shared_ptr & weak_ptr
本次源码采用的是vs2015 c++11的源码进行分析
sizeof(shard_ptr)=?
回答这个问题，就需要看shared_ptr 的数据结构，下边进行源码分析
shared_ptr
| C++
template
class shared_ptr
: public _Ptr_base<_Ty>
|
_Ptr_base<_Ty>
| C++
template
class _Ptr_base
{ // base class for shared_ptr and weak_ptr
public:
typedef _Ptr_base<_Ty> _Myt;
typedef _Ty element_type;
private:
_Ty *_Ptr;
_Ref_count_base *_Rep;
} |
也就是说 shared_ptr 内部数据就是两个指针_Ptr 与 _Rep
那就很好理解了，在32位字节上4字节对齐情况下是8个字节
在64位机器上，8字节对齐请下是16个字节
shared_ptr 引用计数为0的时候做了什么
shared_ptr::_Reset0
| C++
template
class shared_ptr
: public _Ptr_base<_Ty>
{
void _Reset()
{ // release resource
_Reset(0, 0);
}
template
void _Reset(const _Ptr_base<_Ty2>& _Other)
{ // release resource and take ownership of _Other._Ptr
_Reset(_Other._Ptr, _Other._Rep);
}
template
void _Reset(const _Ptr_base<_Ty2>& _Other, bool _Throw)
{ // release resource and take ownership from weak_ptr _Other._Ptr
_Reset(_Other._Ptr, _Other._Rep, _Throw);
}
template
void _Reset(auto_ptr<_Ty2>&& _Other)
{ // release resource and take _Other.get()
_Ty2 *_Px = _Other.get();
_Reset0(_Px, new _Ref_count<_Ty>(_Px));
_Other.release();
_Enable_shared(_Px, _Rep);
}
void _Reset0(_Ty *_Other_ptr, _Ref_count_base *_Other_rep)
{ // release resource and take new resource
if (_Rep != 0)
_Rep->_Decref();
_Rep = _Other_rep;
_Ptr = _Other_ptr;
}
}
|
_Ref_count_base::_Decref
| C++
class _Ref_count_base
{
// common code for reference counting
_Atomic_counter_t _Uses;
_Atomic_counter_t _Weaks;
void _Decref()
{ // decrement use count
if (_MT_DECR(_Uses) == 0)
{ // destroy managed resource, decrement weak reference count
_Destroy();
_Decwref();
}
}
virtual void _Delete_this() = 0；
virtual void _Destroy() _NOEXCEPT
{ // destroy managed resource
delete _Ptr;
}
void _Decwref()
{ // decrement weak reference count
if (_MT_DECR(_Weaks) == 0)
_Delete_this();
}
}； |
_Ref_count_del::_Delete_this
| C++
template class _Dx>
class _Ref_count_del
: public _Ref_count_base
{ // handle reference counting for object with deleter
public:
_Ref_count_del(_Ty *_Px, _Dx _Dt)
: _Ref_count_base(), _Mypair(_One_then_variadic_args_t(), _Dt, _Px)
{ // construct
}
virtual void *_Get_deleter(
const _XSTD2 type_info& _Typeid) const _NOEXCEPT
{ // return address of deleter object
return ((void *)(_Typeid == typeid(_Dx)
? _STD addressof(_Mypair._Get_first()) : 0));
}
private:
virtual void _Destroy() _NOEXCEPT
{
// _Get_first 拿到的是自定义析构方法
//_Mypair._Get_second() 管理的指针
_Mypair._Get_first()(_Mypair._Get_second());
}
virtual void _Delete_this() _NOEXCEPT
{ // destroy self
delete this;
}
_Compressed_pair<_Dx, _Ty *> _Mypair;
}; |
_Ref_count::_Delete_this
| C++
template
class _Ref_count
: public _Ref_count_base
{ // handle reference counting for object without deleter
public:
_Ref_count(_Ty *_Px)
: _Ref_count_base(), _Ptr(_Px)
{ // construct
}
private:
virtual void _Destroy() _NOEXCEPT
{ // destroy managed resource
delete _Ptr;
}
virtual void _Delete_this() _NOEXCEPT
{ // destroy self
delete this;
}
_Ty * _Ptr;
};
|
当shared ptr 创时候带了自定义析构函数会使用_Ref_count_del这个类
默认使用的是_Delete_this
通过上边分析可以看出，当智能指针的引用计数清0的时候首先会销毁ptr，其次在判断弱引用的技术是否也清0了，如果清0了就一并释放了
weak_ptr 分析
weak_ptr构造
| C++
template
class weak_ptr
: public _Ptr_base<_Ty>
{ // class for pointer to reference counted resource
public:
typedef weak_ptr<_Ty> _Myt;
typedef _Ptr_base<_Ty> _Mybase;
constexpr weak_ptr() _NOEXCEPT
{ // construct empty weak_ptr object
}
weak_ptr(const weak_ptr& _Other) _NOEXCEPT
{ // construct weak_ptr object for resource pointed to by _Other
this->_Resetw(_Other);
}
//is_convertible 会判断原weak_ptr持有的指针跟构造的指针是否同一个类型
//如果是同一个类型使用下边方法，不是会调用其他的构造芳芳
//这也就是SFINAE典型应用
template class = typename enable_if::value,
void>::type>
weak_ptr(const shared_ptr<_Ty2>& _Other) _NOEXCEPT
{ // construct weak_ptr object for resource owned by _Other
this->_Resetw(_Other);
}
} |
通过shared_ptr构造weak_ptr 过程
_Ptr_base::_Resetw
| C++
template
class _Ptr_base
{ // base class for shared_ptr and weak_ptr
void _Resetw()
{ // release weak reference to resource
_Resetw((_Ty *)0, 0);
}
template
void _Resetw(const _Ptr_base<_Ty2>& _Other)
{ // release weak reference to resource and take _Other._Ptr
_Resetw(_Other._Ptr, _Other._Rep);
}
template
void _Resetw(_Ty2 *_Other_ptr, _Ref_count_base *_Other_rep)
{ // point to _Other_ptr through _Other_rep
if (_Other_rep)
_Other_rep->_Incwref();
if (_Rep != 0)
_Rep->_Decwref();
_Rep = _Other_rep;
_Ptr = const_cast *>(_Other_ptr);
}
public:
typedef _Ptr_base<_Ty> _Myt;
typedef _Ty element_type;
private:
_Ty *_Ptr;
_Ref_count_base *_Rep;
} |
通过上边的_Resetw 可以发现 它是从shared_ptr中的_Ptr与_Rep 构建出来的，为了防止构建过程中出现野指针的情况，它以上来就会调用_Other_rep->_Incwref();增加一个引用计数
std::enable_shared_from_this
enable_shared_from_this 定义
| C++
template
class enable_shared_from_this
{ // provide member functions that create shared_ptr to this
public:
typedef _Ty _EStype;
shared_ptr<_Ty> shared_from_this()
{ // return shared_ptr
return (shared_ptr<_Ty>(_Wptr));
}
shared_ptr shared_from_this() const
{ // return shared_ptr
return (shared_ptr(_Wptr));
}
private:
weak_ptr<_Ty> _Wptr;
} |