Question

Here is the problem — I have two classes like this:

class A
{
  //some fields and methods 
   void niceMethod();
};

class B : public A
{
   void niceMethod();
};

class C : public A
{
   void niceMethod();
};

and function

void myFunc(A** arrayOfABC);

//Double * is to notice that I am going to modify the argument.

And I want to do:

(*arrayOfABC)[i].niceMethod();

in my function, getting different things done when I pass array of Bs or Cs to function.

But then I try to call it like

 B* bees = NULL;
    myFunc(&bees);

I have “Argument type of B** is incompatible with parameter of type A**”.

I know that I can pass B or C as A to functions like f(A), but what’s wrong with pointers?

Answer 1

The compiler is right, it is indeed incompatible. Consider this:

B* bees = new B[2];
myFunc(&bees); // Imagine that it's allowed

Now inside myFunc you do this:

void myFunc(A** arrayOfABC) {
    // This is OK:
    arrayOfABC[0] = new C();
}

This should be allowed, because C extends A. However, upon return from myFunc your bees would contain a C, which is not good.

To fix this, create an array of A*, and populate it with pointers to B.

P.S. Don’t forget to make niceMethod virtual, otherwise it’s not going to work the way you expect.