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From: Hyman Rosen <hymie@prolifics.com>
Subject: Re: Ada OO Mechanism
Date: 1999/05/27
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Samuel Mize <smize@imagin.net> writes:
> I hope you find this of some interest.

I'm trying to wrap my head around this class-wide type stuff.
I'm still at that stage of pre-clarity confusion :-)

Here's some code I threw at gnat, based on Richard D. Riehle's example.

-- in ft.ads
package ft is
	m: boolean;
	type S is null record;
	a_f: S;
	type T is abstract tagged null record;
	function Get(F: S) return T is abstract;
	function GetC(F: S) return T'Class;
	type A is new T with null record;
	function Get(F: S) return A;
	a_a: A;
	type B is new T with null record;
	function Get(F: S) return B;
	a_b: B;
end;

-- in ft.adb
package body ft is
	function GetC(F: S) return T'Class is
	begin if m then return a_a; else return a_b; end if; end;
	z: T'Class := GetC(a_f);
	function Get(F: S) return A is begin return a_a; end;
	function Get(F: S) return B is begin return a_b; end;
begin
	z := Get(a_f);
end;

Gnat complains about the assignment to z:
ft.adb:8:14: ambiguous expression (cannot resolve "Get")
ft.adb:8:14: possible interpretation at ft.ads:12
ft.adb:8:14: possible interpretation at ft.ads:9
ft.adb:8:14: possible interpretation at ft.ads:6

>From reading the Rationale, I thought that the assignment to z would
result in a dispatching call to Get, using the tag of z that was set
at its initialization. What am I doing wrong here? How do controlling
return types work?