[Rivet] Dangerous casting to FinalState

[Frank Siegert]

Hi David,

Thanks for your feedback.

> Just thinking out loud:
> addProjection is templated on a reference, is it possible to make a
> specialized template for the non-reference version?
>
> Something like:
>     template <typename PROJ>
>     const PROJ& addProjection(const PROJ proj, const std::string& name) {
>       std::cerr<<"Please be careful about using references!"<<std::endl;
>       return proj;
>     }

But we would not want to prevent *registering* projections by copy,
since that's in general absolutely fine, right? It's rather the
copying of a derived projection, e.g. VetoedFinalState, into a
FinalState that seems dangerous to me.

I have just tested that it's possible to hide the copy constructor,
and am attaching a patch for Rivet which shows how this would work in
real life. It's a bit ugly to have to hide this for each derived
FinalState explicitly, I don't know whether there is any better way to
disallow such "lossy" copying altogether for a base class?

With this patch Rivet compiles fine, and a few of the typical ZFinder
analyses seem to work as expected. Furthermore, an accidental VFS ->
FS copying like in my Example 1 now fails as expected and 2a and 2b
work as expected. But 2c is a bit strange... it compiles fine, but at
runtime fails with:
Error in MY_RFSTEST2c::init method: No projections registered for
parent 0x7fff33dc1630

I don't at all understand where that's coming from, but there is a lot
of casting in the projection registration going on. Any ideas? I'm
attaching a simple analysis that demonstrates this behaviour, together
with one Zee event for testing.

Cheers,
Frank
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