C++智能指针 shared_ptr 与 weak_ptr 原理

• 注：源代码摘自 GNU C++，除此之外为原创，转载请注明出处。

// Copyright (C) 2007-2016 Free Software Foundation, Inc.//// This file is part of the GNU ISO C++ Library. 

一、weak_ptr 的 lock() 函数原理

/*  当每次有新的 shared_ptr 生成时，会增加 _Sp_counted_base 的 _M_use_count (+1);    当每次有新的 weak_ptr 生成时，会增加 _Sp_counted_base 的 _M_weak_count (+1);    对应的，shared_ptr 析构时，会调用 _Sp_counted_base 的 _M_release 来使 _M_use_count (-1);    weak_ptr 析构时，会调用 _Sp_counted_base 的 _M_release 来使 _M_weak_count (-1);    当 _M_use_count 的数量为 0 的时候 就会释放，原始的对象(用户自定义对象) 的内存，但是不会释放     _Sp_counted_base 的内存。    当  _M_use_count 和 _M_weak_count 都为 0的时候， 才会释放 _Sp_counted_base 的内存。*/    // __shared_count的构造函数会 new 一个 计数对象(_Sp_counted_base),但是__weak_count的构造函数不会new。    __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;        }    }    // lock 函数    __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&) {                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    bool  expired() const // never throws    {            return _M_refcount._M_get_use_count() == 0;    }

二、类成员图：

三、下面源码有点长，不差资源分的话可以点击链接下载。
*注：所有的源码参看连接: http://download.csdn.net/download/u013005025/10155393

四、源码 （//version： Copyright (C) 2007-2016 Free Software Foundation, Inc.）

1 weak_ptr 源码

// weak_ptrtemplate<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    }};

2  __weak_ptr 源码

template<typename _lock_policy = "" _lp = "">class __weak_ptr{public:    typedef _Tp element_type;    __weak_ptr() : _M_ptr(0), _M_refcount() // never throws    { }    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&) {            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 _lock_policy = "" _lp1 = ""> friend class __shared_ptr;    template<typename _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.};

3 __weak_count 源码

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;};

五、shared_ptr 源码

// The actual shared_ptr, with forwarding constructors and// assignment operators.// shared_ptrtemplate<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 _deleter = "" typename = "">    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};// __shared_ptrstruct __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 _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 _deleter = "" typename = "">    __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 _deleter = "" typename = "">    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 _lock_policy = "" _lp1 = "">    bool _M_less(const __shared_ptr<_Tp1, _Lp1>& __rhs) const    {        return _M_refcount < __rhs._M_refcount;    }    template<typename _lock_policy = "" _lp1 = ""> friend class __shared_ptr;    template<typename _lock_policy = "" _lp1 = ""> friend class __weak_ptr;    template<typename _lock_policy = "" _lp1 = "" typename = "">    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.};// __shared_counttemplate<_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 _deleter = "" typename = "">    __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;};// _Sp_counted_basetemplate<_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 https://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 = "">(_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)};

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