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From: Kevin Ingalls <kevin.l.ingalls@boeing.com>
Subject: Dispatching in Generics
Date: 1997/09/11
Message-ID: <34183CD3.7F4@boeing.com>#1/1
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Organization: Boeing
Reply-To: kevin.l.ingalls@boeing.com
Newsgroups: comp.lang.ada
Date: 1997-09-11T00:00:00+00:00
List-Id: <comp.lang.ada>


Hello,
I am trying to write a generic queue package in Ada95 will allow me to
queue up objects that are of a type hierarchy. For example, I might have
a parent tagged type A that has a derived type B and C. I want to be
able to put an object of either type A, B, or C onto a queue. So, I use
tagged types, hierarchical packages, generics, and classwide access
types. Almost everything works. What doesn't work is dispatching with
generics.
Let's say that in this A,B,C hierarchy I have a primative operation
"put". I want to be able to walk through my queue and "put" each
element, and, of course, I want the correct "put" to be called (i.e., I
want dispatching). How is this done? Here is a small example of the
problem (the error message is from ObjectAda 7.1)

------------------Code Fragment-----------------------
generic
    type T is tagged private;
    with procedure put (T1 : T);
package G is
    procedure cant_dispatch(TC : T'class);
end G;

package body G is
    procedure cant_dispatch(TC : T'class) is
    begin
        put (TC);	-- the error occurs here
    end cant_dispatch;
end G;
-----------------Error Message-------------------------    
cant_dispatch.ada: Error: line 11 col 14 LRM:3.9.2(9), If the expected
type of a name or expression is a tagged type, then it shall not be
dynamically tagged unless it is a controlling operand in a call on a
dispatching operation, Continuing
-----------------End of Error Message------------------

I understand why this doesn't work, but I don't know how to get around
it. The only work-around that I have found so far is to add a classwide
operation to the A,B,C hierarchy. The classwide operation simply does a
dispatching call to the primative operation. I then change the generic
formal parameter to take the classwide operation, instead of the
primitive operation. However, this violates the spirit of generics in
that it requires the generic actual type to know that a classwide
operation is needed for the generic to work correctly. That is, my goal
was to write a generic queue that could take objects of *any* hierarchy,
not only those that have a classwide operation bolted on.

My second question is why is this so difficult to do? Maybe when the
answer is pointed out to me, I'll take that back :-), but this seems
like a fairly straight-forward use to the language. I'm hardly a
proponant of C++, but I wrote the C++ version of this problem in about
1/2 an hour. What important software engineering concept is being
violated in what I am trying to do?

Thanks for any insight you might have.

-- 
Kevin Ingalls
kevin.l.ingalls@boeing.com
(253)773-9404