Hi,
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?
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!
A rough example.
Suppose you have a text box. On each letter user types on the text box you want to perform some background task. You can setup a QThread for that. Emit something whenever the contents of text box changes. Assign a slot from your QThread that handles the background task. Emit something from QThread when the task finished. Handle this signal from main thread. Connect them. Start the thread when the text box is created (or any appropriate time). If you call exec() from your QThread::run() then you don't need to start() the thread multiple times.
If you don't use this mechanism, you may need to create (and/or start()) a QThread each time the content of text box changes, perform the background task and get result. This time you can still use signal/slot between main thread and this thread, but you need to start() the thread multiple times.