Both std::set and std::map are associative containers. The difference is that std::sets contain only the key, while in std::map there is an associated value. Choosing one over the other depends mainly on what the task at hand is. If you want to build a dictionary of all the words that appear in a text, you could use a std::set<std::string>, but if you also want to count how many times each word appeared (i.e. associate a value to the key) then you would need an std::map<std::string,int>. If you don’t need to associate that count, it does not make sense to have the int that is unnecessary.