Question

take this simple code:

class A{ 
  public: 
  virtual void foo() = 0; 
  void x(){ foo(); }
};
class B: public A{ foo(){ ... } };

main(){
  B b;
  b.x();
}

What I want is to build an abstract class that will have a function that will call a function expecting it to be implemented in the derived class

The question is that I can’t seem to make that work, the compiler says it can’t compile because it can’t find the reference(or something like that) to the foo() to be executed in x() of the base class. Can this work? Can anyone give me an example of this?

EDIT: It seems that it just doesn’t work when the “foo();” is inside the destructor of class A(the base one)…
It just got me confused. =[

EDIT2: how interesting this got. I just created a callfoo(){ foo(); } and now it compiles ok, but if I try to call the pure abstract function directly from within the destructor of Base class A, it gives me errors… weird. Anyone has any idea of this? O_o

any help on this please?

Thanks,
Jonathan

Update

It worked outside the destructor. Now I just got confused.

Try putting the “foo()” inside the destructor of the A(base) class, at least for me is not compiling…

any help plz?

Answer 1

There is nothing preventing you from doing that:

struct A {
    virtual ~A() {}
    virtual void f() = 0;
    virtual void g() { f(); }
};

struct B : A {
    void f() { std::cout << "B::f()" << std::endl; }
};

// ...
A* a = new B;
a->g(); // prints "B::f()"

---

As for calling a pure virtual function from the destructor (or constructor): Don’t! It invokes undefined behaviour.

§10.4/6:

Member functions can be called from a constructor (or destructor) of an abstract class; the effect of making a virtual call (10.3) to a pure virtual function directly or indirectly for the object being created (or destroyed) from such a constructor (or destructor) is undefined.