Re: Invoking abstract subprograms in Ada95

["Grein, Christoph" <christoph.grein@eurocopter.com>]

> "Grein, Christoph" <christoph.grein@eurocopter.com> wrote in message 
news:<mailman.1019449992.18314.comp.lang.ada@ada.eu.org>...
> > The following compiles fine (with Rational Apex):
> > 
> > 
> > package Abstract_Class_Wide is
> > 
> >     type T is abstract tagged null record;
> > 
> >     procedure P (X : T) is abstract;
> > 
> >     procedure C (X : T'Class);
> > 
> > end Abstract_Class_Wide;
> > 
> > 
> > 
> > package body Abstract_Class_Wide is
> > 
> >     procedure C (X : T'Class) is
> >     begin
> > 	P (X);
> >     end C;
> > 
> > end Abstract_Class_Wide;
> 
> Hmm - the only differences I can see here are that my abstract method
> also takes a classwide type, and is implemented in a child package.
> Here's the distilled problem:
> 
> --
> package abstract_bit is
> 
> type A is abstract tagged null record;
> 
> procedure generalOperation( this : in out A'class );
> procedure specificOperation( this : in out A'class ) is abstract;


Class-wide operations cannot be abstract, they also cannot be overrridden.

OK, they can be abstract, this is why your code compiles, but abstractness here 
has a devastating meaning: There can never be such an operation!


> end abstract_bit;
> 
> --
> package body abstract_bit is
> 
> procedure generalOperation( this : in out A'class ) is
> begin
>     specificOperation( this => this );

You said it was abstract, so does not exist! Therefore you cannot call it here.


> end generalOperation;
> 
> end abstract_bit;
> 
> --
> package abstract_bit.concrete_bit is
> 
> type C is new A with null record;
> 
> procedure specificOperation( this : in out C );
> 
> end abstract_bit.concrete_bit;
> 
> 
> --
> package body abstract_bit.concrete_bit is
> 
> procedure specificOperation( this : in out C ) is
> begin
>     null;
> end specificOperation;
> 
> end abstract_bit.concrete_bit;
> 
> -- main program
> with abstract_bit.concrete_bit;
> 
> procedure annoying is
> 
> thingy : abstract_bit.concrete_bit.C;
> 
> begin
>     abstract_bit.generalOperation( thingy );
> end annoying;
> 
> 
> Mike
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