How to show a QLabel in fullscreen?

I have a ui with a QPushButton and a QLabel. I would like the QLabel to be displayed in full screen on my laptop whenever I push that button.

I tried the following code:

void MainWindow::on_fullScreenBtn_clicked()
{
    // ui->myImage is a QLabel*
    ui->myImage->setText("going full screen");
    ui->myImage->showMaximized();
    ui->myImage->QWidget::showFullScreen(); 
}

I can see this function is executed, but it doesn't display my label in full screen. What am I doing incorrectly/missing?

I am using Qt Creator 3.5.1 and made my GUI using the built-in graphical interface.

Solution

The problem is that your QLabel is part of another widget and therefore displayed as a child widget instead due to the default Qt::Widget flag:

This is the default type for QWidget. Widgets of this type are child widgets if they have a parent, and independent windows if they have no parent. See also Qt::Window and Qt::SubWindow.

Therefore, the solution is to change the flag to Qt::Window manually as follows:

ui->myImage->setWindowFlag(Qt::Window);

Pre Qt 5.9 users can use the following:

ui->myImage->setWindowFlags(ui->myImage->windowFlags() | Qt::Window);

Note that you should call showFullScreen after you changed the window flags as documented by Qt:

Note: This function calls setParent() when changing the flags for a window, causing the widget to be hidden. You must call show() to make the widget visible again..

Undo maximisation

The window flag should be removed to undo the maximisation. This can be done as follows:

ui->myImage->setWindowFlag(Qt::Window, false);
ui->myImage->show();

Pre Qt 5.9 users can use the following:

ui->myImage->setWindowFlags(ui->myImage->windowFlags() & ~Qt::Window);
ui->myImage->show();

Note that the call to show is needed as pointed out above.