Re: point by point advantages of Ada

["Warren W. Gay VE3WWG" <ve3wwg@cogeco.ca>]

Richard Riehle wrote:
> "Warren W. Gay VE3WWG" wrote:
>>I thought that this might work:
>>
>>    declare
>>       V : T'Class := ...
>>    begin
>>       ...
>>       V := Some_Other_Func;  - Dispatching
>>    end;
>>
>>But GNAT (at least) reports that this assignment is "ambiguous"
>>and will not compile it. I would have expected that it would
>>resolve it at runtime, knowing the current value of V and its
>>current tag. Is this consistent with the LRM (IANAL)?
> 
> I am not sure if you have tried this, but I often create a function
> such as,
> 
>              function Get ( ... ) return T'Class;
> 
> Then, I initialize the declaration with
> 
>              V : T'Class := Get (... );
> 
> where the parameter list might be the name of a data structure,
> a file, or even a simple array along with the next index.   This
> works really well in solving a lot of problems with dispatching.
> 
> Richard Riehle

Absolutely -- I love this feature. In fact the Booch components
rely on this as well, for the Iterators (I use BC a lot). For
example:

    procedure Close_Widget_List(List : in out WPD.List) is
       Iter :   WPC.Iterator'Class := WPD.New_Iterator(List);
       W :      Widget_Handle;
    begin
       while not WPC.Is_Done(Iter) loop
          W := WPC.Current_Item(Iter); -- Handle of current widget in list
          Close(W);                    -- Destroy the widget and its children
          WPC.Next(Iter);              -- Advance to the next widget in the list
       end loop;
       WPD.Clear(List);                -- Empty the list
    end Close_Widget_List;

Note the WPC.Iterator'Class type.

I was just trying to see if true runtime dispatching could occur
on the return type. It would seem that in my earlier example,
that the V := Some_Other_Func dispatch is fixed at compile
time, if it is not ambiguous.  I suppose that this is the
"safe" thing to do, and probably why it works that way.

But again, I did not try it with AONIX or any other compiler, and
I've not researched the LRM position on this.

-- 
Warren W. Gay VE3WWG
http://home.cogeco.ca/~ve3wwg