C / C++ / MFC Discussion Boards

Re: delete in ctor !

Roger Stoltz2-Jun-06 5:42

Roger Stoltz

2-Jun-06 5:42

Chris Losinger wrote:
as i noted in my first reply

Sorry Chris, didn't pay enough attention.

Chris Losinger wrote:
looking over the web for someone who can definitively show that it (as written in the OP) isn't safe - haven't found one yet

I think you can stop looking.
IMO it's quite clear what happens, the same way you described it previously.
Whether it's "safe" or not depends on how we define "safe"; It will run and the behaviour is predictable and in that aspect it's considered safe. But it has to be used with certain restrictions, otherwise it's not safe.

Better safe than sorry...;)
--
Roger

It's supposed to be hard, otherwise anybody could do it!

Regarding CodeProject: "resistance is pointless; you will be assimilated"

Re: delete in ctor !

C^hris L^osinger2-Jun-06 5:48

C^hris L^osinger

2-Jun-06 5:48

Roger Stoltz wrote:
Sorry Chris, didn't pay enough attention

and i probably wasn't as clear about what i was saying as i should have been. Beer | [beer]

Roger Stoltz wrote:
But it has to be used with certain restrictions, otherwise it's not safe.

if i was in charge of writing coding standards for my company (oh wait, i am!), those restrictions would be simple: never do this. Smile | :)

Cleek | Image Toolkits | Thumbnail maker

Re: delete in ctor !

Roger Stoltz2-Jun-06 6:16

Roger Stoltz

2-Jun-06 6:16

Chris Losinger wrote:
never do this

Ahh, the good old KISS! Laugh | :laugh:

Well, I'm off now; have a nice weekend.

Beer | [beer]

It's supposed to be hard, otherwise anybody could do it!

Regarding CodeProject: "resistance is pointless; you will be assimilated"

Re: delete in ctor !

Nemanja Trifunovic2-Jun-06 5:05

Nemanja Trifunovic

2-Jun-06 5:05

Chris Losinger wrote:
of course it will. step through the code in a debugger and see what happens.

Hmmm, works with g++ under Linux as well.

However, I am not sure what would other compilers do.

My programming blahblahblah blog. If you ever find anything useful here, please let me know to remove it.

Re: delete in ctor !

BadKarma2-Jun-06 3:15

BadKarma

2-Jun-06 3:15

Hi,

in the specific case presented there 'should' be no problem.

BUT, I don't think it safe to do this when using virtual destructors.
Because the this pointer isn't fully valid for all dervided class
functionality like the derived destructor, which could be triggered
when calling delete this;

See this article[^] for more info. Smile | :)

codito ergo sum

Re: delete in ctor !

toxcct2-Jun-06 3:35

toxcct

2-Jun-06 3:35

BadKarma wrote:
BUT, I don't think it safe to do this when using virtual destructors

have you noticed that he does delete this in the Constructor ?

TOXCCT >>> GEII power

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Re: delete in ctor !

Zac Howland2-Jun-06 3:47

Zac Howland

2-Jun-06 3:47

toxcct wrote:
have you noticed that he does delete this in the Constructor ?

If the class used inheritence, it is possible to get to a point where all the constructors in the inheritence chain would not have been called yet. Thus, calling the virtual destructors would lead to possible memory issues. That is what he was getting at.

As a side note, the code written in the original post should NEVER be used in any real code. Use a constructor for what it is meant for; the same goes for destructors. Making these functions do things contrary to general practice serves no purpose.

If you decide to become a software engineer, you are signing up to have a 1/2" piece of silicon tell you exactly how stupid you really are for 8 hours a day, 5 days a week

Zac

Re: delete in ctor !

Roger Stoltz2-Jun-06 4:23

Roger Stoltz

2-Jun-06 4:23

Zac Howland wrote:
As a side note, the code written in the original post should NEVER be used in any real code. Use a constructor for what it is meant for; the same goes for destructors. Making these functions do things contrary to general practice serves no purpose.

Absolutely.
Otherwise the code be hard to use, maintain and even understand.
What should be done in this case, since the OP wants to gain control over objects created on the heap, is to override the new operator.

--
Roger

It's supposed to be hard, otherwise anybody could do it!

