Strange manner in multiplying a double value by 100000 in Qt (c )-CodePudding

Can anybody explain this:

double nnn = 2678.87;
QVariant vvv(2678.87);
qDebug() << qRound(nnn*1000.0);    //--> 2678870
qDebug() << qRound(nnn*10000.0);   //--> 26788700
qDebug() << qRound(nnn*100000.0);  //--> 267887000
qDebug() << qRound(nnn*1000000.0); //--> 2147483647 !
qDebug() << qRound(vvv.toDouble()*1000000.0);  //--> -2147483648 !!!

The two last statements also have strange result in following format:

qDebug() << qRound(nnn*1000000); //--> 2147483647 !
qDebug() << qRound(vvv.toDouble()*1000000);  //--> -2147483648 !!!

I need to use last statement but it is wrong apparently!

CodePudding user response：

qRound returns an int, your value is larger than will fit in an int without overflowing. try qround64 instead.

Note you'll still eventually run into precision problems, a double can only exactly represent integers up to around 9 * 10¹⁵.