class Foo
{
public:
  std::unordered_set<Foo>::iterator findClosest(std::unordered_set<Foo> const &others)
  {
    return std::end(others);
  }

  size_t hashValue() const {
    return std::hash<int>()(m_Member);
  }

private:
  int m_Member;
};

namespace std
{
  template <>
  struct hash<Foo>
  {
    size_t operator()(Foo const & bar) const
    {
      return bar.hashValue();
    }
  };
}