From: stt@henning.camb.inmet.com (Tucker Taft)
Subject: Re: type casting tagged types in the wrong direction
Date: 1996/03/28
Date: 1996-03-28T00:00:00+00:00 [thread overview]
Message-ID: <DozKGv.CwJ.0.-s@inmet.camb.inmet.com> (raw)
In-Reply-To: 3159B860.28A6@csehp3.mdc.com
James A. Squire (m193884@CSEHP3.MDC.COM) wrote:
: I have a question:
: Say I have something like:
: type A is tagged
: record
: X : ABC;
: Y : DEF;
: end record;
: type B is new A with
: record
: W : GHI;
: end record;
: type C is new B with
: record
: Z : JKL;
: end record;
: OK. Now, let's say that I forgot which type extends off of which, and I
: write a statement like this:
: Jim : C;
: Jane : B;
: Save : JKL;
: begin
: ...
: Jim := C (Jane);
This is illegal, by RM95 4.6(21-23).
: Save := Jim.Z;
: ...
: end;
: GNAT 3.03 compiles this statement. Is that really right?
No. GNAT should have complained at compile time about the conversion
from B to C. You may convert from B'Class to C (and then a run-time
check is performed), or you may convert from C to B, but a direct
conversion from B to C is illegal.
: ... Is Jim.Z defined?
Jim.Z is "defined," but the conversion on the previous line
is illegal, so who knows what code GNAT is generating for the
assignment to Jim.
: According to the LRM 4.6, the above is called a "view conversion". This
: raises the question, how do you take a type C view of a type B entity
: when type C looks for additional information?
You can't.
: Should a validated Ada95 compiler flag this usage? I realize that GNAT
: is not validated.
GNAT is validated, but validation doesn't guarantee bug-free
(nor does anything else I know of ;-).
This looks like a bug in GNAT, pure and simple. (It also *suggests*
that another ACVC test might be in order ;-).
: I would appreciate any expert or otherwise opinions that any of you have
: to offer.
: Thank you.
: --
: James Squire
: MDA Avionics Tools & Processes
: ja_squire@csehp3.mdc.com
: "one of these days I'm going to better myself by going to Knight school"
-Tucker Taft stt@inmet.com http://www.inmet.com/~stt/
Intermetrics, Inc. Cambridge, MA USA
next prev parent reply other threads:[~1996-03-28 0:00 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
1996-03-27 0:00 type casting tagged types in the wrong direction James A. Squire
1996-03-28 0:00 ` Norman H. Cohen
1996-03-28 0:00 ` Robert Dewar
1996-03-28 0:00 ` Tucker Taft [this message]
1996-03-28 0:00 ` Robert Dewar
1996-03-30 0:00 ` Fergus Henderson
1996-03-31 0:00 ` Robert Dewar
1996-03-29 0:00 ` type casting tagged types in the wrong Scott Moody
[not found] <md5:5AA0EAFF641BB9F461162E7F13E1101F>
1996-03-29 0:00 ` type casting tagged types in the wrong direction James A. Squire
1996-03-29 0:00 ` Robert Dewar
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