What is the fastest way to get a random element from an unordered_set? [duplicate]

This question already has an answer here:

• Select random element in an unordered_map 4 answers
• Faster way to read/write a std::unordered_map from/to a file 5 answers

The question itself is pretty straightforward.

I know that I could generate a random value R between 0 and the size of the container, create an iterator over it and advance it by R, but this seems to be O(R) and I think there could be a faster way to get random elements inside an unordered_set.