In looking at the documentation for the intrinsic metafunction mpl::at, it seems to suggest that a three argument version that returns a default type when a key is not found exists for associative sequences. I don't see this implemented anywhere in the code, and the simple example below doesn't compile. Am I missing something or is this an error in the documentation? Thanks, Nate #include <boost/mpl/map.hpp> #include <boost/mpl/at.hpp> #include <boost/mpl/pair.hpp> #include <boost/mpl/assert.hpp> #include <boost/mpl/int.hpp> #include <boost/type_traits/is_same.hpp> using namespace boost::mpl; int main() { typedef map< pair<int_<2>,int_<3> > > map_t; typedef at<map_t, int_<1>, int_<0> >::type missing; BOOST_MPL_ASSERT(( boost::is_same< missing, int_<0> > )); } With gcc 4.0.1 and Boost 1.37 on OS X I get test.cpp: In function 'int main()': test.cpp:12: error: wrong number of template arguments (3, should be 2) ../BoostInstall/boost_1_37_0/boost/mpl/at_fwd.hpp:20: error: provided for 'template<class Sequence, class N> struct boost::mpl::at' ...