared_count<>&, const enable_shared_from_this<_Tp1>*, const _Tp2*); template<_Lock_policy _Lp> inline void __enable_shared_from_this_helper(const __shared_count<_Lp>&, ...) { } struct __static_cast_tag { }; struct __const_cast_tag { }; struct __dynamic_cast_tag { }; // A smart pointer with reference-counted copy semantics. The // object pointed to is deleted when the last shared_ptr pointing to // it is destroyed or reset. template<typename _Tp, _Lock_policy _Lp> class __shared_ptr { public: typedef _Tp element_type; __shared_ptr() : _M_ptr(0), _M_refcount() // never throws { } template<typename _Tp1> explicit __shared_ptr(_Tp1* __p) : _M_ptr(__p), _M_refcount(__p) { __glibcxx_function_requires(_ConvertibleConcept<_Tp1*, _Tp*>) typedef int _IsComplete[sizeof(_Tp1)]; __enable_shared_from_this_helper(_M_refcount, __p, __p); } template<typename _Tp1, typename _Deleter> __shared_ptr(_Tp1* __p, _Deleter __d) : _M_ptr(__p), _M_refcount(__p, __d) { __glibcxx_function_requires(_ConvertibleConcept<_Tp1*, _Tp*>) // TODO requires _Deleter CopyConstructible and __d(__p) well-formed __enable_shared_from_this_helper(_M_refcount, __p, __p); } // generated copy constructor, assignment, destructor are fine. template<typename _Tp1> __shared_ptr(const __shared_ptr<_Tp1, _Lp>& __r) : _M_ptr(__r._M_ptr), _M_refcount(__r._M_refcount) // never throws { __glibcxx_function_requires(_ConvertibleConcept<_Tp1*, _Tp*>) } template<typename _Tp1> explicit __shared_ptr(const __weak_ptr<_Tp1, _Lp>& __r) : _M_refcount(__r._M_refcount) // may throw { __glibcxx_function_requires(_ConvertibleConcept<_Tp1*, _Tp*>) // It is now safe to copy __r._M_ptr, as _M_refcount(__r._M_refcount) // did not throw. _M_ptr = __r._M_ptr; } #if (__cplusplus < 201103L) || _GLIBCXX_USE_DEPRECATED // Postcondition: use_count() == 1 and __r.get() == 0 template<typename _Tp1> explicit __shared_ptr(std::auto_ptr<_Tp1>& __r) : _M_ptr(__r.get()), _M_refcount() { // TODO requries delete __r.release() well-formed __glibcxx_function_requires(_ConvertibleConcept<_Tp1*, _Tp*>) typedef int _IsComplete[sizeof(_Tp1)]; _Tp1* __tmp = __r.get(); _M_refcount = __shared_count<_Lp>(__r); __enable_shared_from_this_helper(_M_refcount, __tmp, __tmp); } #endif template<typename _Tp1> __shared_ptr(const __shared_ptr<_Tp1, _Lp>& __r, __static_cast_tag) : _M_ptr(static_cast<element_type*>(__r._M_ptr)), _M_refcount(__r._M_refcount) { } template<typename _Tp1> __shared_ptr(const __shared_ptr<_Tp1, _Lp>& __r, __const_cast_tag) : _M_ptr(const_cast<element_type*>(__r._M_ptr)), _M_refcount(__r._M_refcount) { } template<typename _Tp1> __shared_ptr(const __shared_ptr<_Tp1, _Lp>& __r, __dynamic_cast_tag) : _M_ptr(dynamic_cast<element_type*>(__r._M_ptr)), _M_refcount(__r._M_refcount) { if (_M_ptr == 0) // need to allocate new counter -- the cast failed _M_refcount = __shared_count<_Lp>(); } template<typename _Tp1> __shared_ptr& operator=(const __shared_ptr<_Tp1, _Lp>& __r) // never throws { _M_ptr = __r._M_ptr; _M_refcount = __r._M_refcount; // __shared_count::op= doesn't throw return *this; } #if (__cplusplus < 201103L) || _GLIBCXX_USE_DEPRECATED template<typename _Tp1> __shared_ptr& operator=(std::auto_ptr<_Tp1>& __r) { __shared_ptr(__r).swap(*this); return *this; } #endif void reset() // never throws { __shared_ptr().swap(*this); } template<typename _Tp1> void reset(_Tp1* __p) // _Tp1 must be complete. { // Catch self-reset errors. _GLIBCXX_DEBUG_ASSERT(__p == 0 || __p != _M_ptr); __shared_ptr(__p).swap(*this); } template<typename _Tp1, typename _Deleter> void reset(_Tp1* __p, _Deleter __d) { __shared_ptr(__p, __d).swap(*this); } // Allow class instantiation when _Tp is [cv-qual] void. typename std::tr1::add_reference<_Tp>::type operator*() const // never throws { _GLIBCXX_DEBUG_ASSERT(_M_ptr != 0); return *_M_ptr; } _Tp* operator->() const // never throws { _GLIBCXX_DEBUG_ASSERT(_M_ptr != 0); return _M_ptr; } _Tp* get() const // never throws { return _M_ptr; } // Implicit conversion to "bool" private: typedef _Tp* __shared_ptr::*__unspecified_bool_type; public: operator __unspecified_bool_type() const // never throws { return _M_ptr == 0 ? 0 : &__shared_ptr::_M_ptr; } bool unique() const // never throws { return _M_refcount._M_unique(); } long use_count() const // never throws { return _M_refcount._M_get_use_count(); } void swap(__shared_ptr<_Tp, _Lp>& __other) // never throws { std::swap(_M_ptr, __other._M_ptr); _M_refcount._M_swap(__other._M_refcount); } private: void* _M_get_deleter(const std::type_info& __ti) const { return _M_refcount._M_get_deleter(__ti); } template<typename _Tp1, _Lock_policy _Lp1> bool _M_less(const __shared_ptr<_Tp1, _Lp1>& __rhs) const { return _M_refcount < __rhs._M_refcount; } template<typename _Tp1, _Lock_policy _Lp1> friend class __shared_ptr; template<typename _Tp1, _Lock_policy _Lp1> friend class __weak_ptr; template<typename _Del, typename _Tp1, _Lock_policy _Lp1> friend _Del* get_deleter(const __shared_ptr<_Tp1, _Lp1>&); // Friends injected into enclosing namespace and found by ADL: template<typename _Tp1> friend inline bool operator==(const __shared_ptr& __a, const __shared_ptr<_Tp1, _Lp>& __b) { return __a.get() == __b.get(); } template<typename _Tp1> friend inline bool operator!=(const __shared_ptr& __a, const __shared_ptr<_Tp1, _Lp>& __b) { return __a.get() != __b.get(); } template<typename _Tp1> friend inline bool operator<(const __shared_ptr& __a, const __shared_ptr<_Tp1, _Lp>& __b) { return __a._M_less(__b); } _Tp* _M_ptr; // Contained pointer. __shared_count<_Lp> _M_refcount; // Reference counter. }; // 2.2.3.8 shared_ptr specialized algorithms. template<typename _Tp, _Lock_policy _Lp> inline void swap(__shared_ptr<_Tp, _Lp>& __a, __shared_ptr<_Tp, _Lp>& __b) { __a.swap(__b); } // 