使从一个基类派生的类能够使用继承的受保护成员

Question

I created a base class with one protected method returning the id of the object. I want only derived classes to be able to query this id on other derived classes, but hide it from classes outside the inheritance hierarchy.

class Identifiable {
    public:
        virtual ~Identifiable() = default;
    protected:
        virtual auto getId() const noexcept -> unsigned = 0;
    };

    class ObjectA: public Identifiable {
    protected:
        auto getId() const noexcept -> unsigned override { return 0; }
    };

    class SpecificObjectA: public ObjectA {
    protected:
        using ObjectA::getId;
    };

    class ObjectB: Identifiable {
    public:
        explicit ObjectB(const SpecificObjectA& objectA) {
            objectA.getId(); // error C2248: 'SpecificObjectA::getId': cannot access protected member declared in class 'SpecificObjectA'
        }
    protected:
        auto getId() const noexcept -> unsigned override { return 0; }
};

Is there any way to make it work other than adding a next method?

auto getId(const Identifiable& identifiable) const noexcept -> unsigned {
    return getId();
}

Answer 1

No, that's not possible. Otherwise, it would let you use virtual protected members from any class by creating another class derived from the base class in question.

Answer 2

You cannot do as intended, but you might try this:

class Identifiable
{
public:
    virtual ~Identifiable() = default;
protected:
    virtual unsigned getId() const noexcept = 0;
    static unsigned int getId(Identifiable const& other)
    {
        return other.getId();
    }
};

Now you can do:

explicit ObjectB(const SpecificObjectA& objectA)
{
    Identifiable::getId(objectA);
}

I personally wouldn't use trailing return type unless you really need it, otherwise it just reduces readability.