How to create a QIcon with Qt programmatically

First, create a 32×32 QImage and make it transparent (or whatever background color you need):

QImage img(32, 32, QImage::Format_ARGB32);
img.fill(QColor(0, 0, 0, 0));

I use QImage instead of QPixmap, because the documentation says that QImage is optimized for I/O, e.g. painting, and the first attempts with QPixmap looked like crap. Next, create a QPainter for the QPaintDevice and set some render hints to make it look nice:

QPainter *p = new QPainter(&img);
p->setRenderHint(QPainter::Antialiasing);
p->setRenderHint(QPainter::TextAntialiasing);
p->setRenderHint(QPainter::SmoothPixmapTransform);

QPainter::setBrush() sets the background color for shapes, QPainter::setPen() the foreground color for text:

p->setBrush(QColor(Qt::red));
p->setPen(QColor(Qt::white));

Then select a font the size of our future Icon:

QFont f("courier new");
f.setPixelSize(32);
p->setFont(f);

Now we need some background. White on transparent isn’t really readable, so let’s draw a circle:

p->drawEllipse(img.rect());

Since our QImage is actually a square and not an rectangle, QPainter::drawEllipse will draw a circle. Print the QChar, letter, or whatever:

p->drawText(img.rect(), Qt::AlignCenter, QChar(char));

Now clean up and return the QImage as QIcon:

delete p;
return QIcon(QPixmap::fromImage(img));

The whole shebang can be marveled at here. It looks like this:

Have fun!