Segmentation Fault

[Krish_ng]

When trying to destroy an object i am getting segmentation fault.I hav created some labels
when destroying the same in the destructor of respective class i am getting segmentation fault.I hav used the following statement in the destructr

if(m_pLabel!=NULL)
delete m_pLabel;
m_pLabel=NULL;

is there any dependency we need to check while performing delete...

[aMan]

If you set their parent in the constructor (new QLable("Blub", this) ), you don't need and aren't allowed to delete them. Qt deletes them in the destructor of the parent object..

[ufo-vl]

Please, show your code.
My code:


DialogImpl::DialogImpl( QWidget * parent, Qt::WFlags f)
: QDialog(parent, f)
{
setupUi(this);
lbl = new QLabel("test", this);
}
//
DialogImpl::~DialogImpl()
{
if (lbl != NULL)
delete lbl;
lbl = NULL;
}


working fine.
And when destroying parent widget (in this case Dialog, automatically destroyed childs widgets), may be it's issue your segmentation fault?

[Krish_ng]

I hav used the following statement for the label creation---QLabel *m_pLabel=new QLabel;
But in the class i am inherting QMainWindow--in cpp file i am using the following statement
QClass::QClass():QMainWindow(0);
i am able to use the same delete statemnts for other widgets..But only for some labels i am not able to do so...Is there any other way to get rid of that

[comlink21]

I think you try to assign the object after deleting it's object.

try the following:

if(m_pLabel!=NULL)
m_pLabel=NULL;
delete m_pLabel;

--comlink21

[marcel]

I think you try to assign the object after deleting it's object.

try the following:

if(m_pLabel!=NULL)
m_pLabel=NULL;
delete m_pLabel;

--comlink21

But this makes no sense. You first make it null and then you delete it? Won't work.



I hav used the following statement for the label creation---QLabel *m_pLabel=new QLabel;
But in the class i am inherting QMainWindow--in cpp file i am using the following statement
QClass::QClass():QMainWindow(0);
i am able to use the same delete statemnts for other widgets..But only for some labels i am not able to do so...Is there any other way to get rid of that
As it was suggested earlier in this thread, if you pass a parent when you create your widgets, then that parent will handle disposal.

Since it works for other widgets, then I think that you add that label to a layout. In this case the widget that has that layout takes ownership over the label. So you both try to delete it.
I think that you succeed and then the parent widget tries again, and tries to delete an invalid pointer.

What about posting your code?

Regards

[Krish_ng]

It was my mistake...i had use the statement in the way you specified..i.e.,
if(m_pLabel!=NULL)
delete m_pLabel;
m_pLabel=NULL.

I hav resloved the problem..I was trying delete the label which was already deleted by the
parent.I had added the label to a splitter and deleted splitter object first and was trying to delete the label obj..So i got segmentation fault...Thanks for ur reply

[Gopala Krishna]

if(m_pLabel!=NULL)
delete m_pLabel;
m_pLabel=NULL.


Actually you don't need to check for null while deleting since deleting a null pointer is perfectly valid in c++. Its just redundancy to check for null pointer. However the pointer should be set to null once deleted.
Check this faq (http://www.parashift.com/c++-faq-lite/freestore-mgmt.html#faq-16.8) for complete details.

Therefore, IMHO this would suffice

delete m_pLabel;
m_pLabel = NULL;

[jpn]

I hav used the following statement for the label creation---QLabel *m_pLabel=new QLabel;
Looks like you're creating a new local variable instead of assigning to the member variable.


// QLabel *m_pLabel=new QLabel; // wrong
m_pLabel=new QLabel(this); // correct