Return a reference providing modifiable access to the mapped‐value associated with the specified key; if this map does not already contain a value_type object having an equivalent key, first insert a new value_type object having key and a default‐constructed VALUE object, and return a reference to the newly mapped (default) value. This method requires that the (template parameter) type KEY be copy‐insertable into this map and the (template parameter) type VALUE be default‐insertable into this map (see {Requirements on KEY and VALUE}).