delete on delete ! - Page 2

Sandeep Grover

Hi,

If I do another delete on an object which has been deleted earlier, then
how is
the system expected to behave ?

Is it an unpredictable behavior ??

Thanks
Sandeep

Jul 19 '05

Subscribe Reply

2695

Chris $ Val $

"Ioannis Vranos" <iv*@nothis.ema ils.ru> wrote in message
news:3f******** *************@r ead.news.gr.uu. net...
| "Chris ( Val )" <ch******@bigpo nd.com.au> wrote in message
| news:be******** ****@ID-110726.news.uni-berlin.de...
| >
| >
| > | Yes it is undefined behaviour. It may die, it may continue living and
| die
| > | later, it may truncate data, everything.
| >
| > Again it depends.
| >
| > If you set the pointer to zero straight after you have deleted
| > it, then the behaviour is very defined, and safe.
|
|
| He/she said:
|
| "If I do another delete on an object which has been deleted earlier, then
| how is the system expected to behave ?".

Yes, I know that.

| That means that the pointer will not point to 0.

That is why I said:

*If you set the pointer to zero *straight after* you have deleted it*

Meaning, the first time you delete it, set it to zero, making the
behaviour of deleting it again defined and safe.

Cheers.
Chris Val

Jul 19 '05 #11

Alf P. Steinbach

On Sun, 13 Jul 2003 02:21:37 +1000, "Chris $ Val $" <ch******@bigpo nd.com.au> wrote:

"Ioannis Vranos" <iv*@nothis.ema ils.ru> wrote in message
news:3f******* **************@ read.news.gr.uu .net...
| "Chris ( Val )" <ch******@bigpo nd.com.au> wrote in message
| news:be******** ****@ID-110726.news.uni-berlin.de...
| >
| >
| > | Yes it is undefined behaviour. It may die, it may continue living and
| die
| > | later, it may truncate data, everything.
| >
| > Again it depends.
| >
| > If you set the pointer to zero straight after you have deleted
| > it, then the behaviour is very defined, and safe.
|
|
| He/she said:
|
| "If I do another delete on an object which has been deleted earlier, then
| how is the system expected to behave ?".

Yes, I know that.

| That means that the pointer will not point to 0.

That is why I said:

*If you set the pointer to zero *straight after* you have deleted it*

Meaning, the first time you delete it, set it to zero, making the
behaviour of deleting it again defined and safe.

It's possibly not meaningful to discuss the finer points of language
here, but at least regarding the last few postings there seems to be
confusion about the concept of deleting an object (which cannot be
done without a non-null pointer value) and "deleting a pointer",
probably meaning to use that pointer in a delete expression, which if
it has a technical meaning would be a totally _different_ meaning.

Since the original poster wrote about deleting objects, not pointers,
and furthermore about deleting the same object twice, the only
reasonable interpretation is that what was meant was first deleting
the object, and then performing a delete operation again on the now
invalid pointer value (which for this technically can't be zero).

Setting a pointer to zero after the object it denoted has been
deleted is not an evil practice, though, but don't rely on it to catch
all errors associated with deallocation -- some argue that it might
give a false sense of security + complicates the code + may hide some
errors/bugs, and therefore should be _avoided_ one hundred percent.

Jul 19 '05 #12

Chris $ Val $

"Alf P. Steinbach" <al***@start.no > wrote in message
news:3f******** ********@News.C IS.DFN.DE...
| On Sun, 13 Jul 2003 02:21:37 +1000, "Chris $ Val $" <ch******@bigpo nd.com.au> wrote:
|
| >
| >"Ioannis Vranos" <iv*@nothis.ema ils.ru> wrote in message
| >news:3f******* **************@ read.news.gr.uu .net...
| >| "Chris ( Val )" <ch******@bigpo nd.com.au> wrote in message
| >| news:be******** ****@ID-110726.news.uni-berlin.de...
| >| >
| >| >
| >| > | Yes it is undefined behaviour. It may die, it may continue living and
| >| die
| >| > | later, it may truncate data, everything.
| >| >
| >| > Again it depends.
| >| >
| >| > If you set the pointer to zero straight after you have deleted
| >| > it, then the behaviour is very defined, and safe.
| >|
| >|
| >| He/she said:
| >|
| >| "If I do another delete on an object which has been deleted earlier, then
| >| how is the system expected to behave ?".
| >
| >Yes, I know that.
| >
| >| That means that the pointer will not point to 0.
| >
| >That is why I said:
| >
| >*If you set the pointer to zero *straight after* you have deleted it*
| >
| >Meaning, the first time you delete it, set it to zero, making the
| >behaviour of deleting it again defined and safe.
|
| It's possibly not meaningful to discuss the finer points of language
| here, but at least regarding the last few postings there seems to be
| confusion about the concept of deleting an object (which cannot be
| done without a non-null pointer value) and "deleting a pointer",
| probably meaning to use that pointer in a delete expression, which if
| it has a technical meaning would be a totally _different_ meaning.

