Question

I have two classes as follows in a header file

template<size_t N>
class Parent{
    protected:
    char array[N];
    size_t i;
    public:
    virtual void operator()(int i);
};

template<size_t N>
void Parent<N>::operator()(int i){
    this->i = i;
}

class Child: public Parent<16>{
    public:
    virtual void operator()();
};

Child has operator()() defined elsewhere in a cpp file. Whenever I include this header file from another cpp file I can access operator()() but operator()(int) is not even defined. Why is this? I thought since I inherit from a specific instance of Parent, all the methods of it should be instanced as well and available.

Answer 1

Apart from the errors in your code, this is an example of hiding: Your derived class declares a function of the same name but with different signature as a base class. Thus the base function is hidden:

class A { virtual void foo(); };

class B : public A { virtual void foo(int); /* hides A::foo() ! */ };

Inheritance only affects functions that have the same signature (with some mild exceptions).

Your base class function is declared as void Parent<N>::operator()(int), while in your derived class you declare void Child::operator()().

In C++11 you can explicitly say virtual void foo(int) override to trigger a compiler error if the function isn’t overriding anything.

If you intentionally want to define a new function with the same name as an existing one but with different signature, and not overriding the base function, then you can make the base function visible with a using directive:

class C : public A
{
  using A::foo();
  void foo(int);
};  // now have both C::foo(int) and C::foo()