In the code snippet below (which is inside a vis object)  if the line    //g[*ei].edge_Weight = "Heavy"; is uncommented, compiling gives :error const discarded ( or something to that effect) because of the const in  (const Graph & g) . But then how come put(m_eWeight_map, *ei, "Heavy"); compiles successfully and successfully changes the value? Does   put(m_eWeight_map, *ei, "Heavy");  override the const in void Change_data(const Graph & g) ?