[boost] Re: Learning boost::MPL

From: Maxim Yegorushkin (e-maxim_at_[hidden])
Date: 2004-07-01 02:58:17

Dan <dan_at_[hidden]> wrote:

> I'm trying to use iter_fold with a vector_c to compute the sum of all of the
> integers in the container.
>
> What I have is:
>
> template<typename Sequence>
> struct sum
> {
> 	typedef typename mpl::iter_fold<
> 		Sequence,
> 		typename mpl::begin<Sequence>::type,
> 		mpl::plus< mpl::deref<_1>, mpl::deref<_2> >;
> 		// ^ what should this really be?
> 	> // what do i put after here?
> };
>
> Which obviously won't work. I don't know how to keep a running total as I
> iterate over the sequence with plus.

You are very close:

template<typename Sequence>
struct sum
{
        typedef typename mpl::iter_fold<
                  mpl::iterator_range< // [begin + 1, end)
                            typename mpl::next<typename mpl::begin<Sequence>::type>::type
                          , typename mpl::end<Sequence>::type
		  >
                , typename mpl::deref<typename mpl::begin<Sequence>::type>::type
                , mpl::plus<mpl::deref<mpl::_2>, mpl::_1>
	>::type type;
};

Note that there must be a special case for empty sequences.

-- 
Maxim Yegorushkin