qt点击按钮退出界面

#include "mainwindow.h"

#include <QApplication>

#include<QPushButton>

int main(int argc, char *argv[])

{
  
  

    QApplication a(argc, argv);

    QPushButton *b = new QPushButton("Quit");

    QObject::connect(b,SIGNAL(clicked()),&a,SLOT(quit()));

    b->show();

//    MainWindow w;

//    w.show();

    return a.exec();

}