Re: How to avoid?

From:

blargg.h4g@gishpuppy.com (blargg)

Newsgroups:

comp.lang.c++

Date:

Wed, 30 Jul 2008 06:29:34 -0500

Message-ID:

<blargg.h4g-3007080629340001@192.168.1.4>

In article
<86fb018f-43bc-4480-86ec-56ef8745af84@e53g2000hsa.googlegroups.com>, James
Kanze <james.kanze@gmail.com> wrote:

On Jul 30, 11:58 am, blargg <blargg....@gishpuppy.com> wrote:

In article <6MSgk.3157$KI7.1...@newsfe08.iad>,
"Jim Langston" <tazmas...@rocketmail.com> wrote:

A *a = new A;
A *b, *c;
b = a;
c = b;

delete a;
a = NULL;
// At this point what are b and c? NULL? Nope.
// They still point to the original address which
// has already been deleted.

(sorry, a little late of a reply) Actually, b and c have
indeterminate contents after delete a. Doing ANYTHING with
them is undefined, other than assigning a new value. See
3.7.3.2 in the C++03 standard. Even doing "if ( b != NULL )"
is undefined.

...
And there are other things but assigning a new value which are
allowed: taking the address or passing by reference, for
example, or accessing the underlying raw bytes.

OK, I was glossing over a few things when I said that anything is invalid.
I should have noted that. I do believe that 3.7.3.2 indicates that using
the pointer's value is undefined.

int* p = new int;
*p = 0; // OK
delete p; // OK
delete p; // undefined
*p = 0; // undefined
if ( p ) { } // undefined
int* p2 = p; // undefined
int& r = p; // OK
memcpy( &p2, &p, sizeof p2 ); // OK
p = 0; // OK