Std :: unordered_set <Foo> as a member of the Foo class

Question

I am writing a class that has a unordered_set of its type as a member. Therefore, I need to write a specialization for hash<Foo>. This specialization should be determined after the declaration of Foo. But it seems to me that I already need a specialization for hash<Foo>, before defining a member unordered_set<Foo>. At least it does not compile and does not work. I tried declaring a hash template ahead, but couldn't get it to work.

Corresponding code snippet:

class Foo {
public:
    int i;
    std::unordered_set<Foo> dummy;
    Peer(std::unordered_set<Foo>);
};

namespace std {
    template<> struct hash<Foo>
    {
        size_t operator()(const Foo& f) const
        {
            return hash<int>()(f.i);
        }
    };
}

Thanks in advance

Answer 1

Foocannot have a member variable of type std::unordered_set<Foo>.

. , , , , } .

- (, std::unique_ptr<Foo>), , , (, Boost ​​ ).

Answer 2

, :

class Foo;

namespace std {
  template<> struct hash<Foo> {
    size_t operator()(const Foo& f) const;
  };
}

class Foo {
public:
  int i;
  std::unordered_set<Foo> dummy;
  Foo(std::unordered_set<Foo>);
};

namespace std {
  size_t hash<Foo>::operator()(const Foo& f) const {
    return hash<int>()(f.i);
  }
}

, dummy undefined.

; operator== Foo.

, Foo const-reference.