Hi! I would like to write code like int main() { namespace l = boost::lambda; std::map<int, double> M; M[1] = 5.0; M[3] = 7.0; std::for_each(M.begin(), M.end(), l::select2nd(l::_1) *= 6.0); } Is it hard to implement this for boost::lambda? Markus
Accessing data members can be done, but it's unfortunately a bit more complicated than: _1.second I just wish . could be overloaded. Example code below. Cheers, Jaakko -------------------- #include "boost/lambda/lambda.hpp" #include "boost/lambda/bind.hpp" #include <algorithm> #include <map> #include <iostream> // A function object class that takes returns a second argument of a pair. // This can be used with bind. struct select2nd { // Tell LL how to get the return type. Args will be tuple<select2nd, pair<int, double> >, // so get the element at index 1. template <class Args> struct sig { typedef typename boost::tuples::element<1, Args>::type pair_t; typedef typename boost::add_reference<typename pair_t::second_type>::type type; }; template <class T> typename boost::add_reference<typename T::second_type>::type operator()(T& t) const { return t.second; } }; int main() { namespace l = boost::lambda; std::map<int, double> M; M[1] = 5.0; M[3] = 7.0; std::for_each(M.begin(), M.end(), std::cout << l::bind(select2nd(), l::_1) << " "); std::cout << std::endl; std::for_each(M.begin(), M.end(), l::bind(select2nd(), l::_1) *= 6.0); std::for_each(M.begin(), M.end(), std::cout << l::bind(select2nd(), l::_1) << " "); std::cout << std::endl; // alternatively, use bind to refer directly to the data member. You need to // spell out the full type of the pair though. std::for_each(M.begin(), M.end(), l::bind(&std::map<int, double>::value_type::second, l::_1) *= 6.0); std::for_each(M.begin(), M.end(), std::cout << l::bind(select2nd(), l::_1) << " "); std::cout << std::endl; } -------------------