What makes something not thread safe?

[tgreaves]

If you use signals and slots or events to connect the thread and the widget then yes.

What about if I pass a pointer to the widget to the thread and have it do a "widget*->setText()" there?

[wysota]

No, that's a guaranteed crash sooner or later. You can't access any of the widget methods not explicitely mentioned as thread-safe from worker threads.