What is the difference between them?
Every one of them is a unique QLineEdit instance: each one has a unique pointer value. If you are connecting all of them to the same slot that will be called when the line edit emits the editingFinished() signal, then you tell them apart based on their pointers. The pointer is accessible if you call the sender() method inside the slot. If you store these pointers in a list or vector as you create them and add them to your form, then you can look up this pointer value in this list and its position in the list is the index.

Or, as I (and anda_skoa also) have suggested, you can use QSignalMapper to directly map the QLineEdit's pointer value to any integer. You connect to the QSignalMapper::mapped() signal. In the slot that connects to this, you use the integer that is passed in as the argument to QSignalMapper::mapping() to get the QLineEdit instance that triggered the signal in the first place.