2.2.3.9 shared_ptr casts /* The seemingly equivalent * shared_ptr<_Tp, _Lp>(static_cast<_Tp*>(__r.get())) * will eventually result in undefined behaviour, * attempting to delete the same object twice. */ template<typename _Tp, typename _Tp1, _Lock_policy _Lp> inline __shared_ptr<_Tp, _Lp> static_pointer_cast(const __shared_ptr<_Tp1, _Lp>& __r) { return __shared_ptr<_Tp, _Lp>(__r, __static_cast_tag()); } /* The seemingly equivalent * shared_ptr<_Tp, _Lp>(const_cast<_Tp*>(__r.get())) * will eventually result in undefined behaviour, * attempting to delete the same object twice. */ template<typename _Tp, typename _Tp1, _Lock_policy _Lp> inline __shared_ptr<_Tp, _Lp> const_pointer_cast(const __shared_ptr<_Tp1, _Lp>& __r) { return __shared_ptr<_Tp, _Lp>(__r, __const_cast_tag()); } /* The seemingly equivalent * shared_ptr<_Tp, _Lp>(dynamic_cast<_Tp*>(__r.get())) * will eventually result in undefined behaviour, * attempting to delete the same object twice. */ template<typename _Tp, typename _Tp1, _Lock_policy _Lp> inline __shared_ptr<_Tp, _Lp> dynamic_pointer_cast(const __shared_ptr<_Tp1, _Lp>& __r) { return __shared_ptr<_Tp, _Lp>(__r, __dynamic_cast_tag()); } // 2.2.3.7 shared_ptr I/O template<typename _Ch, typename _Tr, typename _Tp, _Lock_policy _Lp> std::basic_ostream<_Ch, _Tr>& operator<<(std::basic_ostream<_Ch, _Tr>& __os, const __shared_ptr<_Tp, _Lp>& __p) { __os << __p.get(); return __os; } // 2.2.3.10 shared_ptr get_deleter (experimental) template<typename _Del, typename _Tp, _Lock_policy _Lp> inline _Del* get_deleter(const __shared_ptr<_Tp, _Lp>& __p) { #if __cpp_rtti return static_cast<_Del*>(__p._M_get_deleter(typeid(_Del))); #else return 0; #endif } template<typename _Tp, _Lock_policy _Lp> class __weak_ptr { public: typedef _Tp element_type; __weak_ptr() : _M_ptr(0), _M_refcount() // never throws { } // Generated copy constructor, assignment, destructor are fine. // The "obvious" converting constructor implementation: // // template<typename _Tp1> // __weak_ptr(const __weak_ptr<_Tp1, _Lp>& __r) // : _M_ptr(__r._M_ptr), _M_refcount(__r._M_refcount) // never throws // { } // // has a serious problem. // // __r._M_ptr may already have been invalidated. The _M_ptr(__r._M_ptr) // conversion may require access to *__r._M_ptr (virtual inheritance). // // It is not possible to avoid spurious access violations since // in multithreaded programs __r._M_ptr may be invalidated at any point. template<typename _Tp1> __weak_ptr(const __weak_ptr<_Tp1, _Lp>& __r) : _M_refcount(__r._M_refcount) // never throws { __glibcxx_function_requires(_ConvertibleConcept<_Tp1*, _Tp*>) _M_ptr = __r.lock().get(); } template<typename _Tp1> __weak_ptr(const __shared_ptr<_Tp1, _Lp>& __r) : _M_ptr(__r._M_ptr), _M_refcount(__r._M_refcount) // never throws { __glibcxx_function_requires(_ConvertibleConcept<_Tp1*, _Tp*>) } template<typename _Tp1> __weak_ptr& operator=(const __weak_ptr<_Tp1, _Lp>& __r) // never throws { _M_ptr = __r.lock().get(); _M_refcount = __r._M_refcount; return *this; } template<typename _Tp1> __weak_ptr& operator=(const __shared_ptr<_Tp1, _Lp>& __r) // never throws { _M_ptr = __r._M_ptr; _M_refcount = __r._M_refcount; return *this; } __shared_ptr<_Tp, _Lp> lock() const // never throws { #ifdef __GTHREADS // Optimization: avoid throw overhead. if (expired()) return __shared_ptr<element_type, _Lp>(); __try { return __shared_ptr<element_type, _Lp>(*this); } __catch(const bad_weak_ptr&) { // Q: How can we get here? // A: Another thread may have invalidated r after the // use_count test above. return __shared_ptr<element_type, _Lp>(); } #else // Optimization: avoid try/catch overhead when single threaded. return expired() ? __shared_ptr<element_type, _Lp>() : __shared_ptr<element_type, _Lp>(*this); #endif } // XXX MT long use_count() const // never throws { return _M_refcount._M_get_use_count(); } bool expired() const // never throws { return _M_refcount._M_get_use_count() == 0; } void reset() // never throws { __weak_ptr().swap(*this); } void swap(__weak_ptr& __s) // never throws { std::swap(_M_ptr, __s._M_ptr); _M_refcount._M_swap(__s._M_refcount); } private: // Used by __enable_shared_from_this. void _M_assign(_Tp* __ptr, const __shared_count<_Lp>& __refcount) { _M_ptr = __ptr; _M_refcount = __refcount; } template<typename _Tp1> bool _M_less(const __weak_ptr<_Tp1, _Lp>& __rhs) const { return _M_refcount < __rhs._M_refcount; } template<typename _Tp1, _Lock_policy _Lp1> friend class __shared_ptr; template<typename _Tp1, _Lock_policy _Lp1> friend class __weak_ptr; friend class __enable_shared_from_this<_Tp, _Lp>; friend class enable_shared_from_this<_Tp>; // Friend injected into namespace and found by ADL. template<typename _Tp1> friend inline bool operator<(const __weak_ptr& __lhs, const __weak_ptr<_Tp1, _Lp>& __rhs) { return __lhs._M_less(__rhs); } _Tp* _M_ptr; // Contained pointer. __weak_count<_Lp> _M_refcount; // Reference counter. }; // 2.2.4.7 weak_ptr specialized algorithms. template<typename _Tp, _Lock_policy _Lp> inline void swap(__weak_ptr<_Tp, _Lp>& __a, __weak_ptr<_Tp, _Lp>& __b) { __a.swap(__b); } template<typename _Tp, _Lock_policy _Lp> class __enable_shared_from_this { protected: __enable_shared_from_this() { } __enable_shared_from_this(const __enable_shared_from_this&) { } __enable_shared_from_this& operator=(const __enable_shared_from_this&) { return *this; } ~__enable_shared_from_this() { } public: __shared_ptr<_Tp, _Lp> shared_from_this() { return __shared_ptr<_Tp, _Lp>(this->_M_weak_this); } __shared_ptr<const _Tp, _Lp> shared_from_this() const { return __shared_ptr<const _Tp, _Lp>(this->_M_weak_this); } private: template<typename _Tp1> void _M_weak_assign(_Tp1* __p, const __shared_count<_Lp>& __n) const { _M_weak_this._M_assign(__p, __n); } template<typename _Tp1> friend void __enable_shared_from_this_helper(const __shared_count<_Lp>& __pn, const __enable_shared_from_this* __pe, const _Tp1* __px) { if (__pe != 0) __pe->_M_weak_assign(const_cast<_Tp1*>(__px), __pn); } mutable __weak_ptr<_Tp, _Lp> _M_weak_this; }; // The actual shared_ptr, with forwarding constructors and // assignment operators. template<typename _Tp> class shared_ptr : public __shared_ptr<_Tp> { public: shared_ptr() : __shared_ptr<_Tp>() { } template<typename _Tp1> explicit shared_ptr(_Tp1* __p) : __shared_ptr<_Tp>(__p) { } template<typename _Tp1, typename _Deleter> shared_ptr(_Tp1* __p, _Deleter __d) : __shared_ptr<_Tp>(__p, __d) { } template<typename _Tp1> shared_ptr(const shared_ptr<_Tp1>& __r) : __shared_ptr<_Tp>(__r) { } template<typename _Tp1> explicit shared_ptr(const weak_ptr<_Tp1>& __r) : __shared_ptr<_Tp>(__r) { } #if (__cplusplus < 201103L) || _GLIBCXX_USE_DEPRECATED template<typename _Tp1> explicit shared_ptr(std::auto_ptr<_Tp1>& __r) : __shared_ptr<_Tp>(__r) { } #endif template<typename _Tp1> shared_ptr(const shared_ptr<_Tp1>& __r, __static_cast_tag) : __shared_ptr<_Tp>(__r, __static_cast_tag()) { } template<typename _Tp1> shared_ptr(const shared_ptr<_Tp1>& __r, __const_cast_tag) : __shared_ptr<_Tp>(__r, __const_cast_tag()) { } template<typename _Tp1> shared_ptr(const shared_ptr<_Tp1>& __r, __dynamic_cast_tag) : __shared_ptr<_Tp>(__r, __dynamic_cast_tag()) { } template<typename _Tp1> shared_ptr& operator=(const shared_ptr<_Tp1>& __r) // never throws { this->__shared_ptr<_Tp>::operator=(__r); return *this; } #if (__cplusplus < 201103L) || _GLIBCXX_USE_DEPRECATED template<typename _Tp1> shared_ptr& operator=(std::auto_ptr<_Tp1>& __r) { this->__shared_ptr<_Tp>::operator=(__r); return *this; } #endif }; // 2.2.3.8 shared_ptr specialized algorithms. template<typename _Tp> inline void swap(__shared_ptr<_Tp>& __a, __shared_ptr<_Tp>& __b) { __a.swap(__b); } template<typename _Tp, typename _Tp1> inline shared_ptr<_Tp> static_pointer_cast(const shared_ptr<_Tp1>& __r) { return shared_ptr<_Tp>(__r, __static_cast_tag()); } template<typename _Tp, typename _Tp1> inline shared_ptr<_Tp> const_pointer_cast(const shared_ptr<_Tp1>& __r) { return shared_ptr<_Tp>(__r, __const_cast_tag()); } template<typename _Tp, typename _Tp1> inline shared_ptr<_Tp> dynamic_pointer_cast(const shared_ptr<_Tp1>& __r) { return shared_ptr<_Tp>(__r, __dynamic_cast_tag()); } // The actual weak_ptr, with forwarding constructors and // assignment operators. template<typename _Tp> class weak_ptr : public __weak_ptr<_Tp> { public: weak_ptr() : __weak_ptr<_Tp>() { } template<typename _Tp1> weak_ptr(const weak_ptr<_Tp1>& __r) : __weak_ptr<_Tp>(__r) { } template<typename _Tp1> weak_ptr(const shared_ptr<_Tp1>& __r) : __weak_ptr<_Tp>(__r) { } template<typename _Tp1> weak_ptr& operator=(const