Specializing std::hash for nested class in a template class

Question

I have a template class Baz which contains a nested class Sub. I'd like to define a hash function for this subclass by specializing std::hash. However, it doesn't seem to work.

#include <functional>

struct Foo {
    struct Sub {
    };
};

template <class T>
struct Bar {
};

template <class T>
struct Baz {
    struct Sub {
        int x;
    };
};

// declare hash for Foo::Sub - all right
namespace std {
    template <>
    struct hash< Foo::Sub >;
}

// declare hash for Bar<T> - all right
namespace std {
    template <class T>
    struct hash< Bar<T> >;
}

// declare hash function for Baz<T>::Sub - doesn't work!
namespace std {
    template <class T>
    struct hash< Baz<T>::Sub >;
}

// Adding typename produces a different error.
namespace std {
    template <class T>
    struct hash< typename Baz<T>::Sub >;
}

Gcc 4.5.3 complains:

$ g++ -std=c++0x -c hash.cpp
hash.cpp:34:30: error: type/value mismatch at argument 1 in template parameter list for ‘template<class _Tp> struct std::hash’
hash.cpp:34:30: error:   expected a type, got ‘Baz<T>::Sub’
hash.cpp:40:12: error: template parameters not used in partial specialization:
hash.cpp:40:12: error:         ‘T’

UPDATE

What I'm really trying to do is implement a container which supports stable references (not in the C++ sense) to elements within it. I want to allow the user to insert these references into std::unordered_set and similar, and use them to access or modify existing elements efficiently. The following is just a mockup, not the exact container I'm implementing. The problem is in defining a hash function for the reference type.

template <class T>
class Container {
public:
    class Reference {
    public:
        // operator==, operator!=, operator< ...., isNull()
    private:
        size_t index; // index into m_entries (or could be anything else)
        // possibly more stuff
    };

    Reference insert (const T &value);
    Reference find (const T &value);
    void remove (Reference r);
    Reference first ();
    Reference next (Reference prev);

private:
    struct Entry { T value, ... };

    std::vector<Entry> m_entries;
};

Answer 1

Just pull the Reference class out of Container.

template <class Container>
class Reference {
public:
    typedef typename Container::value_type value_type; // etc...

    // operator==, operator!=, operator< ...., isNull()
private:
    size_t index; // index into m_entries (or could be anything else)
    // possibly more stuff
};

template <class T>
class Container {
public:
    typedef ::Reference<Container> Reference;
    friend class Reference; // If you cannot help it

    typedef T value_type;

    Reference insert (const T &value);
    Reference find (const T &value);
    void remove (Reference r);
    Reference first ();
    Reference next (Reference prev);

private:
    struct Entry { T value, ... };

    std::vector<Entry> m_entries;
};

Specialize like this:

namespace std {
    template <typename Container>
    struct hash<Reference<Container>>;
}