関連ワード:  how to empty map in c++    how to clear map in c++    how to erase map in c++    how to delete map in c++    how to empty unordered_map in c++    how to initialize empty map in c++    how to make map empty in c++    how to create an empty map in c++    how to declare an empty map in c++    how to return an empty map in c++