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Subject: Re: A bad counterintuitive behaviour of Ada about OO
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On 05-Aug-14 15:07, Victor Porton wrote:
> Pleas explain what exactly you mean saying "re-dispatch".

The wiki-book has a pretty good explanation:
http://en.wikibooks.org/wiki/Ada_Programming/Object_Orientation#Redispatching

The difference between Dispatching and non-Dispatching is, essentially, 
the same as that of static- and dynamic-linking -- Re-Dispatching is 
when the dispatching-call makes another dispatching call.

I'm probably not explaining it well but let's step-through -- given the 
following:

     Package Example is
         Type Base is tagged null record;
         Procedure Call_Op1  ( T1 : Base'Class ); -- A Dispatching Call
         Procedure Call_Op2  ( T1 : Base );   -- Not A Dispatching Call
         Procedure Operation ( T1 : Base );   -- Not A Dispatching Call

         Type Current is new Base with null record;
         Overriding Procedure Call_Op2 ( T1 : Current ); -- Not A 
Dispatching Call
         Overriding Procedure Operation( T1 : Current ); -- Not A 
Dispatching Call

         Type Child is new Current with null record;
         Overriding Procedure Call_Op2 ( T1 : Child );   -- Not A 
Dispatching Call
         Overriding Procedure Operation( T1 : Child );   -- Not A 
Dispatching Call
     End Example;

     Package Body Example is
         Procedure Call_Op1 ( T1 : Base'Class ) is
         begin
             T1.Operation; -- Re-Dispatching.
         end;

         Procedure Call_Op2 ( T1 : Base ) is
         begin
           Base'Class(T1).Operation; -- Dispatching
         end;

         Overriding Procedure Call_Op2 ( T1 : Current ) is
         begin
             Ada.Text_IO.Put_Line( "-- Current.Op2 --" );
             Ada.Text_IO.Put( ASCII.HT );
             Base'Class(T1).Operation;
         end;

         Overriding Procedure Call_Op2 ( T1 : Child ) is
         begin
             Ada.Text_IO.Put_Line( "-- Child.Op2 --" );
             Ada.Text_IO.Put( ASCII.HT );
             Base(T1).Call_Op2;
         end;


         Procedure Operation( T1 : Base       ) is
         begin
             Ada.Text_IO.Put_Line( "-- BASE --" );
         End Operation;

         Overriding Procedure Operation( T1 : Current ) is
         begin
             Ada.Text_IO.Put_Line( "-- CURRENT --" );
         End Operation;


         Overriding Procedure Operation( T1 : Child ) is
         begin
             Ada.Text_IO.Put_Line( "-- CHILD --" );
         End Operation;
     End Example;

The execution of
     declare
         X : Example.Child;
     begin
         X.Call_Op1;
         X.Call_Op2;
         Example.Current(X).Operation;
         Example.Current(X).Call_Op1;
         Example.Current(X).Call_Op2;
     end;
produces:
-- CHILD --
-- Child.Op2 --
	-- CHILD --
-- CURRENT --
-- CHILD --
-- Current.Op2 --
	-- CHILD --