what is the effective scope of setCodecfForTr()

Question

I'm have the following in main.cpp file,

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);


    QTextCodec::setCodecForCString(QTextCodec::codecForName("UTF-8"));

    MainWindow w;
    w.show();

    return a.exec();
}

And the following is the mainwindow.cpp

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);    

    result = new QLabel("Check Result Here:",this);

    result->setGeometry(QRect(QPoint(180,30),QSize(161,161)));

    QString m("宁愿看见两个恶魔在拔河，也不愿看见一只天使在跳舞。");

    result->setText(m);

}

MainWindow::~MainWindow()
{
    delete ui;
}

The Chinese character on the QLabel can not be displayed normally.

If I modify them into follows, the character can be displayed normally.

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);

    MainWindow w;
    w.show();

    return a.exec();
}

The following is the mainwindow.cpp

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);    
    QTextCodec::setCodecForCString(QTextCodec::codecForName("UTF-8"));

    result = new QLabel("Check Result Here:",this);

    result->setGeometry(QRect(QPoint(180,30),QSize(161,161)));

    QString m("宁愿看见两个恶魔在拔河，也不愿看见一只天使在跳舞。");

    result->setText(m);

}

MainWindow::~MainWindow()
{
    delete ui;
}

I think its the problem of the scope of function 'setCodeForCString()`, Can someone explain that?