QApplication a (argc, argv) and exec() in the main function of QT getting started

One of the simplest 、 The rudimentary QT The procedure is as follows

int main(int argc,char * argv[])
{
    
    QApplication a(argc,argv);
    MainWindow w;
    w.show();
    return a.exec();
}

（1）QApplication a(argc,argv);
Define a Qt Application object , Its constructor receives and main Function like parameters , yes Qt Graphical interface program entry , It's like main Function is normal C++ The entrance of the program is the same .
（2）return a.exec();
Will enter Qt Application's The event loop function Wait for messages from the user's operation and system And then deal with it .
common c/c++ Language main Functions are all direct return 0 Of , Program exit . But graphic programs usually need to be connected with User interaction , Will not automatically turn off , Instead, it waits for the user to operate . If the user clicks the close button of the window , The program will end and return a value , The default is 0 .
qt in main The function starts with an initialization , Then give control to qt, Then all the events will be handled only qt The event loop of handles .