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QMessageBox question

Feoggou

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Friday, September 30th 2011, 3:36pm

QMessageBox question

Hello

Is there a problem if I write this:

Source code

 
1

QMessageBox::critical(NULL, title, message);


ie with the parent set to NULL.
Is this ok or it is a memory leak?

Thanks.

bst

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Friday, September 30th 2011, 9:03pm

Hi,

This is OK.

QMessageBox normally positions in the middle of the parent window. If you use 0 it will position in the middle of the screen.

cu, Bernd

Feoggou

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Wednesday, October 5th 2011, 12:13pm

Thanks.

I thought that QMessageBox::critical may create a messagebox object on the heap (ie using "new") and that the parent needs to be specified so that the memory would be released after the messagebox has been closed.

bst

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Wednesday, October 5th 2011, 12:52pm

Hi,

no, the messagebox is created via:

Source code

 
1

QMessageBox msgBox(icon, title, text, QMessageBox::NoButton, parent);


in showNewMessageBox. See the source in qt-xyz/src/dialogs/qmessagebox.cpp. Parent is IMHO only used for modality:

Quoted

If \a parent is 0, the message box is an \l{Qt::ApplicationModal}
{application modal} dialog box. If \a parent is a widget, the
message box is \l{Qt::WindowModal} {window modal} relative to \a
parent.

cu, Bernd

Feoggou

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Tuesday, October 11th 2011, 9:22am

thanks :)

By the way, is there a quick method to find out which object you can allocate on the heap (ie via "new") without deleting, and which should be delete-d afterwards?
For instance, it is clear that the QString should be allocated on the stack, because it does not have as parameters a pointer to the parent. So, can all objects of all classes who have constructors that take as parameter a pointer to the parent be allocated on the heap (without the need to use delete afterwards)?

bst

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Thursday, October 13th 2011, 9:07am

Hi,

Qt's 'parent-child model' is implemented in QObject. The objects you have to delete yourself are the objects which are created with new and have no parent.

HTH, Bernd

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