Building on @Kristian's answer, I had a desire to display a fixed number of decimal places. That can be accomplished with other arguments in the QString::number(...) function. For instance, I wanted 3 decimal places:
double value = 34.0495834;
QString strValue = QString::number(value, 'f', 3);
// strValue == "34.050"
The 'f' specifies decimal format notation (more info here, you can also specify scientific notation) and the 3 specifies the precision (number of decimal places). Probably already linked in other answers, but more info about the QString::number function can be found here in the QString documentation