weak_ptr<_Tp1>& __r) // never throws { this->__weak_ptr<_Tp>::operator=(__r); return *this; } template<typename _Tp1> weak_ptr& operator=(const shared_ptr<_Tp1>& __r) // never throws { this->__weak_ptr<_Tp>::operator=(__r); return *this; } shared_ptr<_Tp> lock() const // never throws { #ifdef __GTHREADS if (this->expired()) return shared_ptr<_Tp>(); __try { return shared_ptr<_Tp>(*this); } __catch(const bad_weak_ptr&) { return shared_ptr<_Tp>(); } #else return this->expired() ? shared_ptr<_Tp>() : shared_ptr<_Tp>(*this); #endif } }; template<typename _Tp> class enable_shared_from_this { protected: enable_shared_from_this() { } enable_shared_from_this(const enable_shared_from_this&) { } enable_shared_from_this& operator=(const enable_shared_from_this&) { return *this; } ~enable_shared_from_this() { } public: shared_ptr<_Tp> shared_from_this() { return shared_ptr<_Tp>(this->_M_weak_this); } shared_ptr<const _Tp> shared_from_this() const { return shared_ptr<const _Tp>(this->_M_weak_this); } private: template<typename _Tp1> void _M_weak_assign(_Tp1* __p, const __shared_count<>& __n) const { _M_weak_this._M_assign(__p, __n); } template<typename _Tp1> friend void __enable_shared_from_this_helper(const __shared_count<>& __pn, const enable_shared_from_this* __pe, const _Tp1* __px) { if (__pe != 0) __pe->_M_weak_assign(const_cast<_Tp1*>(__px), __pn); } mutable weak_ptr<_Tp> _M_weak_this; }; _GLIBCXX_END_NAMESPACE_VERSION } } #endif // _TR1_SHARED_PTR_H
其主要的類關係如下所示(省略相關的類模板引數）：
圖1
從上面的類圖可以清楚的看出shared_ptr內部含有一個指向被管理物件(managed object)T的指標以及一個__shared_count物件，__shared_count物件包含一個指向管理物件(manager object)的基類指標，管理物件(manager object)由具有原子屬性的use_count和weak_count、指向被管理物件(managed object)T的指標、以及用來銷燬被管理物件的deleter組成，以下均將用new建立後託管給shared_ptr等智慧指標的物件叫做被管理物件（managed object)；shared_ptr等智慧指標內部建立的用來維護被管理物件生命週期的例項叫做管理物件(manager object)：
圖2
weak_ptr內部組成與shared_ptr類似，內部同樣含有一個指向被管理物件T的指標以及一個__weak_count物件：
圖3
從圖2和圖3對比可以看出，shared_ptr與weak_ptr的差異主要是由__shared_ptr與__weak_ptr體現出來的，而__shared_ptr與__weak_ptr的差異則主要是由__shared_count與__weak_count體現出來。
通過shared_ptr的建構函式，可以發現，在建立一個shared_ptr的時候需要一個new 操作符返回被管理物件的地址來初始化shared_ptr, shared_ptr在內部會構建一個_shared_count物件，由_shared_count物件的建構函式可知，建立shared_ptr的時候也動態的建立了一個管理物件_Sp_counted_base_impl:
template<typename _Tp1> explicit __shared_ptr(_Tp1* __p) : _M_ptr(__p), _M_refcount(__p) { __glibcxx_function_requires(_ConvertibleConcept<_Tp1*, _Tp*>) typedef int _IsComplete[sizeof(_Tp1)]; __enable_shared_from_this_helper(_M_refcount, __p, __p); } template<typename _Ptr> __shared_count(_Ptr __p) : _M_pi(0) { __try { typedef typename std::tr1::remove_pointer<_Ptr>::type _Tp; _M_pi = new _Sp_counted_base_impl<_Ptr, _Sp_deleter<_Tp>, _Lp>(__p, _Sp_deleter<_Tp>()); } __catch(...) { delete __p; __throw_exception_again; } }
