[Rivet] Dangerous casting to FinalState

[Frank Siegert]

Hi Riveters,

just a short warning about a tricky mistake that can happen quite easily.

Example1:

      FinalState remainder = zfinder.remainingFinalState();
      addProjection(remainder, "RFS");

Example2a:

      const FinalState& remainder = zfinder.remainingFinalState();
      addProjection(remainder, "RFS");

Example2b:

      addProjection(zfinder.remainingFinalState(), "RFS");

Example 2c:

      VetoedFinalState remainder = zfinder.remainingFinalState();
      addProjection(remainder, "RFS");


All of them will compile fine, but the first one will behave
differently from the others. Since the object is copied instead of
passed by reference the remainder object will actually not be aware of
its "VetoedFinalState" properties and thus not apply any vetoing at
all.

This is all fine, and Rivet is behaving correctly as expected from
C++, but it's nevertheless tricky to overlook. Does anybody have an
idea how to avoid this for users who don't pay close enough attention?
Is there any reasonable way of hiding/masking the copy constructor for
such cases?

Cheers,
Frank