QVariant, QList, QSettings

[TheKedge]

Hello all,
what's going wrong here:
I'm reading in a list of integers from QSettings and that means doing some juggling with QList<QVariant>, no problem but I get a crash on leaving the function.

QList<QVariant> widths;
//fill the QList with some default value
	for(int col=0; col<tableWidget->columnCount(); ++col){
		widths << 80;//some standard width
	}
//read a QVariant from QSettings and convert it to a QList<QVariant>
	widths = settings.value("column widths").toList();
//or with a default value
	widths = settings.value("column widths", QVariant(widths)).toList();
//then I use the widths variable and get a crash on leaving the function

QList<QVariant> widths;
//fill the QList with some default value
	for(int col=0; col<tableWidget->columnCount(); ++col){
		widths << 80;//some standard width
	}
//read a QVariant from QSettings and convert it to a QList<QVariant>
	widths = settings.value("column widths").toList();
//or with a default value
	widths = settings.value("column widths", QVariant(widths)).toList();
//then I use the widths variable and get a crash on leaving the function

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thanks
K

[wysota]

Are you sure the variant is a list? How did you store the value in the settings object?

[TheKedge]

I did this

QList<QVariant> widths;
	for(int col=0; col<tableWidget->columnCount(); ++col){
		widths << header->sectionSize ( header->logicalIndex ( col ) ) ;
	}
	settings.setValue("column widths", widths);

QList<QVariant> widths;
	for(int col=0; col<tableWidget->columnCount(); ++col){
		widths << header->sectionSize ( header->logicalIndex ( col ) ) ;
	}
	settings.setValue("column widths", widths);

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and I get a nice list in the settings (registry)


The list is read correctly. I can see each value with a test variable in the loop. eg.

QHeaderView* header = tableWidget->horizontalHeader();
	for(int col=0; col<tableWidget->columnCount(); ++col){
		header->resizeSection ( col, 80);//widths.at(col).toInt() ) ;
		int thing = widths.at(col).toInt();//have a look at what's in the list - everything ok
	}

QHeaderView* header = tableWidget->horizontalHeader();
	for(int col=0; col<tableWidget->columnCount(); ++col){
		header->resizeSection ( col, 80);//widths.at(col).toInt() ) ;
		int thing = widths.at(col).toInt();//have a look at what's in the list - everything ok
	}

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I have the feeling it's something to do with some allocation or other....

K

[wysota]

Did you try debugging? Most probably some destructor crashes. Also try recompiling the whole project - sometimes the call stack gets corrupted due to recompilation errors (especially if using MSVC) and forcing all modules to be recompiled and relinked helps.

[TheKedge]

absolutely right.
Q_OUTOFLINE_TEMPLATE QList<T>::~QList()
crashes (on leaving the function). But I've no idea why!
(I've rebuilt the whole project)

here's the code again:

QList<QVariant> widths;
	for(int col=0; col<tableWidget->columnCount(); ++col){
		widths << 80;//some standard width
	}
//this is the line that when commented in causes the crash on leaving the function
	widths = settings.value("column widths").toList();
//replacing all the above with the following is no better
QList <QVariant>widths(settings.value("column widths").toList());
//widths is filled with the correct values - I can see that here... variable 'thing'
	QHeaderView* header = tableWidget->horizontalHeader();
	for(int col=0; col<tableWidget->columnCount(); ++col){
		int thing = widths.at(col).toInt();
	}

QList<QVariant> widths;
	for(int col=0; col<tableWidget->columnCount(); ++col){
		widths << 80;//some standard width
	}
//this is the line that when commented in causes the crash on leaving the function
	widths = settings.value("column widths").toList();
//replacing all the above with the following is no better
QList <QVariant>widths(settings.value("column widths").toList());
//widths is filled with the correct values - I can see that here... variable 'thing'
	QHeaderView* header = tableWidget->horizontalHeader();
	for(int col=0; col<tableWidget->columnCount(); ++col){
		int thing = widths.at(col).toInt();
	}

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my feeling is that QSettings is working fine. But is there something going wrong with the copy constructor of QList<QVariant>. Maybe with deep copy?
any ideas?

thanks
K

[jpn]

Which exact version of Qt are you using? I somehow recall a relevant bug, QList destructor crash, being fixed in one of the recent past versions of Qt 4.[1-2].x.

[TheKedge]

I downloaded Qt4.2.3 today and installed it.

K