如何访问其他班级的私人成员？

Question

我正在实现自己的SharedPtr和WeakPtr，有许多方法需要从WeakPtr的方法访问SharedPtr的私有成员。例如：

template<typename Resource>
WeakPtr<Resource>::WeakPtr(const SharedPtr<Resource>& rhs)
{
    //how can i get access to private members of rhs there?
    //only way is make them public?
    //I need something like this:
    resource = rhs.resource;
    control_block = rhs.control_block;
}