This is just replacing a pointer with another pointer. nix1 = nix2

That was a total thinking failure. What I actually wanted to test was something like that:

{
     Nix nix4(4, "Hello, I'm four");

     *nix1 = nix4;
}
qWarning() << "nix1: " << nix1->display(4);

And that works.

solved, actually with help from SGaist (qt.forum)