Inheritance mucking up polymorphism in C++?

Question

Perhaps my knowledge of inheritance and polymorphism isn't what I thought it was. Can anyone shed some light?

Setup (trivialization of problem):

class X {
};

class Y {
};

class Base {
  public:
    void f( X* ) {}
};

class Child: public Base {
  public:
    void f( Y* ) {}
};

Question: This should work, right?

int main( void ) {
  X* x = new X();
  Y* y = new Y();
  Child* c = new Child();
  c->f( x );
  c->f( y );
  return 0;
}

I get errors (GCC 4.4) to the tune of:

`no matching function for call to 'Child::f(X*&)'`
`note: candidates are: void Child::f(Y*)`

Name hiding. This question gets asked like 3 times a day. See the FAQ http://www.parashift.com/c++-faq-lite/strange-inheritance.html#faq-23.9 — AnT stands with Russia, Dec 11 '09 at 18:32
Perhaps I'm misreading, but are you sure that the arguments to f should be A and B? — tloach, Dec 11 '09 at 18:33
His error message shows that he has no `A` and `B` in his real code. It is just a typo. I corrected the source. — AnT stands with Russia, Dec 11 '09 at 18:35
Thanks, @AndreyT - I had A, B, C, and D originally but it was too confusing. — Chris Tonkinson, Dec 11 '09 at 18:39
possible duplicate of [Why does an overridden function in the derived class hide other overloads of the base class?](http://stackoverflow.com/questions/1628768/why-does-an-overridden-function-in-the-derived-class-hide-other-overloads-of-the) — Mooing Duck, Feb 03 '14 at 17:28

score 9 · Accepted Answer · answered Dec 11 '09 at 18:35

9

The virtual keyword will not help you here.

Your base class Base::f is being hidden by your derived type. You need to do the following:

class Child: public Base {
  public:
    using Base::f;
    void f( Y* ) {}
};

answered Dec 11 '09 at 18:35

Brian R. Bondy

Okay, I didn't know you could do 'using' in a class like that - thanks. At any rate, I'm well familiar with the concept of name hiding - but is it really **ONLY** based upon function name, and not the entire signature? – Chris Tonkinson Dec 11 '09 at 18:49
2

Yes, the look-up by name happens before overload resolution, thus `Base::f` isn't considered for overloads without pulling them in explicitly. – Georg Fritzsche Dec 11 '09 at 19:41

score 2 · Answer 2 · answered Dec 11 '09 at 18:37

2

Your derived class' f() hides the base class' f(). You can prevent this by explicitly bringing Base::f() into the derived class' scope:

class Child: public Base {
  public:
    using Base::f;
    void f( Y* ) {}
};

answered Dec 11 '09 at 18:37

sbi

score 0 · Answer 3 · edited May 23 '17 at 10:27

0

It was already answered at:

When you declare a method on a derived class, it hides any method with the same name from the base class.

edited May 23 '17 at 10:27

Community

answered Dec 11 '09 at 18:41

J. Calleja

3 Answers3