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From: Kevin Ingalls <kevin.l.ingalls@boeing.com>
Subject: Dispatching In Generics
Date: 1997/06/05
Message-ID: <3396EB36.3D14@boeing.com>#1/1
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Sender: nntp@bcstec.ca.boeing.com (NNTP News Access)
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Organization: The Boeing Company
Newsgroups: comp.lang.ada
Date: 1997-06-05T00:00:00+00:00
List-Id: <comp.lang.ada>


First, I can't find the Ada FAQ, so perhaps the answer to my question is
there. If you know where it is I would appreciate the address.

Second, here is my coding question: How do I code a generic so that it
can take the base type of a hierarchy and perform dispatching on objects
of that type?

Specifically, given:
	package A with an abstract tagged type X
	a primitive operation "operation" on type X
	child package A.B with a derived type Y
	a primitive operation "operation" on type Y
How do I write a generic that can handle objects of X'class, including
dispatching operations correctly?

Here is the skeleton of a first try:
	
----------------------------
with ada.finalization;
package A is
	type X is abstract tagged private;
	procedure operation (anX : X);
private
	type X is abstract new ada.finalization.controlled with...
end A;

package A.B is
	type Y is new X with private;
	procedure operation (aY : Y);
private
	type Y is new X with record...
end A.B;

generic
	type T is abstract tagged private;
	with procedure op (aT : T);
package gPackage is
	procedure P;
end gPackage;

package body gPackage is
	procedure P is
	begin
		op (object_of_type_T); -- here is the syntax error
	end P;
end gPackage;

with A, gPackage;
package instance is new gPackage(A.X, A.operation);
----------------------------

The syntax error occurs while compiling the generic on the marked line.
I understand that I have no subprograms with class-wide parameters, and
so "op" cannot do dispatching (which is what the error message tells
me). The question is, what is the correct way to accomplish this?

As a kludge, I added a class-wide operation to package A for type X:
----
	procedure operation_class_wide (aClassWideX : X'class);
----
	procedure operation_class_wide (aClassWideX : X'class) is
	begin
		operation(aClassWideX);
	end operation_class_wide;

And changed the instantiation to:

package instance is new gPackage (A.X, A.operation_class_wide);

But this is hardly a clean solution. So, the question is how can I write
a generic package to handle a class hierarchy, including
dispatching-like behavior, cleanly?

-- 

Kevin Ingalls, (206)773-9404, kevin.l.ingalls@boeing.com