Why doesn't my Qt Creator know QPolygon?

Question

I have the following simple code. I just start writing my application and get stuck on this problem: Qt Creator tells me, that QPolygon file or directory doesn't exists. I checked it with QT Documentation and it says, ther is.

#include <QPolygon>

class IHledani
{
public:
    virtual IHledani();
    virtual ~IHledani();

    virtual QPolygon najdiKonturu(IObraz image, int startx, int starty);
};

Thank you