Asp Forum - NULL pointer in overloaded operator delete []

Rahul

11/13/2008 12:28:00 PM

Please read the following code

class Test{
public:
void * operator new [] (size_t t)
{ return malloc(t); }

void operator delete [] (void *p)
{ free(p); }
};

void main () {

Test *p= 0;

delete [] p;

/* What should happen here, Should the call go inside Test::operator
delete []. Because what I learned from books is that deleting a NULL
pointer is safe (calling ::operator delete[] on a NULL pointer does
not crash). But here it causes a crash on Sun CC because it gets
inside Test::operator delete [] whereas on VC++ and g++ it doesn't.

Should I put a check in Test::operator delete [] for "p!
=NULL"? Is Sun CC behaving as per the C++ standard?
*/
}

Thanks in advance

8 Answers

Victor Bazarov

11/13/2008 1:51:00 PM

Rahul wrote:
> Please read the following code
>
> class Test{
> public:
> void * operator new [] (size_t t)
> { return malloc(t); }
>
> void operator delete [] (void *p)
> { free(p); }
> };
>
> void main () {
>
> Test *p= 0;
>
> delete [] p;
>
> /* What should happen here, Should the call go inside Test::operator
> delete []. Because what I learned from books is that deleting a NULL
> pointer is safe (calling ::operator delete[] on a NULL pointer does
> not crash). But here it causes a crash on Sun CC because it gets
> inside Test::operator delete [] whereas on VC++ and g++ it doesn't.
>
> Should I put a check in Test::operator delete [] for "p!
> =NULL"? Is Sun CC behaving as per the C++ standard?

The behaviour of your program is undefined. 'main' must return the type
'int':

int main() {

Seriously, though, everything should be OK because 'free' is explicitly
specified as a NOP if its argument is a null pointer.

You need to investigate further why on Sun it doesn't go into the
overloaded operator delete[]. And even if you don't overload, the
deletion (using 'delete' or 'delete[]') is explicitly defined as OK if
the argument is a null pointer.

> */
> }
>
>
> Thanks in advance

V
--
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Andrey Tarasevich

11/13/2008 3:14:00 PM

Rahul wrote:
> Please read the following code
>
> class Test{
> public:
> void * operator new [] (size_t t)
> { return malloc(t); }
>
> void operator delete [] (void *p)
> { free(p); }
> };
>
> void main () {
>
> Test *p= 0;
>
> delete [] p;
>
> /* What should happen here, Should the call go inside Test::operator
> delete []. Because what I learned from books is that deleting a NULL
> pointer is safe (calling ::operator delete[] on a NULL pointer does
> not crash). But here it causes a crash on Sun CC because it gets
> inside Test::operator delete [] whereas on VC++ and g++ it doesn't.
>
> Should I put a check in Test::operator delete [] for "p!
> =NULL"? Is Sun CC behaving as per the C++ standard?
> */
> }

Unfortunately, the language specification is (was?) not sufficiently
clear on whether the control should go into the overloaded 'operator
delete' when the delete-expression is invoked on the null-pointer of
corresponding type, even though the standard does say that
delete-expression on null-pointer is a no-op. Apparently Sun compiler
thinks that it should be called.

Meanwhile, I don't understand why it crashes on Sun, even if it gets
into the above 'operator delete[]'. The standard 'free' function is also
a no-op on a null-pointer argument. If it crashes, that would mean that
there's a bug in Sun's 'free' implementation.

BTW, how do you know that VC and g++ don't call it? You really checked
it or you just assumed it because it didn't crash?

