|  | Home | Libraries | People | FAQ | More | 
      boost::hash is written to be as portable
      as possible, but unfortunately, several older compilers don't support argument
      dependent lookup (ADL) - the mechanism used for customisation. On those compilers
      custom overloads for hash_value
      needs to be declared in the boost namespace.
    
      On a strictly standards compliant compiler, an overload defined in the boost
      namespace won't be found when boost::hash
      is instantiated, so for these compilers the overload should only be declared
      in the same namespace as the class.
    
Let's say we have a simple custom type:
namespace foo
{
    template <class T>
    class custom_type
    {
        T value;
    public:
        custom_type(T x) : value(x) {}
        friend std::size_t hash_value(custom_type x)
        {
            boost::hash<int> hasher;
            return hasher(x.value);
        }
    };
}
      On a compliant compiler, when hash_value
      is called for this type, it will look at the namespace inside the type and
      find hash_value but on a compiler
      which doesn't support ADL hash_value
      won't be found. To make things worse, some compilers which do support ADL won't
      find a friend class defined inside the class.
    
So first move the member function out of the class:
namespace foo
{
    template <class T>
    class custom_type
    {
        T value;
    public:
        custom_type(T x) : value(x) {}
        std::size_t hash(custom_type x)
        {
            boost::hash<T> hasher;
            return hasher(value);
        }
    };
    template <class T>
    inline std::size_t hash_value(custom_type<T> x)
    {
        return x.hash();
    }
}
Unfortunately, I couldn't declare hash_value as a friend, as some compilers don't support template friends, so instead I declared a member function to calculate the hash, and called it from hash_value.
For compilers which don't support ADL, hash_value needs to be defined in the boost namespace:
#ifdef BOOST_NO_ARGUMENT_DEPENDENT_LOOKUP namespace boost #else namespace foo #endif { template <class T> std::size_t hash_value(foo::custom_type<T> x) { return x.hash(); } }
Full code for this example is at /libs/container_hash/examples/portable.cpp.