Re: Usefulness of classwide "out" parameter

["Nick Roberts" <Nick.Roberts@dial.pipex.com>]

I think 'out' parameters of a classwide type will be useful in some
situations.  I am particularly thinking of a 'wrapper' procedure which
redispatches on the classwide parameter.  For example, consider an abstract
tagged type T, and suppose that it has an abstract procedure:

   procedure Create (Obj: out T) is abstract;

so that all types derived from T must have a 'Create' procedure, which
presumably initialises an object of that type in the appropriate way.

Now, supposing you wanted to monitor the creation of all objects of types
derived from T (perhaps writing a message onto a log file for each
occurrence).  You could do this by declaring a 'wrapper' procedure:

   procedure Monitored_Create (Obj: out T'Class) is
   begin
      Put_Line(Log_File,"Creating object of type " &
Ada.Tags.Expanded_Name(Obj'Tag));
      Create(Obj);
   end;

This procedure outputs a message onto a log file (using the known tag of the
object actually passed), and then redispatches on that object to the
requisite 'Create' procedure.

It may be that 'out' parameters of a classwide type are not likely to be
used very often, but, like many aspects of the language, I suspect they will
be useful every now and then!  (A bit like me, really ;-)

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Nick Roberts
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