I just tried to make a zip iterator out of two vector back_insert_iterators and got a very curious compile failure: /usr/include/boost/iterator/iterator_facade.hpp:653: error: invalid parameter type 'void' /usr/include/boost/iterator/iterator_facade.hpp:653: error: in declaration 'typename boost::detail::operator_brackets_result<Derived, Value, Reference>::type boost::iterator_facade<I, V, TC, R, D>::operator[](Difference) const' /usr/include/boost/iterator/iterator_facade.hpp:693: error: invalid parameter type 'void' /usr/include/boost/iterator/iterator_facade.hpp:693: error: in declaration 'Derived& boost::iterator_facade<I, V, TC, R, D>::operator+=(Difference)' /usr/include/boost/iterator/iterator_facade.hpp:699: error: invalid parameter type 'void' /usr/include/boost/iterator/iterator_facade.hpp:699: error: in declaration 'Derived& boost::iterator_facade<I, V, TC, R, D>::operator-=(Difference)' /usr/include/boost/iterator/iterator_facade.hpp:705: error: invalid parameter type 'void' /usr/include/boost/iterator/iterator_facade.hpp:705: error: in declaration 'Derived boost::iterator_facade<I, V, TC, R, D>::operator-(Difference) const' It seems that even though back_insert_iterator has an output_iterator tag, iterator_facade still trying to define random access operators. If assignment and incrementing are the only valid operations on an iterator (as is the case with output iterators), then doesn't it make sense to support a difference_type of void? Is this a curiosity of my stdlib implementation? A Boost.Iterator library oversight? A Boost.Iterator conscious design decision? Or a loose nut behind the keyboard trying something he shouldn't? I'm using version 1.35.0 on Ubuntu 8.04 x86_64 with g++ version 4.2.3. Here's the code I used to get this error: #include <iterator> #include <iostream> #include <boost/iterator/zip_iterator.hpp> #include <vector> #include <algorithm> using namespace std; using namespace boost; int main() { vector<int> ha1; vector<double> ha2; make_zip_iterator( make_tuple( back_inserter(ha1), back_inserter(ha2) ) ); return 0; } Thanks, Kris