"Dan" <dan@eloff.info> wrote
How does this _1 and _2 stuff work anyway?
From my experience, in order to understand this stuff, you first have to temporarily forget about _1 and _2 (unless you are already fluent with Boost.Lambda).
An MPL algorithm (fold, iter_fold, etc.) actually accepts 3 parameters: 1) sequence; 2) initial type (state); 3) metafunction class. The third parameter defines how to create a new state from the old state and the current element (or iterator) of the sequence, for your example something like this: struct adder { template<class State, class Current> struct apply { typedef typename mpl::plus<State, Current>::type type; }; }; (this may not compile -- I am not sure about the order of template parameters) The adder is analogous to a functor in STL, and apply is analogous to the function call operator. Having defined this "functor", you can get your result something like this: typedef mpl::fold<MySequence, mpl::int_<0>, adder>::type MyResult; As far as _1, _2, etc., is concerned -- this is MPL Lambda facility, it allows to define metafunction classes right in-place, in the same way Boost.Lambda does for runtime functors. I believe (I may be wrong of course) that MPL Lambda is more complicated than the rest of MPL, but, in short, _1 stands for the first parameter of apply, and _2 -- for the second parameter. When you write: mpl::plus<mpl::_2, mpl::_1> this effectively (by the means of MPL Lambda) defines a metafunction class ("functor") that acts in the same way as the "adder" does. HTH, Arkadiy