Have a look at your compare operator! It orders two pairs of values (A,B)(C,D) like this: (A

... typedef multiset Edges; ... struct ltSubgraph : public std::binary_function { bool operator() (const DataEdge& s1, const DataEdge& s2) const { return s1.m_source < s2.m_source || (!(s1.m_source < s2.m_source) && s1.m_target < s2.m_target); } };