Bar code scanner is just a device.
So first you need to know what kind of a device you have.
If its serial, then you can use QExtSerial.
If its parallel, you can use parapin, and I made a Qt wrapper class for it you want I can send it to you.
I don't know my self about any USB C++ or Qt wrappers, so either you will have to seach for it, or write it your self.
(I am talking about linux here)

Again, Qt is a GUI tool kit, and if your app needs to talk to any hardware, you will have to link agains that hardware driver, but it has nothing to do with the application being builed with Qt.
Some times it can be convinent to wrap API of drivers with Qt, and enjoy signal and slots to communicate with the hardware.
I have done it with frame grabbers, parapin, and also custom made (not by me) USB driver.