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From: "Marin David Condic" <dont.bother.mcondic.auntie.spam@[acm.org>
Newsgroups: comp.lang.ada
Subject: Ambiguous reference - What is wrong with this?
Date: Tue, 14 Aug 2001 11:14:30 -0400
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Date: 2001-08-14T15:14:31+00:00
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O.K. I've got a compilation problem that is most likely due to my
misunderstanding of some language rule. Below is some code that shows a
simplified version of the problem. I'm using Gnat 3.13p on a WinNT PC.

I'm pretty sure that the problem has something to do with the 'Class
parameters creating some kind of ambiguity - but given that I've fully
qualified the names, it seems like the compiler ought to know which
procedure I meant to call. The quest6ion is twofold: Am I doing something
illegal (if so, what?) and if it *is* illegal, then is there an alternate
way of doing essentially the same thing? (One parameter being of a specific
descendent and the other being a class-wide parameter. The class wide
parameter wants to be an increasingly restrictive subset as you move down
the chain of child packages)

package Root is
    type Base is tagged private ;
    procedure Some_Proc (
        Object         : in out Base;
        Object2        : in out Base'Class) ;
private
    type Base is tagged record
        Data_Item_1    : Natural    := 0 ;
    end record ;
end Root ;

package body Root is
    procedure Some_Proc (
        Object         : in out Base;
        Object2        : in out Base'Class) is
    begin
        null ;
    end Some_Proc ;
end Root ;

package Root.Child1 is
    type Derive1 is new Base with private ;
    procedure Some_Proc (
        Object         : in out Derive1;
        Object2        : in out Derive1'Class) ;
private
    type Derive1 is new Base with record
        Data_Item_2    : Natural    := 0 ;
    end record ;
end Root.Child1 ;

package body Root.Child1 is
    procedure Some_Proc (
        Object         : in out Derive1;
        Object2        : in out Derive1'Class) is
    begin
        Root.Some_Proc (
            Object     => Root.Base (Object),
            Object2    => Object2) ;
    end Some_Proc ;
end Root.Child1 ;

package Root.Child1.Child2 is
    type Derive2 is new Derive1 with private ;
    procedure Some_Proc (
        Object         : in out Derive2;
        Object2        : in out Derive2'Class) ;
private
    type Derive2 is new Derive1 with record
        Data_Item_3    : Natural    := 0 ;
    end record ;
end Root.Child1.Child2 ;

package body Root.Child1.Child2 is
    procedure Some_Proc (
        Object         : in out Derive2;
        Object2        : in out Derive2'Class) is
    begin
        --
        --    Dies here with the following error message:
        --
        --    root-child1-child2.adb:15:20: ambiguous expression (cannot
resolve "Some_Proc")
        --    root-child1-child2.adb:15:20: possible interpretation at
root-child1.ads:4
        --    root-child1-child2.adb:15:20: possible interpretation at
root-child1.ads:3
        --    gnatmake: "root-child1-child2.adb" compilation error
        --
        Root.Child1.Some_Proc (
            Object     => Root.Child1.Derive1 (Object),
            Object2    => Object2) ;
    end Some_Proc ;
end Root.Child1.Child2 ;

Thanks for any help you can offer.

MDC
--
Marin David Condic
Senior Software Engineer
Pace Micro Technology Americas    www.pacemicro.com
Enabling the digital revolution
e-Mail:    marin.condic@pacemicro.com
Web:      http://www.mcondic.com/