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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Subject: Re: Dynamically tagged expression not allowed. Why?
Newsgroups: comp.lang.ada
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Date: Sat, 29 May 2010 20:23:36 +0200
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Date: 2010-05-29T20:23:37+02:00
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On Sat, 29 May 2010 12:36:33 -0500, Marc A. Criley wrote:

> Stripping out a test case, I see that it has nothing to with the C++ 
> binding per se, but it's an Ada issue that's flummoxing me.  Here's the 
> test code:
> 
> procedure Dyty_Test is
> 
>     type Class_Type is tagged limited record
>        null;
>     end record;
> 
>     function New_Class_Instance return Class_Type'Class;
> 
>     Conn : Class_Type := New_Class_Instance;

Should Conn be mutable? Otherwise you could use a function instead.

>     function New_Class_Instance return Class_Type'Class is
>     begin
>        -- Just a stub
>        return New_Class_Instance;

Infinite recursion. I suppose it is

return X : Class_Type;

>     end New_Class_Instance;
> 
> The error is on the declaration of "Conn", with the error occurring on 
> the invocation of the initializing New_Class_Instance function. I'm just 
> not understanding something here.

You are "returning" a class-wide limited object, which is to "initialize" a
specific object.

A more difficult problem is that to initialize Conn, GNAT wants to know the
body of the initializer.
--------------------------------------------------------------
Variant 1. Two packages (to have the body):

   package P is
      type Class_Type is tagged limited record
         null;
      end record;
      function New_Class_Instance return Class_Type'Class;
      function New_Class_Object return Class_Type;
   end P;

   package P.Instances is
      Conn : Class_Type := New_Class_Object;
   end P.Instances;

   package body P is
      function New_Class_Object return Class_Type is
      begin
         return X : Class_Type;
      end New_Class_Object;
      function New_Class_Instance return Class_Type'Class is
      begin
         return New_Class_Object;
      end New_Class_Instance;
   end P;
----------------------------------------------------------------------------------------------------
Variant 2. Old good Ada initialization (in 80's we knew how to do it right)

   package P is
      type Class_Type is tagged limited record
         null;
      end record;
      function New_Class_Instance return Class_Type'Class;
      Conn : Class_Type; -- It will be OK upon the body elaboration!
   end P;

   package body P is
      procedure Construct (Object : in out Class_Type) is
      begin
         null;
      end Construct;
      function New_Class_Instance return Class_Type'Class is
      begin
         return X : Class_Type do
            Construct (X);
         end return;
      end New_Class_Instance;
   begin
      Construct (Conn); -- Fix it now
   end P;

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