template specialization for all subclasses

Question

I would like to define a C++ template specialization that applies to all subclasses of a given base class. Is this possible?

In particular, I'd like to do this for STL's hash<>. hash<> is defined as an empty parametrized template, and a family of specializations for specific types:

template<class _Key>
  struct hash { };

template<>
  struct hash<char>
  {
    size_t
    operator()(char __x) const
    { return __x; }
  };

template<>
  struct hash<int>
  {
    size_t
    operator()(int __x) const
    { return __x; }
  };
...

I would like to define something like this:

template<class Base>
  struct hash {
    size_t operator()(const Base& b) const {
      return b.my_hash();
    }
  };

class Sub : public Base {
  public:
    size_t my_hash() const { ... }
};

and be able to use it like this:

hash_multiset<Sub> set_of_sub;
set_of_sub.insert(sub);

However, my hash template conflicts with the generic one from STL. Is there a way (perhaps using traits) to define a template specialization that applies to all subclasses of a given base class (without modifying the STL definitions)?

Note that I know I can do this with some extra template parameters whenever this hash specialization is needed, but I'd like to avoid this if possible:

template<>
  struct hash<Base> {
    size_t operator()(const Base& b) const {
      return b.my_hash();
    }
  };

....

// similar specialization of equal_to is needed here... I'm glossing over that...
hash_multiset<Sub, hash<Base>, equal_to<Base> > set_of_sub;
set_of_sub.insert(sub);

Answer 1

Since C++ 11 you can use SFINAE together with standard library enable_if and is_base_of to solve the problem.