Re: [Boost-users] mpl::fold problems

Subject: Re: [Boost-users] mpl::fold problems
From: Stefan Strasser (strasser_at_[hidden])
Date: 2010-08-23 09:47:18

Zitat von sergey kostanbaev <sergey.kostanbaev_at_[hidden]>:
> template <class T, unsigned space=1> struct tr_field2
> {
>    static const unsigned count = space;
> }
>
> typedef boost::mpl::vector4<
>    tr_field2<int,  4>,
>    tr_field2<int,  7>,
>    tr_field2<char, 1>,
>    tr_field2<char, 1>
>> Test_description;
>
>
> I want to calculate a sum of all tr_field2::count
> I've read in the docs mpl::fold is for the case.
>
> I tried to compose
>
>    typedef boost::mpl::fold<Test_description,
>       boost::mpl::int_<0>,
>       boost::mpl::plus<boost::mpl::_, boost::mpl::_::count > >
>    >::type fcount;
>
> but it won't compile. It seems I misundestood something.

template<typename T>
struct get_count{
     typedef mpl::int_<T::count> type;
};

typedef boost::mpl::fold<Test_description,
   boost::mpl::int_<0>,
   boost::mpl::plus<boost::mpl::_1, get_count<mpl::_2> >
> ::type fcount;

the placeholders are not evaluated by mpl::_::count.
and mpl::plus expects a mpl integral constant, not a value.