I hope the value of str is 1.235.
But in fact, the value is 1.234 by qt. Why?
What happened in theory?
double d=1.2345;
auto str=QString::number(d,'f',3);
As per the documentation, QString::number() doesn't do any rounding, just formatting. That is, rounding isn't mentioned anywhere that I can see. I would not expect it to do any rounding.
To round a number to a certain number of decimals, one could use, for example (3 decimal places):
double rounded = std::round(d * 1000.0) / 1000.0;