Add Portability section to AsciiDoc

doc/hash/portability.adoc

@@ -2,3 +2,81 @@
 = Portability
 
 :idprefix: portability_
+
+xref:#ref_hash[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 xref:#ref_hash[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:
+
+[list,subs="+quotes,+macros"]
+----
+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)
+        {
+            xref:#ref_hash[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:
+
+[listing,subs="+quotes,+macros"]
+----
+namespace foo
+{
+    template <class T>
+    class custom_type
+    {
+        T value;
+    public:
+        custom_type(T x) : value(x) {}
+
+        std::size_t hash(custom_type x)
+        {
+            xref:#ref_hash[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:
+
+[listing]
+----
+#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 link:../../examples/portable.cpp[/libs/container_hash/examples/portable.cpp^].