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：
这里写图片描述

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

1 weak_ptr 源码

// weak_ptr
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
    }
};

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_ptr
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 _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_ptr
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 _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_count
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 _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_base
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 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)
};

End —————————————————————————————————-

C++智能指针 shared_ptr 与 weak_ptr 原理

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