looks like i nearly got it
But I think I have to use const_cast.

With a const_cast to Project it seems to work, but I realised that I made a big mistake.
My Projectclass inherits QStandardItem AND QObject.
So I can't make an QStandardItem from a Project. Or am I wrong?

So I now face the descision, to implement a &operator<< with the relevant Attributes of the QStandarditem on my own or to get rid of the QObject inheritance what would make necessary massive changes to my information flux.
Is it bad Style to Make QStandardItems to QObjects?