A slightly old version of the flat_map documentation <http://www.boost.org/doc/libs/1_49_0/doc/html/boost_container_header_reference.html#header.boost.container.flat_map_hpp> defines flat_map's Pred wrong: template<typename Key, typename T, typename Pred = std::less< std::pair< Key, T> >, typename A = std::allocator<T> > class flat_map; It should be std::less<Key>. Why do I care about a slightly old version, you might ask? Well, the current version doesn't seem to document it at all! If I go to the "latest version" link, <http://www.boost.org/doc/libs/1_55_0/doc/html/boost_container_header_reference.html#header.boost.container.flat_set_hpp> doesn't show any definition for flat_map itself, just for comparison operators and other non-member templates. I can't find any means of navigating into the actual documentation for the class members, assuming it still exists. This is generally true for the other containers on that web page.