Qt中把浮点数转为QString

double d = 1.2222;
QString.setNum(d,'f',2);   // 1.22

QString &QString::setNum(double n, char format = 'g', int precision = 6)
This is an overloaded function.
Sets the string to the printed value of n, formatted according to the given format and precision, and returns a reference to the string.
The format can be 'e', 'E', 'f', 'g' or 'G' (see Argument Formats for an explanation of the formats).