QFile::fileName() Get filename only

I have made file using abs path, so fW.fileName() returns abs path, but I want to get "W.txt" only... I suggest to add build-in mechanism of QFile to get filename only.

My example

QFile fW("/home/user/W.txt");
fW.open(QIODevice::WriteOnly);
fW.close();
qDebug() << fW.fileName(); //print "/home/user/W.txt" instead of "W.txt"

Of cause I can use this code to solve my problem

qDebug() << QFileInfo(fW.fileName()).fileName(); //print "W.txt"

But why I can't use my fW object only without QFileInfo() or QString?