Factory Pattern

[shaunpatterson@gmail.com]

I'm just starting out learning Ada and I'm having some trouble
converting some C++ code.

The last hurtle I'm having trouble with is implementing a Factory in
Ada

I basically have a base class Message and all other messages are
derived from this.
I have a function call createMessage that I would like to return the
appropriate
subclass type.


My code looks something like this:

type Message is abstract tagged;
type Message_Class is access all Message'Class;

derived type:

package MessageInteger_Pkg is

-- I want this to act like a constructor
-- Just fills in the value
function create (i : Integer) return MessageInteger;

private
type MessageInteger with new Message with
record
      value : Integer;
end record;


end MessageInteger_Pkg;

-- create looks like this:
function create (i : Integer) return MessageInteger is
     msg : MessageInteger;
begin
     msg.value := i;
     return msg;
end create;




Factory method:

-- I would like it to work like this...
function createMessage (type : Integer) return Message_Class is
begin
         case type is
                 when 0 =>
                      return MessageInteger_Pkg.create (1234);
                 when others =>
                       return MessageSomethingElse_Pkg.create
("blah");
         end case

end createMessage


However - it doesn't like me returning the MessageInteger from a
class
that returns a Message_Class (obviously..)

Now if I do this -- changing the constructor to take in a
Message_Class....


function createMessage (type : Integer) return Message_Class is
      decodedMessage : Message_Class;
begin
         case type is
                 when 0 =>
                      decodedMessage := new
MessageInteger_Pkg.MessageInteger;
                       MessageInteger_Pkg.create (decodedMessage,
1234);        -- change the decoded message this way
                       return decodedMessage;
                 when others =>
                       return MessageSomethingElse_Pkg.create
("blah");
         end case

end createMessage


-- That method works... but I find the intermediary step of making a
new object decodedMessage
to someone unneeded?  There must be another way of doing this.
Perhaps I should move the "new"
to the create function?

And... I'm not completely sure I'm doing all this correctly in Ada.
LIke I said,  I'm very new to
Ada.

Thanks

--
Shaun