Fair enough.

| Since the original poster wrote about deleting objects, not pointers,
| and furthermore about deleting the same object twice, the only
| reasonable interpretation is that what was meant was first deleting
| the object, and then performing a delete operation again on the now
| invalid pointer value (which for this technically can't be zero).

I understand the context in which the OP posted his/her question.

I only provided an suggestion. That is why I used words such as:
'depends', 'if' and 'after'.

| Setting a pointer to zero after the object it denoted has been
| deleted is not an evil practice, though, but don't rely on it to catch
| all errors associated with deallocation -- some argue that it might
| give a false sense of security + complicates the code + may hide some
| errors/bugs, and therefore should be _avoided_ one hundred percent.

I've never heard anyone suggest that before.
Can you give me an example of where such circumstances can arise ?.

Cheers.
Chris Val

Jul 19 '05 #13

Alf P. Steinbach

On Sun, 13 Jul 2003 03:29:50 +1000, "Chris $ Val $" <ch******@bigpo nd.com.au> wrote:

"Alf P. Steinbach" <al***@start.no > wrote in message
news:3f******* *********@News. CIS.DFN.DE...
| Setting a pointer to zero after the object it denoted has been
| deleted is not an evil practice, though, but don't rely on it to catch
| all errors associated with deallocation -- some argue that it might
| give a false sense of security + complicates the code + may hide some
| errors/bugs, and therefore should be _avoided_ one hundred percent.

I've never heard anyone suggest that before.
Can you give me an example of where such circumstances can arise ?.

Not that this is my argument, I think the pros and cons nearly cancel
out, but here is one scenario. A firm or shop or project standardizes
on setting pointers to zero after delete. Such pointers can safely be
passed to functions that check for null, and so they're passed around.
In one function there is an incorrect delete via the passed pointer.
This isn't detected because the pointer is null. Later that function
is used with a more valuable real pointer value. It then incorrectly
deletes an object it shouldn't. The effect only shows up much later
in the code execution, and it's then very hard to track down the cause.

Jul 19 '05 #14

Ioannis Vranos

"Chris ( Val )" <ch******@bigpo nd.com.au> wrote in message
news:be******** ****@ID-110726.news.uni-berlin.de...

I've never heard anyone suggest that before.
Can you give me an example of where such circumstances can arise ?.

There are many philosophical approaches. Many believe -including me- that
the second deletion of an object is the last thing of improper use one will
do on it, before he will have tried to read or write to it.

A better use is always use the standard library containers instead of
allocating and deallocating memory explicitly. So for example, instead of
using an array of pointers, use a vector of objects or whatever container of
the standard library fits your purpose better (e.g. a list).

--
Ioannis

* Programming pages: http://www.noicys.freeurl.com
* Alternative URL 1: http://run.to/noicys
* Alternative URL 2: http://www.noicys.cjb.net

Jul 19 '05 #15

Chris $ Val $

"Ioannis Vranos" <iv*@nothis.ema ils.ru> wrote in message
news:3f******** *************@r ead.news.gr.uu. net...
| "Chris ( Val )" <ch******@bigpo nd.com.au> wrote in message
| news:be******** ****@ID-110726.news.uni-berlin.de...
| >
| >
| > I've never heard anyone suggest that before.
| > Can you give me an example of where such circumstances can arise ?.
|
|
|
| There are many philosophical approaches. Many believe -including me- that
| the second deletion of an object is the last thing of improper use one will
| do on it, before he will have tried to read or write to it.

