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Set a pointer to null when deleting?

When you delete a pointer, you should set it to NULL, right?

Joe
Apr 10 '06
51 10590

peter koch wrote:
Noah Roberts skrev:
peter koch wrote:
Noah Roberts skrev: > And there it is. There is no reason NOT to beyond linespace fetishes.

There is one very good reason: to verify your program logic. Accessing
a resource that is no longer present is an obvious programming error,
and setting the pointer to 0 might just prevent that situation from
being identified.
Not true.

Sure? Examples given in this thread would specifically behave that way.
Far to often you write if (p != 0) p->action() instead of assert(p !=
0); p->action();


How are you applying logic here?

1) what example of pointer access would work if ptr is 0 and not
otherwise?
2) what is the relevance of the assert statement you make above?
If you are going to set the pointer to anything, it is a far better
idea to set it to some illegal value the same way as operator delete
sets memory to an often illegal pattern in some debug builds.


This idea is very non-portable for one and doesn't follow standard
practice. Talk about being difficult to understand...

There is absolutely no reason to go looking for some other value to set
a pointer to. All that does is introduce problems in both
understandabili ty, platform dependance, and undefined behavior.

That is a small problem as this is only something to do while
debugging.


Yes, it is good to introduce hard to read, platform dependant,
undefined behavior during debugging and then replace it with different
code on release.

The arguments against setting a pointer to 0 after deletion just keep
getting dumber and dumber...I don't really find that surprising.

Apr 11 '06 #41
Noah Roberts wrote:
The arguments against setting a pointer to 0 after deletion just keep
getting dumber and dumber...I don't really find that surprising.


Smart code...

- uses smart pointers
- uses references
- use objects with fail-safe behaviors
- reduce 'if' statements on types*
- practices RAII
- generally only delete pointers about to go out of scope
- assign NULL to the tiny fraction of remaining pointers

*If I said "NULL is a type" here, I would get an entire thread of yacking
"NULL is a macro typically defined as 0 which is an integer,
blah-blah-blah". The book Design Patterns calls this topic "program to the
interface". Checking a pointer for NULL is a sign the design could be
generally better. It's using the interface to a pointer instead of an
interface to an object.

--
Phlip
http://www.greencheese.org/ZeekLand <-- NOT a blog!!!
Apr 11 '06 #42
Joe Van Dyk wrote:
Victor Bazarov wrote:
class B {
public:
void foo(A* aptr);

private:
A* aptr_;
};

void B::foo(A* aptr)
{
aptr_ = aptr;
}

B::~B()
{
delete aptr_;
}


OK, this is the case where setting aptr_ to NULL won't have any effect
because B (hence aptr_) is itself being destroyed. (Assuming that it
isn't B that is getting destroyed twice.)

The bug is that B::foo requires an A object that it can own, and delete,
and doesn't have a name/comment/document making that obvious to you.

No doubt you have passed the same A to multiple calls of B::foo, or
kept the A pointer and eventually deleted it, which would be the right
thing to do if B::foo did not have this undocumented requirement.
Apr 12 '06 #43
Robert Mabee wrote:
The bug is that B::foo requires an A object that it can own, and delete,
and doesn't have a name/comment/document making that obvious to you.


It would be nice we could indeed make that problem a "bug". std::auto_ptr
can only get as far as making its constructor explicit, which is as much a
nuisance as a compilable comment. ;-)

Nice thread busting on the evils of RAII, guys...

--
Phlip
http://www.greencheese.org/ZeekLand <-- NOT a blog!!!
Apr 12 '06 #44
Victor Bazarov wrote:
Phlip wrote:
If you must, use std::auto_ptr or boost::shared_p tr, and reset(NULL)
that (if I recall correctly).


'boost' is non-standard. 'std::auto_ptr' is not suitable for some
cases due to its particular copy semantics.


No reason not to use boost. Besides, boost smart pointers have been accepted
into TR1, so they are standard and will be supported by newer compilers.

--
Paul M. Dubuc
Apr 12 '06 #45
Paul M. Dubuc wrote:
No reason not to use boost.


Also no reason not to suggest or (lightly) discuss Boost here.

--
Phlip
http://www.greencheese.org/ZeekLand <-- NOT a blog!!!
Apr 12 '06 #46

Paul M. Dubuc wrote:
Victor Bazarov wrote:
Phlip wrote:
If you must, use std::auto_ptr or boost::shared_p tr, and reset(NULL)
that (if I recall correctly).


'boost' is non-standard. 'std::auto_ptr' is not suitable for some
cases due to its particular copy semantics.


No reason not to use boost. Besides, boost smart pointers have been accepted
into TR1, so they are standard and will be supported by newer compilers.


wrong.

"This technical report is non-normative. Some of the library components
in this technical report may be considered for standardization in a
future version of C++, but they are not currently part of any C++
standard. Some of the components in this technical report may never be
standardized, and others may be standardized in a substantially changed
form."

Apr 12 '06 #47
Phlip wrote:
Paul M. Dubuc wrote:
No reason not to use boost.


Also no reason not to suggest or (lightly) discuss Boost here.


Do you apply that to all non-standard, third-pary libraries?

Brian
Apr 12 '06 #48
Default User wrote:
Also no reason not to suggest or (lightly) discuss Boost here.


Do you apply that to all non-standard, third-pary libraries?


Specific questions about some library X belong on X's forum.

The general question "what [portable] C++ library does X" belong in
the best platform-neutral C++ forum possible.

--
Phlip
http://www.greencheese.org/ZeekLand <-- NOT a blog!!!
Apr 12 '06 #49
Default User wrote:
Phlip wrote:
Paul M. Dubuc wrote:
> No reason not to use boost.


Also no reason not to suggest or (lightly) discuss Boost here.


Do you apply that to all non-standard, third-pary libraries?


Topicality, as per our FAQ, includes planned extensions to the standard. Can
you get any closer than tr1? Even though not everything in boost is up for
standardization , boost happens to be the default route of an idea into the
standard. I consider that topical.
Best

Kai-Uwe Bux

Apr 13 '06 #50

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