Re: Invoking abstract subprograms in Ada95

[Dmitry A. Kazakov <mailbox@dmitry-kazakov.de>]

On 22 Apr 2002 06:24:15 -0700, michael.jackson5@virgin.net (Mike)
wrote:

>"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,

That's the point. It is not a method [in the meaning "overridable"],
when the formal parameter is class wide. It could be a method only
relatively to some specific formal parameter[s]. Class wide means "one
for all derived types", so it cannot be overridden [at least till an
introduction "class class wide types" (:-))]

> 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;

If your intention is to override specificOperation for derived types,
then it should be exactly as in Christoph's example:

procedure specificOperation( this : in out A ) is abstract;

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

---
Regards,
Dmitry Kazakov
www.dmitry-kazakov.de