QT 4.4.0 Upgrade QPainter Problem

[ChrisReath]

Hello all,

I recently upgraded from QT 4.3.4 to 4.4.0. When I upgraded, my GUI started giving me an "ASSERT: SharedPainter ? SharedPainter->isActive() : true" error and then the program craps out.

Here is the stack trace:


QPainter::begin: A paint device can only be painted by one painter at a time.
QPainter::begin: A paint device can only be painted by one painter at a time.
QPainter::begin: A paint device can only be painted by one painter at a time.
QPainter::begin: A paint device can only be painted by one painter at a time.
QPainter::begin: A paint device can only be painted by one painter at a time.
QPainter::begin: A paint device can only be painted by one painter at a time.
QPainter::begin: A paint device can only be painted by one painter at a time.
QPainter::begin: A paint device can only be painted by one painter at a time.
QPainter::begin: A paint device can only be painted by one painter at a time.
QPainter::begin: A paint device can only be painted by one painter at a time.
QPainter::restore: Painter not active
ASSERT: "sharedPainter ? sharedPainter->isActive() : true" in file kernel\qwidget.cpp, line 4398

Anyone have any idea?

Thanks,

Chris

[thomir]

...

Note sure what the problem is, but perhaps you have more than one painter trying to paint the same device at once?

Sounds obvious, but are you calling QPainter::begin and QPainter::end correctly? If you're using the constructor instead of begin(), make sure the QPainter object is really being destroyed.

Also, the output you posted isn't a stack trace - if you still can't work it out, posting a backtrace at the point of the assert might help debug the issue (I'm guessing that it'd lead you back to one of the conflicting QPainter objects anyway).

HTH!

[ChrisReath]

Hey -- Tried your advice above and still got the same result. This is what the call stack looks like at the point of the Assert:

QTCored4.dll!67037ad()
QtCored4.dll!670375a1()

and then nothing else. Strange indeed.

[ChrisReath]

Actually, you were absolutely right. You can only have one QPainter Object in existence at one time.

If you use this method to create QPainter Objects:


void MyWidget::paintEvent(QPaintEvent *)
{
QPainter p(this);
p.drawLine(...); // drawing code
}

You have to be careful because you can't use another QPainter p(this) object in the same function. So you are almost always better off using this method:


void MyWidget::paintEvent(QPaintEvent *)
{
QPainter p;
p.begin(this);
p.drawLine(...); // drawing code
p.end();
}

to create QPainter objects so you can begin() and end() Painter Objects appropriately. Thanks for your help.

Chris

[ChrisReath]

Also, for some reason, QT didn't bother with this assert error in 4.3.4, it is something new they added in 4.4.0 to protect the user.