c++新特性11 (10)shared_ptr三”创建计数器“

Posted 2022-04-04 thefist11

1. 创建计数器_Ref_count类

构造函数中new

template <class _Ux,
    enable_if_t<conjunction_v<conditional_t<is_array_v<_Ty>, _Can_array_delete<_Ux>, _Can_scalar_delete<_Ux>>,    _SP_convertible<_Ux, _Ty>>,   int> = 0>
explicit shared_ptr(_Ux* _Px)  // construct shared_ptr object that owns _Px
    if constexpr (is_array_v<_Ty>) 
        _Setpd(_Px, default_delete<_Ux[]>);
     else 
        _Temporary_owner<_Ux> _Owner(_Px);
        _Set_ptr_rep_and_enable_shared(_Owner._Ptr, new _Ref_count<_Ux>(_Owner._Ptr));
        _Owner._Ptr = nullptr;
    

1.1 _Ref_count类

// CLASS TEMPLATE _Ref_count
template <class _Ty>
class _Ref_count : public _Ref_count_base  // handle reference counting for pointer without deleter
public:
    explicit _Ref_count(_Ty* _Px) : _Ref_count_base(), _Ptr(_Px) 

private:
    virtual void _Destroy() noexcept override  // destroy managed resource
        delete _Ptr;
    

    virtual void _Delete_this() noexcept override  // destroy self
        delete this;
    

    _Ty* _Ptr;
;


// CLASS TEMPLATE shared_ptr
template <class _Ux>
struct _Temporary_owner 
    _Ux* _Ptr;

    explicit _Temporary_owner(_Ux* const _Ptr_) noexcept : _Ptr(_Ptr_) 
    _Temporary_owner(const _Temporary_owner&) = delete;
    _Temporary_owner& operator=(const _Temporary_owner&) = delete;
    ~_Temporary_owner() 
        delete _Ptr;
    
;

1.2 _SP_convertible

// TYPE TRAIT _SP_convertible
template <class _Yty, class _Ty> struct _SP_convertible : is_convertible<_Yty*, _Ty*>::type ;

template <class _Yty, class _Uty> 
     struct _SP_convertible<_Yty, _Uty[]> : is_convertible<_Yty (*)[], _Uty (*)[]>::type ;
     
template <class _Yty, class _Uty, size_t _Ext> struct _SP_convertible<_Yty, _Uty[_Ext]> : is_convertible<_Yty (*)[_Ext], _Uty (*)[_Ext]>::type ;

is_convertible是一个编译器内部支持的函数，能够获取到两个类之间是否能够转换