Regarding CodeProject: "resistance is pointless; you will be assimilated"

Re: delete in ctor !

BadKarma2-Jun-06 4:32

BadKarma

2-Jun-06 4:32

a simpler/better solution would be to make the constructor protected or private and use the Factory Pattern to create an object if the correct params are given, otherwise just return NULL.

In this case the programmer has a mean to validate if the construction of the object worked

codito ergo sum

Re: delete in ctor !

Roger Stoltz2-Jun-06 4:38

Roger Stoltz

2-Jun-06 4:38

BadKarma wrote:
make the constructor protected or private and use the Factory Pattern

Could be. I was thinking about the factory pattern, but the OP said that would always be using new to create the objects. That's why I suggested overriding the new operator.

As always, there's more than one solution to a problem and I took the question as hypothetical.

It's supposed to be hard, otherwise anybody could do it!

Regarding CodeProject: "resistance is pointless; you will be assimilated"

Re: delete in ctor !

Zac Howland2-Jun-06 5:04

Zac Howland

2-Jun-06 5:04

Roger Stoltz wrote:
Could be. I was thinking about the factory pattern, but the OP said that would always be using new to create the objects. That's why I suggested overriding the new operator.

My recommendation (instead of overriding the new/delete operators) would be to use a smart pointer (since it appears he wants it to self-destruct):

class MyClass<br />
{<br />
MyClass() { cout << "I got created!" << endl; }<br />
~MyClass() { cout << "I got destroyed!" << endl; }<br />
};<br />
<br />
void main()<br />
{<br />
   shared_ptr<MyClass> pClass(new MyClass);<br />
}

If you decide to become a software engineer, you are signing up to have a 1/2" piece of silicon tell you exactly how stupid you really are for 8 hours a day, 5 days a week

Zac

Re: delete in ctor !

BadKarma2-Jun-06 5:13

BadKarma

2-Jun-06 5:13

Zac Howland wrote:
If you decide to become a software engineer, you are signing up to have a 1/2" piece of silicon tell you exactly how stupid you really are for 8 hours a day, 5 days a week

couldn't be more correct;P

codito ergo sum

Migrating from VC6 to VS2005 [modified]

Obliterator2-Jun-06 1:36

Obliterator

2-Jun-06 1:36

I'm trying to compile my old application in VS2005.
But the compiler no longer accepts the default assignment of NULL to a vector iterator. How do I resolve this?

I declare my function like this:

vector<CSomeClass*>::iterator MyFunction(vector<CSomeClass*>::iterator itStartPos = NULL);

The compiler says:

error C2440: 'default argument' : cannot convert from 'int' to 'std::_Vector_iterator<_Ty,_Alloc>'

I can't default the parameter to myVectorList.begin() because the function declaration is not aware of which list the iterator belongs.

Thanks

--

The Obliterator

-- modified at 7:40 Friday 2nd June, 2006

Re: Migrating from VC6 to VS2005 [modified]

C^hris L^osinger2-Jun-06 1:58

C^hris L^osinger

2-Jun-06 1:58

Obliterator wrote:
function declaration is not aware of which list the iterator belongs.

does the function itself know ? if not, how do you know the iterator is valid ? (can't test against vec.end() if you don't have the vector)

a quick run through Google tells me that it's not possible, in general, to set iterators to NULL. iterators are not pointers, for one thing.

i think you'll need to re-work that function a bit

Cleek | Image Toolkits | Thumbnail maker

Re: Migrating from VC6 to VS2005 [modified]

Obliterator2-Jun-06 2:17

Obliterator

2-Jun-06 2:17

Good point. Actually, I simplified the function declaration for the purposes of the queston. The end vector is also supplied as a parameter to the function.

However, the function is indeed aware of the list and simply defaults to the beginning of the list if itStartPos parameter is NULL.

I guess your right - I'm going to have to drop the default parameter here and update each call to the function. Serves me right for using a hack to assign NULL to it in the first place. Damn this new strict checking!!!

Thanks for your response. Much appreciated.

--

The Obliterator

Re: Migrating from VC6 to VS2005

Michael Dunn2-Jun-06 7:46

Michael Dunn

2-Jun-06 7:46

Starting with VC7, STL iterators were type-checked better. You can't treat a collection<T>::iterator as a T* (or any other pointer value, like NULL).

