Re: Ada Objects Help!

[bobduff@world.std.com (Robert A Duff)]

In article <1995Jan24.230910.2241@nbivax.nbi.dk>,
Jacob Sparre Andersen <sparre@meyer.fys.ku.dk> wrote:
>I am pretty sure I know how Borland Pascal and C++ decides which virtual 
>method to call, but as I've understood Ada95, the decision could be based on 
>more than one object. Am I right? And how is it done?

Yes, it is possible for more than one parameter to be "controlling".  In
that case, there's a run time check that the tags of all controlling
parameters are equal.  If they are, then that tag determines what
subprogram body to dispatch to.

It's not like CLOS, which has "multi-methods", which can dispatch based
more than one parameter, according to a rather complicated set of rules.

By the way, dispatching on result is an interesting twist that most OOP
languages don't have:

    function Union(X, Y: Set) return Set;
    function Empty_Set return Set;

    X: Set'Class := ...;
    Y: Set'Class := Union(X, Empty_Set);

The appropriate version of function Empty_Set to call is determined (at
run time) based on the tag of Y, since Empty_Set itself doesn't have any
controlling parameters.

- Bob