How to centralize widget automatically with qt designer?

Question

I am creating an application in full screen, however when editing in qt design there is no such option. So when I center the widget on the qt designer it looks like this:

It looks like it was centered, but it centered to the standard window size:

Now look what happens when I increase the size of the window:

Answer 1

You are almost there. If you add 2 vertical spacers, put them and your horizontal layout to a vertical layout and finally put everything on mainwindow to a grid layout, you'll achieve what you are looking for. It looks like so:

This way the widget is placed right in the center of your window no matter the size of the window. Just make sure the sizeType property of spacers are set to Expanding.

Alternatively, you can achieve the same functionality by overriding QMainWindow::resizeEvent. Something like this:

//In your .h file
protected:
    void resizeEvent(QResizeEvent *event) override;
//In your .cpp file
void MainWindow::resizeEvent(QResizeEvent *event)
{
    ui->pushButton->setGeometry(this->width()/2 - ui->pushButton->width()/2,
                                this->height()/2 - ui->pushButton->height()/2,
                                ui->pushButton->width(),
                                ui->pushButton->height());
}

It requires for more coding, but allows for more flexibility.