Dear all, in our company we use the STL map frequently. However one often wants to iterate over the key_type only (or mapped type, or make desicions for the key type, and take action on its corresponded map type). With the transform_iterator this is possible, but the resulting code is not an improvement over writing the loop explicitly. For example I want to search for the maximum string length: typedef std::map<std::string, double> StringDoubleMap; typedef boost::function<StringDoubleMap::key_type (const StringDoubleMap::value_type&)> Function; typedef boost::transform_iterator<Function, StringDoubleMap::const_iterator> TransformIterator; StringDoubleMap mp = boost::assign::map_list_of("George", 0.0) ("Bill", 0.0) ("Ronald", 0.0) ("Jimmy", 0.0); Function fc = boost::bind(&StringDoubleMap::value_type::first, _1); TransformIterator itBegin(mp.begin(), fc); TransformIterator itEnd(mp.end(), fc); TransformIterator it = std::max_element(itBegin, itEnd, boost::bind (&std::string::length, _1) < boost::bind(&std::string::length, _2)); if (it != itEnd) { std::cout << it->c_str() << ' ' << it->length() << std::endl; } A select1st or select2nd would already a great help. Wkr, me