Difference between QString::number() & QString::setNum()?

Hi, I want to convert a double variable to a QString. I want to have a very strict control over the precision & accuracy. A faster method would be convenient. I have found these 2 functions : QString::number() & QString::setNum(). Both perform the conversion from double to a QString with user defined precision & formatting. So I wanted to know what is the difference between these 2 methods. There must be some difference that is why they have provided these 2 functions, right? Thank You.