Confused about virtual overloaded functions

Question

I am confused by compiler errors regarding the code below:

class Base { public: virtual ~Base () { } virtual void func () { } virtual void func (int) { } virtual void another () { } virtual void another (int) { } }; class Derived : public Base { public: void func () { } }; int main () { Derived d; d.func(); d.func(0); d.another(); d.another(0); } 

Using gcc 4.6.3, the above code throws an error in d.func (0) saying that Dervied :: func (int) is undefined.

When I also add a definition for func (int) in Derived, then it works. It also works (as in the case of the “other”) when I do not define either func () or func (int) in Derived.

It’s clear that there is some rule regarding virtual overloaded functions, but this is the first time I came across this, and I don’t quite understand it. Can someone tell me what exactly is going on here?

Answer 1

When you override func() in Derived , it hides func(int) .

gcc can warn you about this:

 $ g++ -Wall -Woverloaded-virtual test.cpp test.cpp:5:16: warning: 'virtual void Base::func(int)' was hidden [-Woverloaded-virtual] test.cpp:12:8: warning: by 'virtual void Derived::func()' [-Woverloaded-virtual] 

You can fix this with using :

 class Derived : public Base { public: using Base::func; void func () { } }; 

For a discussion of why this happens, see Why does an overridden function in a derived class hide other overloads of the base class?