--
Best regards,
Andrey Tarasevich

Rahul

11/13/2008 3:19:00 PM

On Nov 13, 8:13 pm, Andrey Tarasevich <andreytarasev...@hotmail.com>
wrote:
> Rahul wrote:
> > Please read the following code
>
> > class Test{
> > public:
> > void * operator new [] (size_t t)
> > { return malloc(t); }
>
> > void operator delete [] (void *p)
> > { free(p); }
> > };
>
> > void main () {
>
> > Test *p= 0;
>
> > delete [] p;
>
> > /* What should happen here, Should the call go inside Test::operator
> > delete []. Because what I learned from books is that deleting a NULL
> > pointer is safe (calling ::operator delete[] on a NULL pointer does
> > not crash). But here it causes a crash on Sun CC because it gets
> > inside Test::operator delete [] whereas on VC++ and g++ it doesn't.
>
> > Should I put a check in Test::operator delete [] for "p!
> > =NULL"? Is Sun CC behaving as per the C++ standard?
> > */
> > }
>
> Unfortunately, the language specification is (was?) not sufficiently
> clear on whether the control should go into the overloaded 'operator
> delete' when the delete-expression is invoked on the null-pointer of
> corresponding type, even though the standard does say that
> delete-expression on null-pointer is a no-op. Apparently Sun compiler
> thinks that it should be called.
>
> Meanwhile, I don't understand why it crashes on Sun, even if it gets
> into the above 'operator delete[]'. The standard 'free' function is also
> a no-op on a null-pointer argument. If it crashes, that would mean that
> there's a bug in Sun's 'free' implementation.
>
> BTW, how do you know that VC and g++ don't call it? You really checked
> it or you just assumed it because it didn't crash?
>
> --
> Best regards,
> Andrey Tarasevich- Hide quoted text -
>
> - Show quoted text -

Hi,

Sorry for my ignorance. But that is just a dummy code which I wrote
quickly, so actually its "int main" only
and the operator delete [] does not call free(), It manages a double
linked list internally and de-references the pointer p before actually
freeing the memory. Which causes a crash.

So I wanted to know if I should put a platform specific NULL check in
operator delete [] OR should make it general. Doing that would
penalize other platforms un-necessarily, of-course the cost is not
much considering the whole application. But my point is, why to
penalize other platforms if they are behaving as per the standard (If
at all they are ).”

blargg.h4g

11/13/2008 4:32:00 PM

Rahul wrote:

> > Rahul wrote:
> > > class Test{
> > > public:
> > > void * operator new [] (size_t t)
> > > { return malloc(t); }
> >
> > > void operator delete [] (void *p)
> > > { free(p); }
> > > };
> >
> > > void main () {
> >
> > > Test *p= 0;
> >
> > > delete [] p;
> >
> > > /* What should happen here, Should the call go inside Test::operator
> > > delete []. Because what I learned from books is that deleting a NULL
> > > pointer is safe (calling ::operator delete[] on a NULL pointer does
> > > not crash). But here it causes a crash on Sun CC because it gets
> > > inside Test::operator delete [] whereas on VC++ and g++ it doesn't.
> >
> > > Should I put a check in Test::operator delete [] for "p=
> !
> > > =NULL"? Is Sun CC behaving as per the C++ standard?
> > > */
> > > }
[...]
> Sorry for my ignorance. But that is just a dummy code which I wrote
> quickly, so actually its "int main" only
> and the operator delete [] does not call free(), It manages a double
> linked list internally and de-references the pointer p before actually
> freeing the memory. Which causes a crash.

Which means your code may be at fault.

> So I wanted to know if I should put a platform specific NULL check in
> operator delete [] OR should make it general. Doing that would
> penalize other platforms un-necessarily, of-course the cost is not
> much considering the whole application. But my point is, why to
> penalize other platforms if they are behaving as per the standard (If
> at all they are ).

You should first write a test program to see what is really happening on
your platform. First, see if free( 0 ) is crashing:

int main() { free( 0 ); }

then see if NULL is being passed to your operator delete []:

class Test {
public:
void* operator new [] ( size_t s ) throw() { return malloc( s ); }
void operator delete [] ( void* p ) throw() { assert( p ); free( p ); }
};

int main()
{
Test* p = 0;
delete [] p;
}

Once you know what's going on, you can decide how to handle your
compiler's non-conformance (assuming there is any).

Andrey Tarasevich

11/13/2008 5:15:00 PM

Rahul wrote:
>
> So I wanted to know if I should put a platform specific NULL check in
> operator delete [] OR should make it general. Doing that would
> penalize other platforms un-necessarily, of-course the cost is not
> much considering the whole application. But my point is, why to
> penalize other platforms if they are behaving as per the standard (If
> at all they are ).?

I would recommend you to pot a _general_ null-pointer check into your
overloaded 'operator delete[]'. The reason for this is that technically
'operator delete[]' is by itself a standalone self-sufficient
resource-deallocation function. It can actually be called explicitly by
the user, should the need arise. I'd say that there's a de-facto
standard (or at least a widespread and accepted practice) to write all
resource deallocation functions so that they permit a null argument and
act as a no-op in that case.

Of course, I understand perfectly well that overloaded 'operator delete'
is not normally intended to be used as a self-sufficient function. But
nevertheless it feels like a good idea to preserve the null->no-op
guarantee for this function as well. (Standard library-provided versions
do that, BTW.) The overhead of doing the null check is less than
negligible, so I'd remove the issue of "penalizing the other platforms"
from consideration entirely.

