try to implement a count_if using boost accumulator framework

boost_accumulator_count_if.cpp

#include <iostream>
#include <algorithm>
#include <functional>
#include <boost/mpl/placeholders.hpp>
#include <boost/accumulators/accumulators.hpp>
#include <boost/accumulators/statistics/count.hpp>
#include <boost/foreach.hpp>
#include <boost/parameter/keyword.hpp>


#define array_size(p) (sizeof(p)/sizeof(*p))
#define array_end(p) (p+array_size(p))

namespace boost { namespace accumulators {
BOOST_PARAMETER_NESTED_KEYWORD(tag, count_if_value, if_value)


        namespace impl{
                template<typename Sample,typename Tag >
                struct count_if_impl : accumulator_base{
                        typedef std::size_t result_type;

                        template<typename Args>
                        count_if_impl(Args const & args):
                                cnt(0),
                                if_value(args[count_if_value])
                        {}
                        template<typename Args>
                        void operator()(Args const &args){
                                if((args[parameter::keyword<Tag>::get()])< if_value){
                                        ++cnt;
                                }
                        }
                        result_type result(dont_care) const{
                                return cnt;
                        }
                private:
                        result_type cnt;
                        int if_value;
                };
        }
        namespace tag{
                struct count_if:depends_on<>,count_if_value {
                        typedef std::size_t result_type;
                        typedef accumulators::impl::count_if_impl<mpl::_1, tag::sample> impl;
                };
        }
}}

using namespace boost::accumulators;

int main(){
        int data[] = {8,2,5,1,5,7,2,5,2,6,8,2,2,4,7,1};
        accumulator_set< int, features< tag::count_if > > acc(tag::count_if::if_value=4);
        BOOST_FOREACH( int d ,data ){
                acc(d);
        }
        std::cout<<extract_result<tag::count_if >(acc)<<std::endl;
        return 0;
}