Re: Polymorphism

[slos <new.stephane.los@gmail.com>]

Le jeudi 7 mai 2015 09:28:46 UTC+2, Simon Wright a écrit :
> slos writes:
> 
> > I'd like function Object.Area to stay private but the compiler doesn't
> > see it like that and complains if I try to move it in the private
> > part.
> 
> If you do that, the compiler says
> 
> slos.ada:24:13: this primitive operation is declared too late
> slos.ada:24:13: abstract subprograms must be visible (RM 3.9.3(10))
> 
> The reason for this is that derived types wouldn't know that they needed
> to implement the abstract operations.
> 
> If you make the private subprogram concrete,
> 
>    function Area (O : Instance) return Float
>      is (raise Program_Error with "Area not implemented");
> 
> then Circle.Area isn't overriding (because Object.Area isn't visible).
> 
> If you make Circle a child of Object (package Object.Circle), all is
> OK. This is of curse rather restrictive; it depends on who you want to
> be able to derive from Object.Instance.
> 
> 
> The workround I adopted in a similar situation was to comment the
> subprograms that had to be visible with "private use only". I think now
> I would make the names obviously logically private
> (e.g. "Private_Area").

Thank you for your answer Simon.

I have found this suitable for my needs :

> If you make Circle a child of Object (package Object.Circle), all is
> OK. This is of curse rather restrictive; it depends on who you want to
> be able to derive from Object.Instance.

But I thought about something like a private inner package or interface may have existed.

BR,
Stéphane