Unordered set of pairs, compilation error

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That error message appears when you have failed to specialise std::hash or provide a hasher type for your unordered container (see e.g. Using C++11 unordered_set in Visual C++ and clang). The XCode error in this case is particularly unfriendly!

C++11 does not provide a hash for pairs (or tuples), even of hashable types. This discussion indicates that this was primarily due to a lack of time to get anything better in; however I’m not aware whether there will be anything better in C++14.

Specialising std::hash<std::pair<int, int>> is probably not a good idea (and is not permitted by the language; specialising std templates is only allowed for user-defined types), so you’ll have to provide a hasher:

struct MoveHasher {
    std::size_t operator()(const std::pair<int, int> &val) const { ... }
};
typedef std::unordered_set<Move, MoveHasher> Set;

See How do I combine hash values in C++0x? for how to write the hash function.

Alternatively, you could make Move a user-defined class (probably a good idea!) and then specialising std::hash will be fine.

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