mpl::foldでlengthメタ関数

C++ 
#include <boost/static_assert.hpp>
#include <boost/mpl/list_c.hpp>
#include <boost/mpl/fold.hpp>
#include <boost/mpl/plus.hpp>

using namespace boost::mpl;

template <class Seq>
struct length {
    static const int value = fold<Seq, int_<0>, plus<_1, int_<1> > >::type::value;
};

int main()
{
    typedef list_c<int, 1, 2, 3, 4, 5> ls;

    BOOST_STATIC_ASSERT(length<ls>::value == 5);
}