#include <boost/bind.hpp>
#include <boost/function.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,class A1> struct nargs<_bi::bind_t<R,F,_bi::list1<A1> > >{enum {value=1};};
template<class R, class F,class A1,class A2> struct nargs<_bi::bind_t<R,F,_bi::list2<A1,A2> > >{enum {value=2};};

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,_1));
    bind_nargs(boost::bind(&fn_2,_1,_2));
    // won't compile (needs specialization as above)
    // bind_nargs(boost::bind(&fn_2,_1,_1,_1));
}