[Boost-users] [mpl] C++ Template Metaprogramming book - Question

Hi, I've started reading Abrahams' and Gurtovoy's book, since I'm very interested in boost.mpl. Seems like a great book. I have a question regarding the 3rd chapter (3.1.4 section). Eventually, it says that operator* multiplies the runtime values (resulting in 6.0f) and the code uses transform to sum the meta-seq. template quantity::type> operator*(quantity x, quantity y) { typedef mpl::transform::type dim; return quantity(x.value() * y.value()); } 1. Why passing x and y by value? why not const& 2. How did we got 6f ? 3. How and when the mpl::transform is getting into action here? I cannot spot it. Thanks -- - Kobi