The map & set documentation all refer to the method "at" as having a default return value if no match is found, however I can only get the version without a default to compile and a look at the source at.hpp seems to indicate this is not implemented. Any help would be greatly appreciated. boost_1_33_1/libs/mpl/doc/refmanual/associative-sequence.html at<s,k>::type - works at<s,k,def>::type - does not compile (msvc8.0) Many thanks, Paul.