Create Own Signal send to different class

I have a main gui class QLMain , from there i call another class Callback for each object created. i want to create a signal in Callback, which will send signal to QLMain that a certain variable has changed .

I am trying to create signals for QString, which will be called up when string has changed/deleted/etc. The callback class changes the QString value upon each call and I want to be notified of it when it does.

Is this possible? how can i implement it?

Thanks
Arpit