passing by reference... my darkest enemy actual reason below: In your case (without reference) the compiler generates a temporary object (int) for the first argument of your MyApplication constructor. You pass this temporary int to QApplication’s constructor (with int reference), which saves the address of this reference. Once your MyApplication constructor is done the temporary int is destroyed but QApplication still has its address.

Hedgewars - a turn based strategy game.
Distributed under the terms of the GNU GPL licence.
Images and sounds are distributed under the terms of the GNU FDL licence.

Source:
Copyright 2004-2010 Andrey Korotaev <unC0Rr@gmail.com>
Portions copyright 2006-2008 Igor Ulyanov aka Displacer <iulyanov@gmail.com>