#include <boost/bind.hpp>
#include <boost/function.hpp>
#include <boost/bind/arg.hpp>
#include <boost/mpl/min_max.hpp>
#include <iostream>

void fn_1(int){}
void fn_2(int,int){}
void fn_3(int,int,int){}
void fn_0(){}

namespace _bi=boost::_bi;

template<class BindTType>
struct nargs;

template<class R, class F> struct nargs<_bi::bind_t<R,F,_bi::list0> >{enum {value=0};};
template<class R, class F,int arity> 
struct nargs<
    _bi::bind_t<
    R,
    F,
    _bi::list1<boost::arg<arity> > 
> >
{enum {value=arity};};

template<int lhs, int rhs>
struct max
{
    enum{value=lhs > rhs?lhs:rhs};
};

template<class R, class F,int arity1, int arity2>
struct nargs<
    _bi::bind_t<
    R,
    F,
    _bi::list2<boost::arg<arity1>,boost::arg<arity2> >
> >
{
    enum{value=max<arity1,arity2>::value};
};

template<class BindExpr>
void bind_nargs(BindExpr)
{
    std::cout << nargs<BindExpr>::value << std::endl;
}

int main()
{
    bind_nargs(boost::bind(&fn_0));
    bind_nargs(boost::bind(&fn_1,_4));
    bind_nargs(boost::bind(&fn_2,_8,_9));
    bind_nargs(boost::bind(&fn_2,_1,_2));
    // bind_nargs(boost::bind(&fn_2,_1,_2));
    // won't compile (needs specialization as above)
    // bind_nargs(boost::bind(&fn_2,_1,_1,_1));
}