--Mike--
Visual C++ MVP Cool | :cool:

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Re: Migrating from VC6 to VS2005

Obliterator9-Jun-06 2:33

Obliterator

9-Jun-06 2:33

So I see. This is causing me some real headaches!!!
Is there no assignement that can be made to the iterator which means its invalid?

I used to assign NULL to iterators to mean they wern't being used.
Then I could simply check its NULL status and act accordingly.

There must be a way of saying an iterator is invalid.
If I assign it the value of vectorlist.end() I could test against that - but does initial end assignment remain valid after things are added to the vector list?

--

The Obliterator

Right way to delete a pointer

zahid_ash2-Jun-06 1:31

zahid_ash

2-Jun-06 1:31

CSocket * sock;

is it a right way to delete a pointer

sock = NULL;
delete sock;

Or can I directly do

delete sock;

Regards.

Re: Right way to delete a pointer

Cedric Moonen2-Jun-06 1:40

Cedric Moonen

2-Jun-06 1:40

zahid_ash wrote:
sock = NULL;
delete sock;

This won't do anything !
You first have to delete the pointer THEN set it to NULL:

delete sock;
sock = NULL;

Cédric Moonen
Software developer

Charting control

Re: Right way to delete a pointer

Amar Sutar2-Jun-06 1:41

Amar Sutar

2-Jun-06 1:41

CSocket* pSocket;

// Allocate a memory for pSocket

// Delete a allocated memory
if(pSocket)
{
delete pSocket;
pSocket = NULL;
}

Regards
Amar. Smile | :)

Re: Right way to delete a pointer [modified]

Viorel.2-Jun-06 2:14

Viorel.

2-Jun-06 2:14

I think you can even skip the checking for NULL:

delete pSocket; // without redundant "if(pSocket != NULL) ..."
pSocket = NULL;

This is because delete operator performs itself a test for null pointers. You only have to be sure that pSocked was properly initialized before with a right value or with NULL.

-- modified at 9:42 Friday 2nd June, 2006

Re: Right way to delete a pointer [modified]

Zac Howland2-Jun-06 3:50

Zac Howland

2-Jun-06 3:50

Viorel Bejan wrote:
This is because delete operator performs itself a test for null pointers. You only have to be sure that pSocked was properly initialized before with a right value or with NULL.

This only happens in debug builds. You should ALWAYS check for null before deleting a pointer. Calling delete NULL has undefined behavior and should be avoided.

If you decide to become a software engineer, you are signing up to have a 1/2" piece of silicon tell you exactly how stupid you really are for 8 hours a day, 5 days a week

Zac

Re: Right way to delete a pointer [modified]

Nish Nishant2-Jun-06 3:54

Nish Nishant

2-Jun-06 3:54

Zac Howland wrote:
This only happens in debug builds. You should ALWAYS check for null before deleting a pointer. Calling delete NULL has undefined behavior and should be avoided.

That's not true, Zac.

See 16.8 on http://www.parashift.com/c++-faq-lite/freestore-mgmt.html[^]

Regards,
Nish

Nish’s thoughts on MFC, C++/CLI and .NET (my blog)
Currently working on C++/CLI in Action for Manning Publications.
Also visit the Ultimate Toolbox blog (New)

Re: Right way to delete a pointer [modified]

Zac Howland2-Jun-06 4:58

Zac Howland

2-Jun-06 4:58

Yes, the standard says that. However, most compilers did not meet that standard until recently (and some still don't). It is one of those better safe than sorry things.

If you decide to become a software engineer, you are signing up to have a 1/2" piece of silicon tell you exactly how stupid you really are for 8 hours a day, 5 days a week

Zac

Re: Right way to delete a pointer [modified]

Nemanja Trifunovic2-Jun-06 6:17

Nemanja Trifunovic

2-Jun-06 6:17

Zac Howland wrote:
However, most compilers did not meet that standard until recently (and some still don't). It is one of those better safe than sorry things.

Compilers don't meet the Standard mostly in some areas of template handling. Deleting a zero is perfectly safe and has been for quite a while.

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