how to deal with "no such file error or diretory" error for a new programmer in QT creator

when i try to develop a hello demo in QT creator with the code following :

#include<QApplication>

#include<QLabel>

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

{

    QApplication app(argc,argv);

    QLabel l("hello,word!");

    l.show();

    return app.exec();

}

the IDE inform me that "QApplication no such file or directory", then i search the Internet, and find that i should add the following code in "*.pro":

QT       += core gui widgets

or add the following:

QT       += core gui

greaterThan(QT_MAJOR_VERSION, 4): QT += widgets

these code tell the qmake to include the header.