Question

class a //my base class
 {
public:
    a()
    {
        foo();
    }
  virtual void foo() = 0;
};



class b : public a
{
    public:
    void foo()
    {
    }
};

int main()
{
    b obj; //ERROR:  undefined reference to a::foo()
}

Why it gives me error? The pure virtual foo is defined. What do I need to change in my code to make it work? I need pure virtual method from base class be called in its constructor.

Answer 1

Calling virtual functions in a constructor is recognised as a bad thing to do.

During base class construction of a derived class object, the type of
the object is that of the base class. Not only do virtual functions
resolve to the base class, but the parts of the language using runtime
type information (e.g., dynamic_cast (see Item 27) and typeid) treat
the object as a base class type.

So your instantiation of b invokes the a constructor. That calls foo(), but it’s the foo() on a that gets called. And that (of course) is undefined.