Counting keys in a hash_multimap

What is the most efficient way to count all the different keys in a hash_multimap?

E.g. if I have a al开发者_开发技巧ready filled hash_multimap, (e.g. a container where you can store multiple entities with a same key) how can i retrieve the set of keys?

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Insert the keys into a std::set and find the size of the set.