shared_ptr內部包含一個指向被管理物件的指標_M_ptr, _Sp_counted_base_impl內部也含有一個指向被管理物件的指標_M_ptr， 它們是不是重複多餘了呢？實際上不多餘，它們有各自的功能。這首先要從shared_ptr的拷貝構造或者賦值構造說起，當一個shared_ptr物件sp2是由sp1拷貝構造或者賦值構造得來的時候，實際上構造完成後sp1內部的__shared_count物件包含的指向管理物件的指標與sp2內部的__shared_count物件包含的指向管理物件的指標是相等的，也就是說當多個shared_ptr物件來管理同一個物件時，它們共同使用同一個動態分配的管理物件。這可以從下面的__share_ptr的建構函式和__shared_count的建構函式清楚的看出。
template<typename _Tp1> __shared_ptr(const __shared_ptr<_Tp1, _Lp>& __r) : _M_ptr(__r._M_ptr), _M_refcount(__r._M_refcount) // never throws {__glibcxx_function_requires(_ConvertibleConcept<_Tp1*, _Tp*>)} __shared_count& operator=(const __shared_count& __r) // nothrow { _Sp_counted_base<_Lp>* __tmp = __r._M_pi; if (__tmp != _M_pi) { if (__tmp != 0) __tmp->_M_add_ref_copy(); if (_M_pi != 0) _M_pi->_M_release(); _M_pi = __tmp; } }
上面說說當多個shared_ptr物件來管理同一個物件時，它們共同使用同一個動態分配的管理物件，為什麼上面給出的_shared_count的建構函式中出現了__tmp != _M_pi的情形呢？這在sp2未初始化時（_M_pi為0，_r._M_pi非0）便是這樣的情形。
更一般的，也可以考慮這樣的情形：shared_ptr例項sp1開始指向類A的例項物件a1, 另外一個shared_ptr例項sp2指向類A的例項物件a2（a1 != a2)，當把sp2賦值給sp1時便會出現上面的情形。假設初始時有且僅有一個sp1指向a1, 有且僅有一個sp2指向a2; 則賦值結束時sp1與sp2均指向a2, 沒有指標指向a1, sp1指向的a1以及其對應的管理物件均應該被析構。這在上面的程式碼中我們可以很清楚的看到：因為__tmp != _M_pi, __tmp->_M_add_ref_copy()將會增加a2的use_count的引用計數；由於a1內部的_M_pi != 0, 將會呼叫其_M_release()函式：
//************_Sp_counted_base*****************// void _M_add_ref_copy() { __gnu_cxx::__atomic_add_dispatch(&_M_use_count, 1); } //************_Sp_counted_base*****************// void _M_release() // nothrow { // Be race-detector-friendly. For more info see bits/c++config. _GLIBCXX_SYNCHRONIZATION_HAPPENS_BEFORE(&_M_use_count); if (__gnu_cxx::__exchange_and_add_dispatch(&_M_use_count, -1) == 1) { _GLIBCXX_SYNCHRONIZATION_HAPPENS_AFTER(&_M_use_count); _M_dispose(); // There must be a memory barrier between dispose() and destroy() // to ensure that the effects of dispose() are observed in the // thread that runs destroy(). // See http://gcc.gnu.org/ml/libstdc++/2005-11/msg00136.html if (_Mutex_base<_Lp>::_S_need_barriers) { __atomic_thread_fence (__ATOMIC_ACQ_REL); } // Be race-detector-friendly. For more info see bits/c++config. _GLIBCXX_SYNCHRONIZATION_HAPPENS_BEFORE(&_M_weak_count); if (__gnu_cxx::__exchange_and_add_dispatch(&_M_weak_count, -1) == 1) { _GLIBCXX_SYNCHRONIZATION_HAPPENS_AFTER(&_M_weak_count); _M_destroy(); } } } //************_Sp_counted_base*****************// // Called when _M_use_count drops to zero, to release the resources // managed by *this. virtual void _M_dispose() = 0; // nothrow // Called when _M_weak_count drops to zero. virtual void _M_destroy() // nothrow { delete this; } //************_Sp_counted_base_impl*************// virtual void _M_dispose() // nothrow { _M_del(_M_ptr); }
_M_release()函式首先對a1的use_count減去1，並對比減操作之前的值，如果減之前是1，說明減後是0，a1沒有任何shared_ptr指標指向它了，應該將a1物件銷燬，於是呼叫_M_dispose()函式銷燬a1; 同時對a1的weak_count減去1，也對比減操作之前的值，如果減之前是1，說明減後是0，a1沒有weak_ptr指向它了，應該將管理物件銷燬，於是呼叫_M_destroy()銷燬了管理物件。這就可以解答為什麼圖2所示中shared_ptr內部含有兩個指向被管理物件的指標了：__shared_ptr直接包含的裸指標是為了實現一般指標的->,*等操作，通過__shared_count間接包含的指標是為了管