Support using better-enum as key in dictionaries and maps (#77)

This enabled hashing so that unordered_map are supported.

enum.h

@@ -1283,4 +1283,15 @@ BETTER_ENUMS_CONSTEXPR_ map<Enum, T> make_map(T (*f)(Enum))
 
 }
 
+#define BETTER_ENUMS_DECLARE_STD_HASH(type)                                    \
+	namespace std {                                                            \
+    template <> struct hash<type>                                              \
+    {                                                                          \
+        size_t operator()(const type &x) const                                 \
+        {                                                                      \
+            return std::hash<size_t>()(x._to_integral());                      \
+        }                                                                      \
+    };                                                                         \
+	}
+
 #endif // #ifndef BETTER_ENUMS_ENUM_H

test/cxxtest/general.h

@@ -31,7 +31,10 @@ BETTER_ENUM(Namespaced, short, One, Two)
 // be changed to be more precise in the future.
 #ifdef BETTER_ENUMS_HAVE_CONSTEXPR_
 
+BETTER_ENUMS_DECLARE_STD_HASH(Channel)
+
 #include <type_traits>
+#include <functional>
 
 // Type properties.
 static_assert_1(std::is_class<Channel>());
@@ -161,6 +164,24 @@ static_assert_1(same_string(Depth::_names()[0], "HighColor"));
 
 
 // Run-time testing.
+class HashTests : public CxxTest::TestSuite {
+  public:
+    void test_same_values()
+    {
+#ifdef _ENUM_HAVE_CONSTEXPR
+        TS_ASSERT_EQUALS(
+            std::hash<Channel>().operator()(Channel::Red),
+            std::hash<int>().operator()(0));
+        TS_ASSERT_EQUALS(
+            std::hash<Channel>().operator()(Channel::Green),
+            std::hash<int>().operator()(1));
+        TS_ASSERT_EQUALS(
+            std::hash<Channel>().operator()(Channel::Blue),
+            std::hash<int>().operator()(2));
+#endif // #ifdef _ENUM_HAVE_CONSTEXPR
+    }
+};
+
 class EnumTests : public CxxTest::TestSuite {
   public:
     void test_constant_values()