c++ - QPainter not active -

the following code results in bunch of errors:

void mainwindow::displayboard() {     qpixmap pix(0,0);     pix.fill(qt::white);     qpainter painter(&pix);     painter.setbrush(qt::black);     for(int row = 0; row < 8; row++)         for(int col = 0; col < 8; col++)             painter.drawrect(row * 10, col * 10, 10, 10);     ui->label->setpixmap(pix); } 

the errors:

qpainter::begin: paint device returned engine == 0, type: 2 qpainter::setbrush: painter not active qpainter::drawrects: painter not active 

how can resolve error?

the problem that, colin pointed out, pix size zero.