Re: Question about OO programming in Ada

[Jeffrey Carter <spam@spam.com>]

Ekkehard Morgenstern wrote:

> Both the Ada Rationale and the Guide for C/C++ programmers say that
> you have to declare the first parameter of a procedure or the return
> value of a function to be of the class-wide type of the class to get
> the dynamic runtime dispatching behaviour from Ada (like virtual
> functions in C++).

Not at all. With a class-wide type, you get NO dispatching when you call 
the subprogram; you may get dispatching from calls made by the 
subprogram. To get dispatching, you need to call a parent type's 
operation with a value with a tag not known at compile time. Example:

package A is
    type Parent is tagged null record;

    procedure Op (P : in Parent); -- Op 1

     function Get return Parent'Class;
end A;

package A.B is
    type Child is new Parent with null record;

    procedure Op (P : in Child); -- Op 2
end A.B;
...
with A;
...
A.Op (P => A.Get); -- Dispatches to the correct Op

-- 
Jeff Carter
"Sheriff murdered, crops burned, stores looted,
people stampeded, and cattle raped."
Blazing Saddles
35