I have understood the following regarding QApplication's exec function:
QApplication exec starts the main event loop. It launches the GUI. It processes the signals and calls appropriate slots on receiving them. It waits until exit is called and returns the value which was set in exit.
Now, when we say event loop, does it mean that there is some while loop running in the internal code of Qt, and in that while loop the method of handling signals and slots is written?
Now, when we say event loop, does it mean that there is some while loop running in the internal code of Qt, and in that while loop the method of handling signals and slots is written?
In a sense, yes. Most software these days sits and waits for events -- user input, network traffic, timer events, sensors, etc. -- and responds accordingly.
This is not specific to Qt. It's a common design pattern you'll find everywhere from Windows to Android to Arduino.
来源：https://stackoverflow.com/questions/40880949/what-is-an-event-loop-in-qt