I try to compile next code: ===== my_test.cpp ===== #include <functional> #include <boost/bind.hpp> using namespace std; using namespace boost; struct my_assign_if { typedef void result_type; template<class T, class P> void operator()(T& dst, T const& src, P const& to_assign) const { if (to_assign()) dst = src; } }; int main() { int a=1, b=0; bind(my_assign_if(), ref(a), 0, _1)(bind(less<int>(), cref(b), 1)); // compiled bind(my_assign_if(), ref(a), 0, bind(less<int>(), cref(b), 1))(); // not compiled return 0; } ==================== and take next errors: Microsoft (R) 32-bit C/C++ Optimizing Compiler Version 13.10.3077 for 80x86 Copyright (C) Microsoft Corporation 1984-2002. All rights reserved. my_test.cpp my_test.cpp(11) : error C2064: term does not evaluate to a function taking 0 arguments C:\Alnik\heap\boost\boost-dev\boost\bind.hpp(335) : see reference to function template instantiation 'void my_assign_if::operator ()<T,boost::_bi::result_traits<R,F>::type>(T &,const T &,const P &) const' being compiled with [ T=int, R=boost::_bi::unspecified, F=std::less<int>, P=boost::_bi::result_traits<boost::_bi::unspecified,std::less<int>>::type ] C:\Alnik\heap\boost\boost-dev\boost\bind\bind_template.hpp(21) : see reference to function template instantiation 'R boost::_bi::list3<A1,A2,A3>::operator ()<boost::_bi::bind_t<boost::_bi::unspecified,F,L>::result_type,F,boost::_bi ::list0>(boost::_bi::type<T>,F,A &)' being compiled with [ R=boost::_bi::bind_t<boost::_bi::unspecified,my_assign_if,boost::_bi::list3< boost::_bi::list_av_3<boost::reference_wrapper<int>,int,boost::_bi::bind_t<b oost::_bi::unspecified,std::less<int>,boost::_bi::list2<boost::_bi::list_av_ 2<boost::reference_wrapper<const int>,int>::B1,boost::_bi::list_av_2<boost::reference_wrapper<const int>,int>::B2>>>::B1,boost::_bi::list_av_3<boost::reference_wrapper<int>,int ,boost::_bi::bind_t<boost::_bi::unspecified,std::less<int>,boost::_bi::list2 <boost::_bi::list_av_2<boost::reference_wrapper<const int>,int>::B1,boost::_bi::list_av_2<boost::reference_wrapper<const int>,int>::B2>>>::B2,boost::_bi::list_av_3<boost::reference_wrapper<int>,int ,boost::_bi::bind_t<boost::_bi::unspecified,std::less<int>,boost::_bi::list2 <boost::_bi::list_av_2<boost::reference_wrapper<const int>,int>::B1,boost::_bi::list_av_2<boost::reference_wrapper<const int>,int>::B2>>>::B3>>::result_type, A1=boost::_bi::list_av_3<boost::reference_wrapper<int>,int,boost::_bi::bind_ t<boost::_bi::unspecified,std::less<int>,boost::_bi::list2<boost::_bi::list_ av_2<boost::reference_wrapper<const int>,int>::B1,boost::_bi::list_av_2<boost::reference_wrapper<const int>,int>::B2>>>::B1, A2=boost::_bi::list_av_3<boost::reference_wrapper<int>,int,boost::_bi::bind_ t<boost::_bi::unspecified,std::less<int>,boost::_bi::list2<boost::_bi::list_ av_2<boost::reference_wrapper<const int>,int>::B1,boost::_bi::list_av_2<boost::reference_wrapper<const int>,int>::B2>>>::B2, A3=boost::_bi::list_av_3<boost::reference_wrapper<int>,int,boost::_bi::bind_ t<boost::_bi::unspecified,std::less<int>,boost::_bi::list2<boost::_bi::list_ av_2<boost::reference_wrapper<const int>,int>::B1,boost::_bi::list_av_2<boost::reference_wrapper<const int>,int>::B2>>>::B3, F=my_assign_if, L=boost::_bi::list3<boost::_bi::list_av_3<boost::reference_wrapper<int>,int, boost::_bi::bind_t<boost::_bi::unspecified,std::less<int>,boost::_bi::list2< boost::_bi::list_av_2<boost::reference_wrapper<const int>,int>::B1,boost::_bi::list_av_2<boost::reference_wrapper<const int>,int>::B2>>>::B1,boost::_bi::list_av_3<boost::reference_wrapper<int>,int ,boost::_bi::bind_t<boost::_bi::unspecified,std::less<int>,boost::_bi::list2 <boost::_bi::list_av_2<boost::reference_wrapper<const int>,int>::B1,boost::_bi::list_av_2<boost::reference_wrapper<const int>,int>::B2>>>::B2,boost::_bi::list_av_3<boost::reference_wrapper<int>,int ,boost::_bi::bind_t<boost::_bi::unspecified,std::less<int>,boost::_bi::list2 <boost::_bi::list_av_2<boost::reference_wrapper<const int>,int>::B1,boost::_bi::list_av_2<boost::reference_wrapper<const int>,int>::B2>>>::B3>, T=boost::_bi::bind_t<boost::_bi::unspecified,my_assign_if,boost::_bi::list3< boost::_bi::list_av_3<boost::reference_wrapper<int>,int,boost::_bi::bind_t<b oost::_bi::unspecified,std::less<int>,boost::_bi::list2<boost::_bi::list_av_ 2<boost::reference_wrapper<const int>,int>::B1,boost::_bi::list_av_2<boost::reference_wrapper<const int>,int>::B2>>>::B1,boost::_bi::list_av_3<boost::reference_wrapper<int>,int ,boost::_bi::bind_t<boost::_bi::unspecified,std::less<int>,boost::_bi::list2 <boost::_bi::list_av_2<boost::reference_wrapper<const int>,int>::B1,boost::_bi::list_av_2<boost::reference_wrapper<const int>,int>::B2>>>::B2,boost::_bi::list_av_3<boost::reference_wrapper<int>,int ,boost::_bi::bind_t<boost::_bi::unspecified,std::less<int>,boost::_bi::list2 <boost::_bi::list_av_2<boost::reference_wrapper<const int>,int>::B1,boost::_bi::list_av_2<boost::reference_wrapper<const int>,int>::B2>>>::B3>>::result_type, A=boost::_bi::list0 ] C:\Alnik\heap\boost\boost-dev\boost\bind\bind_template.hpp(19) : while compiling class-template member function 'boost::_bi::bind_t<R,F,L>::result_type boost::_bi::bind_t<R,F,L>::operator ()(void)' with [ R=boost::_bi::unspecified, F=my_assign_if, L=boost::_bi::list3<boost::_bi::list_av_3<boost::reference_wrapper<int>,int, boost::_bi::bind_t<boost::_bi::unspecified,std::less<int>,boost::_bi::list2< boost::_bi::list_av_2<boost::reference_wrapper<const int>,int>::B1,boost::_bi::list_av_2<boost::reference_wrapper<const int>,int>::B2>>>::B1,boost::_bi::list_av_3<boost::reference_wrapper<int>,int ,boost::_bi::bind_t<boost::_bi::unspecified,std::less<int>,boost::_bi::list2 <boost::_bi::list_av_2<boost::reference_wrapper<const int>,int>::B1,boost::_bi::list_av_2<boost::reference_wrapper<const int>,int>::B2>>>::B2,boost::_bi::list_av_3<boost::reference_wrapper<int>,int ,boost::_bi::bind_t<boost::_bi::unspecified,std::less<int>,boost::_bi::list2 <boost::_bi::list_av_2<boost::reference_wrapper<const int>,int>::B1,boost::_bi::list_av_2<boost::reference_wrapper<const int>,int>::B2>>>::B3> ] my_test.cpp(18) : see reference to class template instantiation 'boost::_bi::bind_t<R,F,L>' being compiled with [ R=boost::_bi::unspecified, F=my_assign_if, L=boost::_bi::list3<boost::_bi::list_av_3<boost::reference_wrapper<int>,int, boost::_bi::bind_t<boost::_bi::unspecified,std::less<int>,boost::_bi::list2< boost::_bi::list_av_2<boost::reference_wrapper<const int>,int>::B1,boost::_bi::list_av_2<boost::reference_wrapper<const int>,int>::B2>>>::B1,boost::_bi::list_av_3<boost::reference_wrapper<int>,int ,boost::_bi::bind_t<boost::_bi::unspecified,std::less<int>,boost::_bi::list2 <boost::_bi::list_av_2<boost::reference_wrapper<const int>,int>::B1,boost::_bi::list_av_2<boost::reference_wrapper<const int>,int>::B2>>>::B2,boost::_bi::list_av_3<boost::reference_wrapper<int>,int ,boost::_bi::bind_t<boost::_bi::unspecified,std::less<int>,boost::_bi::list2 <boost::_bi::list_av_2<boost::reference_wrapper<const int>,int>::B1,boost::_bi::list_av_2<boost::reference_wrapper<const int>,int>::B2>>>::B3> ] My question: why? or that can not be used by this way? Regards, Alexey.