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Technology Stocks : BORL: Time to BUY!
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To: David R who wrote (8348)1/1/1998 1:12:00 PM
From: Kashish King  Read Replies (1) of 10836
 
David, the compiler doesn't have to know whether a pointer points to a base class or a derived class. Since we're talking about efficiency here, remember that the compiler can often substitute a direct call for a virtual call but that's an optimization issue which we can leave out for now.

class A { virtual void foo( ) ; virtual void bar ( ) ; } ;
class B { virtual void foo( ) ; virtual void zot ( ) ; } ;

When we instantiate an A and a B we get two distinct vtbls, barring optimization as already stipulated. Here's what the vtbls might look like for an object of type A and B respectively, this demonstrates the crucial point:

vtbl for object of type A:
A::foo
A:bar

vtbl for object of type B:
B::foo
A::bar
B::zot

Now, a pointer has to point to an actual instance of an object and that object has to be constructed. There are no vitual constructors, the compiler always knows the exact type of the object being constructed on a given line of code. Since each object gets it's own vtbl it doesn't matter how many pointers there are or what the exact type of the pointer is. If you are pointing to an object type B you will always call B::foo, A::bar and B::zot!

Here's an example using a reference instead of a pointer because you rarely need to use pointers in C++ and you don't have them in Java anyway. Not only does it make the syntax cleaner, it make the code more reliable without doing any null pointer checks and having to decide how to handle that:

Thud ( A & Geek )
{
Geek.foo ( );
}

B Bazola;
Thud( Bazola);
Bazola.zot ( );

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