Point taken.

| A better use is always use the standard library containers instead of
| allocating and deallocating memory explicitly. So for example, instead of
| using an array of pointers, use a vector of objects or whatever container of
| the standard library fits your purpose better (e.g. a list).

Agreed.

Cheers.
Chris Val

Jul 19 '05 #16

Ashish

"Sandeep Grover" <sa*****@magm a-da.com> wrote in message
news:3F******** *******@magma-da.com...

Sandeep Grover wrote:
If I do another delete on an object which has been deleted earlier, then how is the system expected to behave?

Is it an unpredictable behavior ??
It's undefined, so in theory, yes; anything could happen.

S

Thanks !

So, if I have an array of pointers; more than one of the entries could

point to
the same chunk of memory (allocated using new). How do I ensure that I
end up deleting that entry only once. I dont want to use reference-counting kind of thing in constructor.

How can you point two pointers (allocated using new) to the same location???
If you are pointing a pointer to an already allocated piece of memory, you
CANT use the "reference-counting kind of thing in constructor".

The best solution, IMHO, would be to use smart pointers.
--
-Ashish
--------------------------------------------------------------------------
Hi! I'm a shareware signature! Send $5 if you use me, send $10 for manual!
http://www.123ashish.com http://www.softwarefreaks.com

Jul 19 '05 #17

Ashish

"Chris ( Val )" <ch******@bigpo nd.com.au> wrote in message
news:be******** ****@ID-110726.news.uni-berlin.de...

"Ioannis Vranos" <iv*@nothis.ema ils.ru> wrote in message
news:3f******** *************@r ead.news.gr.uu. net...
| "Chris ( Val )" <ch******@bigpo nd.com.au> wrote in message
| news:be******** ****@ID-110726.news.uni-berlin.de...
| >
| >
| > | Yes it is undefined behaviour. It may die, it may continue living and | die
| > | later, it may truncate data, everything.
| >
| > Again it depends.
| >
| > If you set the pointer to zero straight after you have deleted
| > it, then the behaviour is very defined, and safe.
|
|
| He/she said:
|
| "If I do another delete on an object which has been deleted earlier, then | how is the system expected to behave ?".

Yes, I know that.

| That means that the pointer will not point to 0.

That is why I said:

*If you set the pointer to zero *straight after* you have deleted it*
I think you have misunderstood the problem.
The OP's problem is that more than one pointers point to the same location.

Lets assume that p1 and p2 point to location m1.
Delete m1 using p1.
Set p1 to NULL.
So far so good.
Now, delete p2, the behaviour is undefined. And you had no idea that p2 also
points to m1.

Meaning, the first time you delete it, set it to zero, making the
behaviour of deleting it again defined and safe.

Cheers.
Chris Val

--
-Ashish
--------------------------------------------------------------------------
Hi! I'm a shareware signature! Send $5 if you use me, send $10 for manual!
http://www.123ashish.com http://www.softwarefreaks.com

Jul 19 '05 #18

Karl Heinz Buchegger

"Chris ( Val )" wrote:

| I think you have misunderstood the problem.
| The OP's problem is that more than one pointers point to the same location.
|
| Lets assume that p1 and p2 point to location m1.
| Delete m1 using p1.
| Set p1 to NULL.
| So far so good.
| Now, delete p2, the behaviour is undefined. And you had no idea that p2 also
| points to m1.

Yes, that is true.

However, its the programmers job and responsibility to keep
track of such things, and take preventative measures to avoid
this issue all together.

Exactly.

But especially newbies tend to think: If I set the pointer to
0 immediatly after the delete, nothing bad can happen any more.
This (having a second pointer to the same object) is a counter-
example. That's why lots of programmers warn about setting
a pointer to 0: It creates a false sense of security.
Don't get me wrong: setting a pointer to 0 can't do any harm
and is often a good idea. But it doesn't solve all dynamic
memory related problems.

--
Karl Heinz Buchegger
kb******@gascad .at

Jul 19 '05 #19

Chris $ Val $

Jul 19 '05 #20

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