While studying boost.graph, I stumbled upon this constructor definition: template <class EdgeIterator> adjacency_list(EdgeIterator first, EdgeIterator last, vertices_size_type n, edges_size_type = 0, const GraphProperty& p = GraphProperty()) : Base(n, first, last), m_property(new graph_property_type(p)) { } I have 2 questions here: - What does "edges_size_type = 0," means here? Assigning a value to a TYPE, instead of a parameter? Can anyone please point me to an explanation of this grammar usage? - Why the necessary of edges_size_type = 0 parameter, since there's a range pair of "EdgeIterator first, EdgeIterator last"? Does anyone have an answer? Thanks.