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From: stt@houdini.camb.inmet.com (Tucker Taft)
Subject: Re: Invoking parental methods (was: Java vs Ada 95)
Date: 1996/11/05
Message-ID: <E0EIM2.A3p.0.-s@inmet.camb.inmet.com>#1/1
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references: <E0ED6w.24D.0.-s@inmet.camb.inmet.com>
organization: Intermetrics, Inc.
newsgroups: comp.lang.ada
Date: 1996-11-05T00:00:00+00:00
List-Id: <comp.lang.ada>


Mitch Gart (mg@harp.camb.inmet.com) wrote:

: 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.  

No, the call as written does not dispatch at all, because the
actual parameter is of an access-to-specific, rather than an
access-to-class-wide type.  The above call is a statically bound
call to the p_first of the parent type.

*However* the above call does involve a conversion to a library-level
named access type, "super", which results in a run-time accessibility
check.  The check will succeed unless the original caller passed
in a pointer to a local aliased/heap object.

The call which avoids the accessibility check, and accomplishes the
statically bound call on the parent's operation, is:

    p_first(parent(param.all)'access);

: ... 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.

No, that's not right.  It does call the parent's p_first, but it
also performs an accessibility check, which may not be desired.

: - Mitch Gart

-Tucker Taft   stt@inmet.com   http://www.inmet.com/~stt/
Intermetrics, Inc.  Cambridge, MA  USA