Russian Qt Forum

#include <QApplication>
#include <QLabel>
#include <QMainWindow>
#include <QRegularExpression>
#include <QResizeEvent>
#include <QVBoxLayout>
#include <QWidget>

class ChildWidget : public QWidget
{
		Q_OBJECT
	public:
		explicit ChildWidget(QWidget *parent = 0) :
			QWidget(parent)
		{
			lblSize = new QLabel("Child Widget Size: (0 x 0)");
			lblSize->setAlignment(Qt::AlignCenter);
			lblSize->setStyleSheet("QLabel {"
										  "border-style: solid;"
										  "border-width: 1px;"
										  "border-color: black; "
										  "}");

			QVBoxLayout *vbxLayout = new QVBoxLayout();
			vbxLayout->addWidget(lblSize);

			setLayout(vbxLayout);
		}

	protected:
		void resizeEvent(QResizeEvent *event) override
		{
			QString sizeText = QString("%1 x %2").arg(event->size().width()).arg(event->size().height());
			lblSize->setText(lblSize->text().replace(QRegularExpression("\\d+\\sx\\s\\d+"), sizeText));
		}

	private:
		QLabel *lblSize;
};

class MainWindow : public QMainWindow
{
		Q_OBJECT

	public:
		MainWindow(QWidget *parent = 0) :
			QMainWindow(parent)
		{
			setCentralWidget(new ChildWidget());
		}
};

int main(int argc, char *argv[])
{
	QApplication a(argc, argv);

	MainWindow w;
	w.show();

	return a.exec();
}

#include "main.moc"