--
Best regards,
Andrey Tarasevich

James Kanze

11/13/2008 8:54:00 PM

On Nov 13, 2:50 pm, Victor Bazarov <v.Abaza...@comAcast.net> wrote:
> Rahul wrote:
> > Please read the following code

> > class Test{
> > public:
> > void * operator new [] (size_t t)
> > { return malloc(t); }
>
> > void operator delete [] (void *p)
> > { free(p); }
> > };

> > void main () {

> > Test *p= 0;

> > delete [] p;

> > /* What should happen here, Should the call go inside Test::operator
> > delete []. Because what I learned from books is that deleting a NULL
> > pointer is safe (calling ::operator delete[] on a NULL pointer does
> > not crash). But here it causes a crash on Sun CC because it gets
> > inside Test::operator delete [] whereas on VC++ and g++ it doesn't.

> > Should I put a check in Test::operator delete [] for "p!
> > =NULL"? Is Sun CC behaving as per the C++ standard?

> The behaviour of your program is undefined. 'main' must
> return the type 'int':

Not undefined. It's an error which requires a diagnosis.

Of course, that is completely orthogonal to his question.

> int main() {

> Seriously, though, everything should be OK because 'free' is
> explicitly specified as a NOP if its argument is a null
> pointer.

> You need to investigate further why on Sun it doesn't go into
> the overloaded operator delete[]. And even if you don't
> overload, the deletion (using 'delete' or 'delete[]') is
> explicitly defined as OK if the argument is a null pointer.

It's still the responisiblity of operator delete (or delete[])
to check; the standard doesn't guarantee that it won't be given
a null pointer; the standard requires that it be a no-op if
given a null pointer.

But of course, all he's doing is calling free() with the pointer,
and free() is guaranteed to be a no-op when given a null
pointer, so his implementation meets the requirements.

--
James Kanze (GABI Software) email:james.kanze@gmail.com
Conseils en informatique orientée objet/
Beratung in objektorientierter Datenverarbeitung
9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34

James Kanze

11/13/2008 9:07:00 PM

On Nov 13, 4:13 pm, Andrey Tarasevich <andreytarasev...@hotmail.com>
wrote:
> Rahul wrote:
> > Please read the following code

> > class Test{
> > public:
> > void * operator new [] (size_t t)
> > { return malloc(t); }
>
> > void operator delete [] (void *p)
> > { free(p); }
> > };

> > void main () {

> > Test *p= 0;

> > delete [] p;

> > /* What should happen here, Should the call go inside Test::operator
> > delete []. Because what I learned from books is that deleting a NULL
> > pointer is safe (calling ::operator delete[] on a NULL pointer does
> > not crash). But here it causes a crash on Sun CC because it gets
> > inside Test::operator delete [] whereas on VC++ and g++ it doesn't.

> > Should I put a check in Test::operator delete [] for "p!
> > =NULL"? Is Sun CC behaving as per the C++ standard?
> > */
> > }

> Unfortunately, the language specification is (was?) not
> sufficiently clear on whether the control should go into the
> overloaded 'operator delete' when the delete-expression is
> invoked on the null-pointer of corresponding type, even though
> the standard does say that delete-expression on null-pointer
> is a no-op. Apparently Sun compiler thinks that it should be
> called.

Or that the implementation is allowed to call it. According to
the latest draft, "The value of the first argument supplied to a
deallocation function may be a null pointer value; if so, and if
the deallocation function is one supplied in the standard
library, the call has no effect." I'm not quite sure what the
implications of that "is one supplied in the standard library"
are meant to be---taken literally, since his function is not one
provided by the standard library, the sentence wouldn't seem to
apply. But somehow, that doesn't make sense.

It may be worth raising the issue with the committee.

--
James Kanze (GABI Software) email:james.kanze@gmail.com
Conseils en informatique orientée objet/
Beratung in objektorientierter Datenverarbeitung
9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34

Andrey Tarasevich

11/13/2008 9:19:00 PM

James Kanze wrote:
>
> It may be worth raising the issue with the committee.
>

It has been raised already

http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defect...

--
Best regards,
Andrey Tarasevich

comp.lang.c++

NULL pointer in overloaded operator delete []

Rahul

Victor Bazarov

Andrey Tarasevich

Rahul

blargg.h4g

Andrey Tarasevich

James Kanze

James Kanze

Andrey Tarasevich

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