c++新特性11 (10)shared_ptr五”构造函数“

1. 几种构造函数

shared_ptr p(q) p管理内置指针q所指的对象；q必须指向new分配的内存，且能够转换为T类型
shared_ptr p(q,d) p接管了内置指针q所指向的对象的所有权。q必须能转换为T类型。p将使用可调用对象d来代替delete
shared_ptr p(p2,d) p是shared_ptr p2的拷贝,唯一的区别是p将使用可调用对象d来代替delete

1.1 如果参数是shared_ptr引用，那么调用底层的construct函数，计算器加1，否则如果是&&引用，就调用底层的move

 template <class _Ty2>
    shared_ptr(const shared_ptr<_Ty2>& _Right, element_type* _Px) noexcept {
    
    
        // construct shared_ptr object that aliases _Right
        this->_Alias_construct_from(_Right, _Px);
    }

    template <class _Ty2>
    shared_ptr(shared_ptr<_Ty2>&& _Right, element_type* _Px) noexcept {
    
    
        // move construct shared_ptr object that aliases _Right
        this->_Alias_move_construct_from(_STD move(_Right), _Px);
    }

    shared_ptr(const shared_ptr& _Other) noexcept {
    
     // construct shared_ptr object that owns same resource as _Other
        this->_Copy_construct_from(_Other);
    }

    template <class _Ty2, enable_if_t<_SP_pointer_compatible<_Ty2, _Ty>::value, int> = 0>
    shared_ptr(const shared_ptr<_Ty2>& _Other) noexcept {
    
    
        // construct shared_ptr object that owns same resource as _Other
        this->_Copy_construct_from(_Other);
    }

    shared_ptr(shared_ptr&& _Right) noexcept {
    
     // construct shared_ptr object that takes resource from _Right
        this->_Move_construct_from(_STD move(_Right));
    }

    template <class _Ty2, enable_if_t<_SP_pointer_compatible<_Ty2, _Ty>::value, int> = 0>
    shared_ptr(shared_ptr<_Ty2>&& _Right) noexcept {
    
     // construct shared_ptr object that takes resource from _Right
        this->_Move_construct_from(_STD move(_Right));
    }

1.2 底层函数：_Move_construct_from

 template <class _Ty2>
    void _Move_construct_from(_Ptr_base<_Ty2>&& _Right) noexcept {
    
    
        // implement shared_ptr's (converting) move ctor and weak_ptr's move ctor
        _Ptr = _Right._Ptr;
        _Rep = _Right._Rep;

        _Right._Ptr = nullptr;
        _Right._Rep = nullptr;
    }
    
template <class _Ty2>
    void _Alias_move_construct_from(shared_ptr<_Ty2>&& _Other, element_type* _Px) noexcept {
    
    
        // implement shared_ptr's aliasing move ctor
        _Ptr = _Px;
        _Rep = _Other._Rep;

        _Other._Ptr = nullptr;
        _Other._Rep = nullptr;
    }

1.3 底层函数：construct_from

template <class _Ty2>
void _Copy_construct_from(const shared_ptr<_Ty2>& _Other) noexcept {
    
    
    // implement shared_ptr's (converting) copy ctor
    _Other._Incref();

    _Ptr = _Other._Ptr;
    _Rep = _Other._Rep;
}

template <class _Ty2>
void _Alias_construct_from(const shared_ptr<_Ty2>& _Other, element_type* _Px) noexcept {
    
    
    // implement shared_ptr's aliasing ctor
    _Other._Incref();

    _Ptr = _Px;
    _Rep = _Other._Rep;
}