Undefined reference to 'vtable to FocusWatcher'

[YoungProgrammer]

I found this script on the internet but when I try to build it i got this error: "Undefined reference to 'vtable to FocusWatcher' ".

#include<QEvent>
#include<QObject>

class FocusWatcher : public QObject
{
   Q_OBJECT
public:
   explicit FocusWatcher(QObject* parent = nullptr) : QObject(parent)
   {
      if (parent)
         parent->installEventFilter(this);
   }
   virtual bool eventFilter(QObject *obj, QEvent *event) override
   {
      Q_UNUSED(obj)
      if (event->type() == QEvent::FocusIn)
         emit focusChanged(true);
      else if (event->type() == QEvent::FocusOut)
         emit focusChanged(false);
      return false;
   }

   virtual ~FocusWatcher() {}

Q_SIGNALS:
   void focusChanged(bool in);
};

[SGaist]

Hi,

How are you compiling it ?