[Rivet] Dangerous casting to FinalState

[David Bjergaard]

Hi Frank,

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;
    }

I dunno if that will even compile. If its not too ugly, I think you
could do something with pointers instead of references, I know you can
properly specialize on those cases.

    Dave


Frank Siegert <frank.siegert at cern.ch> writes:

> 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
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