Question

I’ve checked a satisfying explanation but could not find. Usually docs mention that in order to use signals/slots between threads, we need to use event loops and start them by calling exec.

However I can see that w/o using exec(), I can still send signals and handle them across threads.

What’s the exact use of it?

Answer 1

Use QThread::exec() when you want to run the event loop Qt provides for you in the QThread class. If you don’t call exec(), you need to create your own event loop that processes Qt events (that is, if you want signals / slots to work). This is almost certainly more work than it’s worth, unless you have very specific needs.

You say you can still send signals / slots? My guess is that you’re not actually running anything on a different thread. This is a very common issue when using QThread. Put a breakpoint inside the code you think is running on a different thread and have a look at the stack trace – you may be in for a shock!