How can I make an unordered set of pairs of integers in C++?

Question

The following program does not compile an unordered set of pairs of integers, but it does for integers. Can unordered_set and its member functions be used on us

Answer 1

You are missing a hash function for std::pair>. For example,

struct bad_hash
{
  std::size_t operator()(const std::pair& p) const
  {
    return 42;
  }
};

....

std::unordered_set< std::pair, bad_hash> u_edge_;

You can also specialize std::hash for std::hash>, in which case you can omit the second template parameter.