Re: Basic Explaination of OO in Ada

["Alex R. Mosteo" <devnull@mailinator.com>]

Ludovic Brenta wrote:

> Access types come in handy if you want a collection of class-wide
> objects; the collection is said to be polymorphic:
> 
> with Pak;
> package Collection is
>    type Ref is access Pak.T'Class; -- access-to-class-wide-objects
>    type Vector is array (Positive range <>) of Ref; -- polymorphic
> vector
>    procedure Traverse (V : in Vector);
> end Collection;
> 
> package body Collection is
>    procedure Traverse (V : in Vector) is
>    begin
>       for J in V'Range loop
>          if V (J) /= null then
>             Pak.P (V (J).all); -- dispatches on the specific type of
> the object
>          end if;
>       end loop;
>    end Traverse;
> end Collection;

Just a note: although what Ludovic writes is right, and you can only have
vanilla arrays of definite types, you can have more sophisticated
containers (for example Ada.Containers.Indefinite_Vectors) that store
class-wide types in Ada 2005:

package Pak_Vectors is 
  new Ada.Containers.Indefinite_Vectors (Positive, Pak.T'Class);

In this case, however, you store copies so some overhead can occur, but in
many cases it can be more convenient than starting to play with pointers.