Question

Consider the following program:

class Base
{
private:
    int m_nID;
public:
    Base()
    {
        m_nID = ClassID();
    }

    // ClassID returns a class-specific ID number
    virtual int ClassID() { return 1; }

    int GetID() { return m_nID; }
};

class Derived: public Base
{
public:
    Derived()
    {
    }

    virtual int ClassID() { return 2; }
};

int main()
{
    Derived cDerived;
    cout << cDerived.GetID(); 
    return 0;
}

In the above example,The derived id is surprisingly 1 instead of 2.I have already found a similar question regarding the same question here.But what i don’t understand is that if this is wrong,Then how are we supposed to identify a (derived) class member and use it?I mean suppose i want to dedicate a unique id or typename to each class (base class, the first derived class , the second derived class which is either a derivation of the base class or the second derived class and so on),In this regard how can i go on and act?
I think the correct way is that i assign an id/name in the constructor upon the instantiation of any class object so that the type is known immediately.The above approach failes,What other options do i have in this regard?

Answer 1

To elaborate on what Karoly said, maybe you can just avoid using the class ID in the constructor, but after construction you can just do:

cout << cDerived.ClassID();

There is no reason to have two functions that return the same thing and there is no reason to waste memory by having your int m_nID; stored in every object.

Also, you should change your base class so that it says:

virtual int ClassID() = 0;

This should make it a compiler error if you try to call ClassID in your Base constructor, though I have not tried it. Also, it will make Base be an abstract class, so you can’t create new instances of it (which is good).