Re: A bad counterintuitive behaviour of Ada about OO

[Maciej Sobczak <see.my.homepage@gmail.com>]

W dniu poniedziałek, 11 sierpnia 2014 09:41:09 UTC+2 użytkownik Dmitry A. Kazakov napisał:

> Nope, it would not work in any language, unless you redefine the word
> dispatching as you tried with covariance. There is no dispatching when
> types are known.

OK, so you still don't get it. The (potential) dispatch is on the factory type:

procedure Use_Whatever_Factory (F : in Factory'Class) is

   P : Product'Class := F.Make_Product;

begin
   -- ...
end;

Now you have dispatch on the factory type.
Of course, I still want all the specific calls work, too, that's why the return type cannot be Product'Class, otherwise I will be forced to cast types.
In other words, this has to work as well:

procedure Use_Specific_Factory (F : in Specific_Factory) is

   SP : Specific_Product := F.Make_Product;

begin
   -- ...
end;

And there is no way to make *both* work in Ada. You can get one or the other but not both.

> Where is a problem? Do this (in another package):
> 
>    not overriding
>       function Make (F : not null access B_Factory) return B;
> 
> Observe: not overriding.

And this is where your idea breaks down. The Make operation *must* be overriding, otherwise I will not be able to use Factory'Class.

> > Useless?
> 
> Yes, useless. If you know all types involved you call a specific subprogram
> or simply declare the object. Work done.

I might not know the specific factory type. That's the whole point of all creational design patterns, which I have mentioned several times already.

> Factories are used *only* when types are unknown in advance.

Not only. I might want to use specific factory from the hierarchy of factories, too. The OOP does not force me to use class-wide types only.

-- 
Maciej Sobczak * http://www.msobczak.com * http://www.inspirel.com