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## Portability

`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/functional/hash/examples/portable.cpp.