Hi Jeffrey, hi all, I believe the usability of counting iterator could be improved, by value-initializing its Iterator type in the default constructor, because integral types won't be initialized by its default constructor. This can lead to hard to recognize user errors, e.g., when wrapping such a counting iterator with a filter iterator. counting_iterator is special in that sense, because to my understanding it is intended to adapt integral types to become iterators. And a default initialized scalar type, isn't. I suggest a one line change: counting_iterator() { } should become counting_iterator() : super_t(Incrementable()) { } The following example program shows the interesting UB, because the 3rd argument of make_filter_iterator, default constructs a counting_iterator<unsigned>(). Any thoughts? Objections? Teachings... Regards Peter. =====>8 #include <iostream> #include <boost/iterator/filter_iterator.hpp> #include <boost/iterator/counting_iterator.hpp> #include <algorithm> #include <iterator> #include <functional> struct is_prime{ bool operator()(unsigned int val) const { if (val < 2) return false; using std::placeholders::_1; auto ce=boost::make_counting_iterator(val); return ce==std::find_if_not(boost::make_counting_iterator(2u),ce, bind(std::modulus<unsigned>{},val,_1)); } }; int main(){ std::copy_n(boost::make_filter_iterator(is_prime{},boost::make_counting_iterator(1u) // if the following line is omitted we have undefined behavior, because of uninitialized unsigned value // ,boost::make_counting_iterator(std::numeric_limits<unsigned>::max()) // must be different from above value ),40,std::ostream_iterator<unsigned>{std::cout,", "}); } =====8< -- Prof. Peter Sommerlad Institut für Software: Bessere Software - Einfach, Schneller! HSR Hochschule für Technik Rapperswil Oberseestr 10, Postfach 1475, CH-8640 Rapperswil http://ifs.hsr.ch http://cute-test.com http://linticator.com http://includator.com tel:+41 55 222 49 84 == mobile:+41 79 432 23 32 fax:+41 55 222 46 29 == mailto:peter.sommerlad@hsr.ch