Reimplement of resizeEvent

[jpn]

The easiest possible way is to let the label scale its contents:

SLD::SLD(QWidget *parent, Qt::WFlags flags)
    : QMainWindow(parent, flags)
{
    ui.setupUi(this);
    QLabel* label=new QLabel(this);
    label->setPixmap(QPixmap("Resources/CB.bmp"));
    label->setScaledContents(true); // let the label scale its contents
    setCentralWidget(label); // label will get automatically resized as a central widget
}

SLD::SLD(QWidget *parent, Qt::WFlags flags)
    : QMainWindow(parent, flags)
{
    ui.setupUi(this);
    QLabel* label=new QLabel(this);
    label->setPixmap(QPixmap("Resources/CB.bmp"));
    label->setScaledContents(true); // let the label scale its contents
    setCentralWidget(label); // label will get automatically resized as a central widget
}

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If you need more intelligent scaling, like keeping aspect ratio or something, you might want to scale it by hand. This would be done by storing the image as a member variable, scaling it for example in resizeEvent() and setting the scaled image on the same label. I'd suggest doing it a bit delayed so that each and every resizeEvent() in a row doesn't scale and set a new image.

Notice that you were creating new QLabel widgets again and again during every resize event! Also, Qt has a concept of layouts to handle geometries. You barely set geometries by hand.