object oriented compiler... question

[Stefan]

Ok... I made first an Scanner... after that an Parser, which return an parse tree. Noew I have to do the part wich tranlate to machine code. I have a problem:
It is an object oriented compiler... I don't know in an hierarchy of classes how to store the offsets of metods...
This problem is very big for me... I don't know how C++ or Java handles it...
For example:

class A {
  public void print() {
    // some code
  }
}

class B extends A {
   public void print() {
     // some ather code
   }
}

void function(A object) {
   object.print();
}

So I have two classes with the method print.
I don't understand how can I handle this case:

public static void main(String args[]) {
   A a=new A();
   B b=new B();
   function(a); // should be called print from class A
   function(b); // should be called print from class B
}

Does anyone have an ideea how can I know witch method print should I call in this case?

[carbonBased]

There's probably many ways to do this.  If I recall correctly, I believe C++ keeps a "virtual table" which contains pointers to each virtual function of each class.  It's, essentially, a database of function pointers.

In this case, then, C++ would mangle the names something like _A_print and _B_print (obviously it's a little more sophisticated then that), and would have function pointers pointing to them.

Obviously, the compiler would have to have _some_ method of differentiating object A from object B, even when they are referenced the same was (as "object" in your above code).  You'd have to incorporate some form of RTTI for this, I would imagine.  I'm not sure how C++ takes care of this.

Initially, with my own compiler, I intended have all objects as structures, at the low level.

For example:

class Stuff {
  int one, two;
  print();
};

would equate to:
stuff:
  size  dd end
  one   dd 0
  two   dd 0
  print dd print_func
end:

The size is just something I added in there, to be able to tell the total size of any object, in case of a copy.

In this instance, your code will always call [stuff+12] to access the print routine.  Wether the object is defined as A or B, the print method will always reside at this location, and can be overrided whenever, without knowledge of the executing code.

Cheers,
Jeff

[Stefan]

I think I have an ideea... I can add an pointer to an object wich represents that class... so, I'll know where an object belongs to.

Thanks!