Added sample with default_ trait.

samples/6-traits.cc

@@ -0,0 +1,49 @@
+// Using traits to capture project conventions on enums.
+
+#include <iostream>
+#include <enum.h>
+
+// Adopt the convention that the first value in an enum is the default value.
+template <typename Enum>
+constexpr const Enum default_()
+{
+    return Enum::_first;
+}
+
+// Make it possible to override the convention for specific enums.
+#define ENUM_DEFAULT(Enum, Default)                                            \
+    template <>                                                                \
+    constexpr const Enum default_<Enum>()                                      \
+    {                                                                          \
+        return Enum::Default;                                                  \
+    }
+
+
+
+// Default will be Red, because it is first.
+ENUM(Channel, int, Red, Green, Blue);
+
+// Default will be TrueColor, even though it is not first.
+ENUM(Depth, int, HighColor, TrueColor);
+ENUM_DEFAULT(Depth, TrueColor);
+
+
+
+int main()
+{
+    // Default construction can now be simulated for some purposes, and the
+    // default value is still declared in one place, not all over the program
+    // code.
+    Depth   depth = default_<Depth>();
+    std::cout << depth.to_string() << std::endl;
+
+    std::cout << default_<Channel>().to_string() << std::endl;
+    std::cout << default_<Depth>().to_string()   << std::endl;
+
+    return 0;
+}
+
+
+
+// Also works at compile-time.
+constexpr auto  value = default_<Channel>();