A question about inheritance

[subjugater]

Suppose I define a base class and a derived class in the following way:

// the base class
class base{
virtual void fa() = 0;
void fb(){fa();}
}

// the derived class
class derived: public base{
void fa() {...}
}

In the main function:

int main(){
derived* p;
p->fb();
...
}

My question: when the main function is calling the function fb() by using a pointer to an instantiation of the derived class, will fb() automatically incur the fa() defined in the derived class?

[LB]

If it compiles, then yes, because if it were to call the pure virtual one...wellllll, you see why that can't work ;)

[subjugater]

Hi LB,

Thank you for your comment. Actually this is only the tentative plan for my current project. I have not coded. But my idea is correct, right?

BTW, what do you mean by "wellllll, you see why that can't work"? I'm confused by this sentence...

[LB]

Do you know what a pure virtual function is? It is a function with no definition, meaning that it is syntactically illegal to call it and it also makes it so you cannot have an instance of the class. The only way to get an instance of the class is to derive from it, implement the function, and THEN you can get an instance.

Obviously the code you posted would do either one of two things:
1. Fail to compile from trying to call a pure virtual function, or
2. Compile and call the most derived virtual-overridden version of the function.

;)

[Disch]

To confirm, what you are trying to do is perfectly legal and acceptable. That's how virtual functions work.

The only caveat is that the object has to be fully constructed in order for it to work. That is, trying to call a virtual function from a constructor won't work -- at least not how you expect. Calling from a destructor has a similar problem and also won't work.

[subjugater]

LB,

Thank you for your further explanation. I'll code up what I planed to see if the compilation succeeds. Again, thank you very much.

[subjugater]

Hi Disch,

Thanks a lot for your comments. It clarifies my concern on if what I am trying to do is legal.

By your second paragraph, do you mean that I have to fully define the ctor and dtor in both the base and derived class? BTW, I don't think that I am implying anything in my pseudo code that I am intent to call the fa() or fb() from the ctor or dtor. Curious why you think that I am going to call these functions from ctor or dtor.

Anyway, thank you very much!!!

[Disch]

By your second paragraph, do you mean that I have to fully define the ctor and dtor in both the base and derived class?

No. You do not have to define ctors/dtor.

But if you do, they can't call virtual functions.

[subjugater]

Hi Disch,

One last question:

Which pair of ctor and dtor can not call the virtual functions? The pair in the base class or the pair in the derived class or both in base and derived class?

Thank you!

[Disch]

You shouldn't do it in any ctor/dtor. What will happen is it will only call the function in the same level class (not in any derived class).

Here's a simple example:

class A
{
public:
  A() { TheVirtual(); }
  virtual ~A() { TheVirtual(); }
  virtual void TheVirtual() { cout << "A" << endl; }
};

class B : public A
{
public:
  virtual void TheVirtual() { cout << "B" << endl; }
};

//...

int main()
{
  A* b = new B;  // prints "A"

  b->TheVirtual();  // prints "B" as expected

  delete b;  // prints "A"
}

In the destructor, the reason for this is because the B portion of the object has already been destructed by the time A's dtor is called, therefore it can't call a B function, so it calls the A version.

Calling it from the B dtor would print "B", but that would have the same problem if there was a 3rd class (C) which derives from B.

The constructor is the same story. The A portion is constructed first, so the B portion hasn't been constructed when TheVirtual is called, so it can't call the B version of that function.