how to center a Qt mainform on the screen?

I've tried these in my mainform's constructor:

QRect desktopRect = QApplication::desktop()->availableGeometry(this);
move(desktopRect.center() - frameGeometry().center());

QRect desktopRect = QApplication::desktop()->availableGeometry(this);
move(desktopRect.center() - rect().center());

but both put the bottom right corner of the fo开发者_运维知识库rm at about the center of the screen, instead of centering the form. Any ideas?

I've tried these in my mainform's constructor

That's likely the problem. You probably don't have valid geometry information at this point because the object isn't visible.

When the object is first constructed, it's essentially positioned at (0,0) with it's expected (width,height), as such:

frame geometry at construction:  QRect(0,0 639x479) 

But, after being shown:

frame geometry rect:  QRect(476,337 968x507) 

Thus, you can't yet rely on your frameGeometry() information.

EDIT: With that said, I presume you can easily move it as desired, but for completeness I'm dropping in Patrice's code which doesn't depend on the frame geometry information:

QRect desktopRect = QApplication::desktop()->availableGeometry(this);
QPoint center = desktopRect.center();

move(center.x() - width() * 0.5, center.y() - height() * 0.5);

The move function (see QWidget doc) takes one QPoint or two int as parameter. This corresponds to the coordinates of the top-left corner of your Widget (relative to its parent; Here OS Desktop). Try:

QRect desktopRect = QApplication::desktop()->availableGeometry(this);
QPoint center = desktopRect.center();

move(center.x()-width*0.5, center.y()-height*0.5);

availableGeometry() is deprecated.

move(pos() + (QGuiApplication::primaryScreen()->geometry().center() - geometry().center()));

#include <QStyle>
#include <QDesktopWidget>

window->setGeometry(
    QStyle::alignedRect(
        Qt::LeftToRight,
        Qt::AlignCenter,
        window->size(),
        qApp->desktop()->availableGeometry()
    )
);

https://wiki.qt.io/How_to_Center_a_Window_on_the_Screen

move(QGuiApplication::primaryScreen()->geometry().center() - rect().center());

PyQT Python Version

# Center Window
desktopRect = QApplication.desktop().availableGeometry(self.window)
center = desktopRect.center();
self.window.move(center.x()-self.window.width()  * 0.5,
                 center.y()-self.window.height() * 0.5);   

Another solution, assuming the window in question is 800×800:

QRect rec = QApplication::desktop()->availableGeometry();
move(QPoint((rec.width()-800)/2, (rec.height()-800)/2));