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From: julius_bip@yahoo.com (Julio Cano)
Newsgroups: comp.lang.ada
Subject: Re: How do I create an object instance from a tag?
Date: 20 Feb 2003 07:38:21 -0800
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"Steve" <nospam_steved94@attbi.com> wrote in message news:<yyb4a.155282$vm2.110486@rwcrnsc54>...
> I would like to create a function that returns a reference to a class wide
> object based on the external name.  Something along the lines of:
> 
> function Create_Object( external_name : String ) return objAcc is
>   returnTag : Ada.Tags.Tag;
> begin
>   returnTag := Ada.Tags.Internal_Tag( external_name );
>   return ??? some function of returnTag ???
> end Create_Object;
> 
> Can this be done?
> 

It depends on what you actually want. You can have "some function of
returnTag" if you have already registered the tagged type constructor
with its tag.
You can do this with a tagged type hierarchy. Just define the
constructor like this:
  function create return obj_class_acc
being
  type obj_class_acc is access obj'class

Defining all subtypes constructors like this:
  function create return obj_class_acc is
  begin
    return new derived_obj'class;
  end;
Note return type is obj_class_acc (base type).

Now you can "register" all the constructors in a package:
   type constr_func is access function return obj_class_acc;
  procedure register_constructor (type_name: string; 
                                   constructor: constr_func);

and look for the constructor like this:
   function objectforname(type_name: string) return constr_func;

Register somewhere (i.e: in the package of the type)
    register_constructor("this_class_name", constructor'access);

And create object based in its name:
    my_obj : obj_class_acc := objectforname("this_class_name").all;
    do_something(my_obj.all);

This solution is obviously restricted to a hierarchy, and
obj_class_acc most be the base type.
I think that one of the problems here is the lack of a generic base
type like (Object in Java, Id in Objective-C, ...). That kind of base
type or generic access type would help to implement a more generic
(and may be more dangerous) solution.






> I've perused the rationale and the RM and haven't run across anything.
> 
> Steve
> (The Duck)