Hi, I'm trying to use boost::make_transform_iterator to create an iterator for a custom class whose data is held in a map and where the iterator uses the vector of keys to access the values. In my problem, the values of the map are containers which hold large data. Since I can't afford to copy the data, I would like to access the data by reference via the iterator. However, when doing this, the data is corrupted, as is exemplified by the output of the simple example I have attached. As far as I can tell the problem lies in the use of the from_key functor, which is initialised using a reference to the map and is corrupted when passed through make_transform_iterator. Any ideas how I could do this properly using boost? Thanks, Patrick #include <iostream> #include <string> #include <vector> #include <boost/unordered_map.hpp> #include <boost/iterator/transform_ iterator.hpp> #include <boost/assign.hpp> #include <boost/assign/std/vector.hpp> #include <boost/lambda/lambda.hpp> #include <boost/lambda/bind.hpp> #include <boost/ref.hpp> using namespace boost::assign; namespace bl = boost::lambda; class holder { public: holder() : v() {}; holder( const std::vector<double>& in ) : v(in) {}; std::vector<double>& vector() { return v; }; const std::vector<double>& vector() const { return v; }; private: std::vector<double> v; }; class from_key { public: typedef holder result_type; from_key( const boost::unordered_map<std::string, holder >& m ) : map_(m) {}; const holder& operator() ( const std::string& in ) const { return map_.at(in); }; private: const boost::unordered_map<std::string, holder >& map_; }; typedef boost::transform_iterator<from_key, std::vector<std::string>::iterator > iterator; int main() { std::vector<std::string> keys; keys += "1","2","3"; std::vector<double> vals; vals += 1.0, 2.0, 3.0; holder h(vals); boost::unordered_map<std::string, holder > m; insert( m ) ( "1", h ) ( "2", h ) ( "3", h ); iterator it = boost::make_transform_iterator( keys.begin(), from_key( m ) ); iterator end = boost::make_transform_iterator( keys.begin(), from_key( m ) ); const std::vector<double>& v = it->vector(); std::for_each( vals.begin(), vals.end(), std::cout << bl::_1 << " " ); std::cout << std::endl; std::for_each( v.begin(), v.end(), std::cout << bl::_1 << " " ); std::cout << std::endl; }