Signals and Slots.

#include <QApplication>
#include <QPushButton> //Allow PushButtons.

int main(int argc, char *argv[])
{
    QApplication prog(argc, argv);
    QPushButton *button = new QPushButton("Quit"); //Declare.
    button->show(); //Display.
    QObject::connect(button, SIGNAL(clicked()), &prog, SLOT(quit()));

    //SIGNAL() = the event that the user does.
    //SLOT() = the code that gets executed after the event.

    return prog.exec();
}