Copying data vs passing by reference about pennylane-lightning HOT 2 CLOSED

Thanks for the info @nquesada!
So far we've been following the discussion on Eigen in the pybind docs: https://pybind11.readthedocs.io/en/stable/advanced/cast/eigen.html#pass-by-reference

They recommend using Eigen::Ref<MatrixType>, e.g., doing

void scale_by_2(Eigen::Ref<Eigen::VectorXd> v) {
    v *= 2;
}

This allows passing by reference, but with some caveats. Luckily we just want to pass a vector so we might get away with this choice and avoid the complications (e.g. row major vs column major).

However it's good to know of this alternative!
Thanks!

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Thanks @nquesada, let us know if you see any other things we need to be mindful about 👍

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