How to get access to private members of other class?

Question

I am implementing my own SharedPtr and WeakPtr and there are many methods where I need access to private members of SharedPtr from WeakPtr's methods. For example:

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;
}