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.