Re: how do i implement double-dispatching?

["Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>]

cl1motorsports wrote:

> On Mon, 22 Dec 2003 16:41:52 -0500, cl1motorsports wrote:
> 
>>> thanks for all of your suggestions. after careful consideration i
>>> decided to change the above Accept_Visitor procedure to:
>>> 
>>> procedure Accept_Vistor(V:in out Visitor_Record'Class; Node : in
>>> Parse_Tree_Node_Record'Class) is begin
>>>   if Node in Equal_Op'Class then
>>> Visit(V, Equal_Op(Node));
>>>   elsif Node in Number_Node'Class then
>>> Visit(V, Number_Node(Node));
>>>   ...
>>>   end if;
>>> end Accept_Visitor;
>>> 
>>> now it only dynamically dispatches on the Visitor_Record'Class and the
>>> Node parameter is determined at compile time. I don't think i would have
>>> figured that out by myself (at least not at this point in time).
>> 
>> Okay. now it works when the Visitor type, Visit() procedures and the
>> Accept_Visitor() procedure is in a separate package from the parse tree.
>> but when i try to combine the 2 into one package i get
>> an error about multiple dispatching on all of the Visit() procedures.
>> what is my scope issue with being in the same package that doesn't apply
>> to them being in 2 packages?

This is a strength of Ada. The compiler indicates a problem in your design.
Visit appears to be multiple dispatching but it is not according to your
design. You should declare it as follows:

procedure Visit
          (  V    : in out Visitor_Record;
             Node : Equal_Op'Class
          )  is abstract;
procedure Visit
          (  V    : in out Visitor_Record;
             Node : Number_Node'Class
          )  is abstract;

Now Visit dispatches in V, but does not in Node, for you do the latter
manually in Accept_Visitor.

-- 
Regards,
Dmitry A. Kazakov
www.dmitry-kazakov.de