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From: mg@harp.camb.inmet.com (Mitch Gart)
Subject: Re: Invoking parental methods (was: Java vs Ada 95)
Date: 1996/11/05
Message-ID: <E0ED6w.24D.0.-s@inmet.camb.inmet.com>#1/1
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sender: news@inmet.camb.inmet.com (USENET news)
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references: <1996Oct31.094136.1@eisner>
organization: Intermetrics, Inc.
newsgroups: comp.lang.ada
Date: 1996-11-05T00:00:00+00:00
List-Id: <comp.lang.ada>


Larry Kilgallen (kilgallen@eisner.decus.org) wrote:

: But the dangling reference seems to be precipitated by the
: the pointer object, not by the the pointer type.
: What is wrong with:

:     type super is access all parent_obj; -- this line may get changed

:     procedure p_first(param: access child_obj) is
:     begin
:        ...
:        p_first(super(param));  -- call the parent's p_first
:        ...
:     end p_first;

This is the trap I fell into the first time I tried to write this code.
The above solution doesn't work because the type conversion super(param)
converts param into a pointer to the parent type, but tag inside the
object that is pointed to still says the object is a child_obj, not a 
parent_obj, so the call dispatches to the wrong place.  This line

:        p_first(super(param));  -- call the parent's p_first

in fact doesn't call the parent's p_first, in spite of the comment.  If
the object pointed to by "param" is a child_obj this call is an infinite
resursion.

- Mitch Gart