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From: psrc@pegasus.ATT.COM (Paul S. R. Chisholm)
Newsgroups: comp.lang.c++
Subject: Re: Change to delete is in order
Summary: it's in there already!
Message-ID: <4538@pegasus.ATT.COM>
Date: 19 Mar 90 04:16:11 GMT
References: <5136@odin.SGI.COM> <25FC0210.26173@paris.ics.uci.edu> <260349F1.24972@paris.ics.uci.edu>
Organization: AT&T Bell Laboratories
Lines: 38

<Krasny Oktyabr:  "The torpedo has acquisition!"  That's the good news?>

In article <5263@odin.SGI.COM> shap@delrey.sgi.com (Jonathan Shapiro)
suggests that "delete p" should set the value of p to a null pointer
after the memory it points at is free.  Later posters suggest that the
pointer may not be an lvalue.  I haven't seen anyone suggest a decent
example where a non-lvalue *should* be passed to delete, and I have a
strong gut feeling that such an expression should probably generate a
warning!  But anyway, if it's an lvalue, yes, I agree, implementors
should be free to zero out the pointer.  (Or not, if the application is
*so* time critical that the assignment might slow things down.  We all
know how expensive assignment is as compared to memory deallocation,
right?-)  Leaving the value alone may also help debugging.

In article <260349F1.24972@paris.ics.uci.edu>, rfg@ics.uci.edu (Ronald
Guilmette) agrees, writing:
> It would certainly be nice if the language definition were to say that:
> "In cases where the expression given in a delete statement is an lvalue,
> the value of that lvalue after the execution of the delete statement is
> undefined."

Well, open your ugly red book ("UNIX(R) System V AT&T C++ Language
System, Release 2.0, Product Reference Manual", select code 307-146) to
page 20, and repeat after me:  "if the expression denoting the object
in a delete expression is a modifiable lvalue its value is undefined
after the deletion."

Well, it's good to know Dr. Stroustrup agrees with us. . . .-)

> // Ron Guilmette (rfg@ics.uci.edu)

Paul S. R. Chisholm, AT&T Bell Laboratories
att!pegasus!psrc, psrc@pegasus.att.com, AT&T Mail !psrchisholm
I'm not speaking for the company, I'm just speaking my mind.
UNIX(R) is a registered trademark of AT&T.

P.S.:  How does one have an unmodifiable lvalue?  By using the const
keyword, of course.  I'd like a warning there, too.