Does a member of a C++ base class really needs to be virtual to be overridden by a derived class?

Question

class A { 
    public: 
    virtual int test()=0; 
};

class B : public A { 
   public: 
   int test(){return 10;}
};


B *b = new B();
b->test(); // would return 10;

whereas:

class A { 
    public: 
    int test(){return 0;}
};

class B : public A { 
   public: 
   int test(){return 10;}
};


B *b = new B();
b->test(); // **would return 0**;

Why does it return "0" here? This makes zero sense to me, because I assume that the (kind of overloaded) members of the derived class (B) come first! What is happening here?

`B->test()` doesn't compile. What makes you think the second example results in zero? — aschepler, Feb 12 '13 at 19:24
Yeah, your conceptions are incorrect. Even if you change the B to b to make it compile, it would return 10. — Seth Carnegie, Feb 12 '13 at 19:25
I'm guessing that he is wondering why `A *b = new B()` yields 0, even though the function has been replaced. Someone please explain vtbls to him — Anya Shenanigans, Feb 12 '13 at 19:28
You should review this question and answers, it should clear things up for you: http://stackoverflow.com/questions/2932909/overriding-vs-virtual — nybbler, Feb 12 '13 at 19:30
its returning 10 check here http://ideone.com/4mzEJO#view_edit_box — exexzian, Feb 12 '13 at 19:36
*What is happening here?* The definition of the language in the standard... — David Rodríguez - dribeas, Feb 12 '13 at 19:44
This question needs to be either edited or closed, since it isn't actually producing the output as described. — JBentley, Feb 12 '13 at 20:28
What is happening if you add using B::test; in the class B ? Or if you call it like that: B::b->test(); — Marc-Emmanuel Ramage, Feb 12 '13 at 19:34

user1610015 · Accepted Answer · 2013-02-12T19:34:43.823

4

Apart from the invalid syntax (B->test(); where it should be b->test();), the second one will also return 10.

If instead you would have written:

A* a = new B();
a->test();

It would have returned 0 or 10 depending on whether A::test is virtual.

edited Feb 12 '13 at 19:34

answered Feb 12 '13 at 19:29

user1610015

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Go to: http://www.cplusplus.com/doc/tutorial/polymorphism/ and Ctrl+F "varify" and see the above example. Now If I make the base virtual funtion not virtual and allocate "ppoly1" not as a Polygon, but as a CRectangle, it will return 0! – user1511417 Feb 12 '13 at 19:39
@user1511417: Take the time to reread the code in the linked page. You will only get the base's implementation if the static type of the pointer/reference is *base* – David Rodríguez - dribeas Feb 12 '13 at 19:46

Does a member of a C++ base class really needs to be virtual to be overridden by a derived class?

1 Answers1