#include <QtWidgets>

int main(int argc, char** argv)
{
	QApplication app(argc, argv);

	QWidget* window = new QWidget();

	window->show();
	window->resize(400, 300);

	QBoxLayout* buttonLayout = new QBoxLayout(QBoxLayout::LeftToRight, window);
	QPushButton* toggleFullscreenButton = new QPushButton("Toggle fullscreen", window);
	QObject::connect(toggleFullscreenButton, &QPushButton::clicked, window, [window]
	{
		if (!window->isFullScreen())
			window->showFullScreen();
		else
			window->showNormal();
	});

	buttonLayout->addWidget(toggleFullscreenButton);

	return app.exec();
}