Virtual non-method members

Question

Is something similar to this possible? This way it produces an error.

class A {
    public:
    virtual std::string key;
};

class B : public A {
    public:
    std::string key;
};

int main()
{
    A a;
    a.key = "Foo";
    return 1;
}

Answer 1

No, because this doesn't really make sense. Remember that a subclass contains all members of its parent class; therefore, B still has A's std::string key. Moreover, since B's std::string key is the same type, it's absolutely identical to A's - so what was the point of overriding it?

Also, note that during construction, when we're running A's constructor, B's virtual methods won't be called. This means that if we access key during A's construction, we'd get A's key - but then when B is constructed, that key would be shadowed, its data completely inaccessible.

That said, if you really do want to do something like this, for some reason, you'll need to use a virtual accessor function:

class A {
    private:
    std::string m_key;
    public:
    virtual std::string &key() { return m_key; }
    virtual const std::string &key() const { return m_key; }
};

class B : public A {
    private:
    std::string m_key;
    public:
    virtual std::string &key() { return m_key; }
    virtual const std::string &key() const { return m_key; }
};

int main()
{
    B b;
    b.key() = "Foo";
    return 0;
}