Re: OOP in Ada: Alternatives for "protected" scope

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

On Fri, 04 May 2012 12:37:01 +0200, Georg Bauhaus wrote:

> On 03.05.12 23:27, Felix Krause wrote:
>> Now there is some calculation to be done while execution Do_Something
>> which cannot be defined in A.
>> (...)
>> In languages like Java and C#, there is a "protected" scope to cope with this situation.
> 
> There are some O-O solutions.
> 
> One solution is outlined by at least on Java guru, and by the Objective-C
> programming model in ***step.  In short, replace procedural style
> with O-O style (sounds odd, but that's really what it does;
> the dependence of A/Do_Something on Calculate is an implicit
> procedural thing).
> 
> with Calculators;
> package P_A is
> 
>     type A is abstract tagged private;
> 
>     procedure Do_Something (Object : in out A);
>     --
>     -- Add your type of computation here to be performed
>     -- at some time during the execution of Do_Something:
>     --
>     procedure Link
>         (Object : in out A;
>          Computation: Calculators.Calculator'Class);

Well, if additional type hierarchies are permitted, then a possible pattern
is mix-in + Rosen's trick:

package P is
   type A (<>) is tagged private;
   procedure Do_Something (Object : in out A);

private
   type Abstract_Formula is abstract tagged null record;
   function Calculate (Formula : Abstract_Formula; Object : A'Class)
      return Integer is abstract;

   type A (Formula : not null access Abstract_Formula'Class) is
      abstract tagged null record;
end P;

package body P is
   procedure Do_Something (Object : in out A) is
   begin
      ...
      Result := Object.Formula.Calculate (Object);
      ...
   end Do_Something;

Clients derive from Abstract_Formula their implementations, these instances
are mixed in with A, which need not to be derived from when only formula
changes.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de