The correct way to do so is:
#include <QDebug>
// snip...
QString letters;
qDebug() << letters;
Be careful to use qDebug()
beginning with a small letter, since it is not the same thing as the QDebug
class.
See http://qt-project.org/doc/qt-5.0/qtcore/qtglobal.html#qDebug. It is a convenience function that returns an already configured QDebug object.