Question

Having MyClass : QPolygonF I would like to create

void bin_write(QDataStream & out ) const
{
   out << (BASE CLASS QPOLYGONF)*this  ??
}

which calls the overload operator

QDataStream& operator<< ( QDataStream & stream, const QPolygonF & polygon )

I dont know how to tell it to call that method.

Answer 1

Simply do:

out << *this;

There’s no need for an explicit upcast here (as is generally the case) unless your subclass is also defining its own overload of operator<< (insertion operator) with QDataStream.

If your class does provide overloaded operator<< and you need to use QPolygonF’s insertion operator (for example, in order to help implement operator<< for your subclass), then you can do:

out << static_cast<const QPolygonF&>(*this);