How can I get a std::set of keys to a std::map?

You can use the versatile boost::transform_iterator to return an iterator that returns only the keys (and not the values). See http://stackoverflow.com/questions/110157/how-to-retrieve-all-keys-or-values-from-a-stdmap/110228#110228.

Map does guarantee order; that's why it's called a sorted associative container. You can use set_intersection with a custom comparator function, the second variant listed here.

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