#include <QApplication>
#include <QWidget>
#include <QLabel>
#include <QKeyEvent>
#include <QString>
#include <QObject>

class Filter: public QObject
{
public:
    Filter(QObject *parent=nullptr): QObject(parent)
    {
    }
protected:
    bool eventFilter(QObject *, QEvent *ev) override
    {
        if(ev->type()==QEvent::KeyPress)
        {
            QKeyEvent *kEv=static_cast<QKeyEvent*>(ev);
            l.setText(QString("You pressed key %1.\nQt::Key_Minus=%2").arg(kEv->key()).arg(Qt::Key_Minus));
            l.show();

        }
        return false;
    }
private:
    QLabel l;
};

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    QWidget w;
    w.installEventFilter(new Filter(&w));
    w.show();
    return a.exec();
}