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From: mheaney@on2.com (Matthew Heaney)
Newsgroups: comp.lang.ada
Subject: Re: new/free of abstract types
Date: 31 Mar 2003 07:59:06 -0800
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"Frank" <franjoe@frisurf.no> wrote in message news:<3e87f119$1@news.wineasy.se>...
> 
> I have an abstract type, and wish to perform a new/free of objects extended
> from this type, inside one of the methods/subprograms
> that has the abstract type as a classwide parameter (in one of the type
> methods, to say it in Java/C++ language).  See example below.
> 
> It seems that I can't to it this way:
> 
> See example below to see how "The_New_Object" is declared:
> 
> The_New_Object := new MyType;
> 
> because the real property of the type is not fully known in the scope.
> 
> What I wish is to create a new object of the same type as the object
> "P_Parameter" (see example below).
> Can it be done - if so, how is the syntax?
> 
> I haven't yet tried to do this with the "Freeing" method, so a tip about that
> would also be appreciated.
> 
> 
> Frank
> 
> example:
> ---------------
> type Pointer_To_MyType is access all MyType;
> 
> type A_Type is record
>    The_New_Object : Pointer_To_MyType;
> end record;
> 
> Something_Global : A_Type;
> 
> ---------------
>   procedure Subprogram (P_Parameter : out MyType'Class) is
>   begin
>       Something_Global.The_New_Object := new MyType; <---?

You have to allocate objects of T'Class, like this:

type Pointer_To_MyType_Class is access all MyType'Class;

procedure Op (Arg : in MyType'Class) is  --in param, not out
begin
   Something_Global.The_New_Object := new MyType'Class'(Arg);

The_New_Object will inherit the type tag of Arg.  This is sort of like
the prototype pattern, except that it's built into the language.

Freeing the object isn't a problem, because Ada.Unchecked_Deallocation
accepts an indefinte type (as T'Class), e.g.

procedure Free is
   new Ada.Unchecked_Deallocation 
     (MyType'Class, 
      Pointer_To_MyType_Class);

Now you can say:

Free (Something_Global.The_New_Object);

Let me know if any of this isn't clear.  You asked an important
question, and it touches upon aspects of the language with which many
programmers aren't familiar.