Hi, In this simple code, I expect that the size of the fusion::set is equal to one, because keys are the same. Why the size is equal to two? #include <boost/fusion/container/set.hpp> #include <boost/fusion/include/set.hpp> #include <boost/fusion/container/set/set_fwd.hpp> #include <boost/fusion/include/set_fwd.hpp> #include <boost/fusion/sequence/intrinsic/size.hpp> #include <boost/fusion/include/size.hpp> #include <iostream> int main() { typedef boost::fusion::set<int, int> set1; set1 s1; std::cout << "size1 = " << boost::fusion::result_of::size<set1>::type::value << ", size2 = " << boost::fusion::size(s1) << std::endl; } http://liveworkspace.org/code/166b54cdd208f7b252bf1f38a1a6ec4b Thanks! -- Regards, niXman ___________________________________________________ Dual-target(i686/x86_64) MinGW compiler for i686/x86_64 hosts: http://sourceforge.net/projects/mingwbuilds/