Consider the following program: #include <iostream> #include <boost/fusion/algorithm.hpp> #include <boost/fusion/container.hpp> #include <boost/fusion/sequence.hpp> int main() { using namespace boost::fusion; map<> m; result_of::push_back<const map<>, pair<int, int> >::type m1 = push_back(m, pair<int, int>(0)); bool b1 = has_key<int>(m1); // b1 == false bool b2 = has_key<int>(as_map(m1)); // b2 == true int sz = size(m1); // sz == 1 return 0; } According to documentation [1], result of push_back is a model of Associative Sequence if its sequence parameter implements the Associative Sequence model. Therefore, I'd expect the variable b1 to be evaluated to true, not false. Is this the intended behavior? I get the same results with push_front and insert. Tested with boost 1.43 and 1.44. [1] http://www.boost.org/doc/libs/1_44_0/libs/fusion/doc/html/fusion/algorithm/t... -- Best regards, Alexander Fokin, ru.elric@gmail.com.