1 files changed, 39 insertions, 5 deletions
diff --git a/TCLAP/tclap/ArgTraits.h b/TCLAP/tclap/ArgTraits.h
index 7ae85a9..0ccf5e3 100644
--- a/TCLAP/tclap/ArgTraits.h
+++ b/TCLAP/tclap/ArgTraits.h
@@ -5,6 +5,7 @@
  *  file:  ArgTraits.h
  *
  *  Copyright (c) 2007, Daniel Aarno, Michael E. Smoot .
+ *  Copyright (c) 2017 Google LLC
  *  All rights reserved.
  *
  *  See the file COPYING in the top directory of this distribution for
@@ -73,13 +74,46 @@ struct ValueLikeTrait {
  * Arg traits are used to get compile type specialization when parsing
  * argument values. Using an ArgTraits you can specify the way that
  * values gets assigned to any particular type during parsing. The two
- * supported types are StringLike and ValueLike.
+ * supported types are StringLike and ValueLike. ValueLike is the
+ * default and means that operator>> will be used to assign values to
+ * the type.
  */
 template<typename T>
-struct ArgTraits {
-    typedef typename T::ValueCategory ValueCategory;
-	virtual ~ArgTraits() {}
-    //typedef ValueLike ValueCategory;
+class ArgTraits {
+	// This is a bit silly, but what we want to do is:
+	// 1) If there exists a specialization of ArgTraits for type X,
+	// use it.
+	//
+	// 2) If no specialization exists but X has the typename
+	// X::ValueCategory, use the specialization for X::ValueCategory.
+	//
+	// 3) If neither (1) nor (2) defines the trait, use the default
+	// which is ValueLike.
+
+	// This is the "how":
+	//
+	// test<T>(0) (where 0 is the NULL ptr) will match
+	// test(typename C::ValueCategory*) iff type T has the
+	// corresponding typedef. If it does not test(...) will be
+	// matched. This allows us to determine if T::ValueCategory
+	// exists by checking the sizeof for the test function (return
+	// value must have different sizeof).
+	template<typename C> static short test(typename C::ValueCategory*);
+	template<typename C> static long  test(...);
+	static const bool hasTrait = sizeof(test<T>(0)) == sizeof(short);
+
+	template <typename C, bool>
+	struct DefaultArgTrait {
+		typedef ValueLike ValueCategory;
+	};
+
+	template <typename C>
+	struct DefaultArgTrait<C, true> {
+		typedef typename C::ValueCategory ValueCategory;
+ 	};
+
+public:
+	typedef typename DefaultArgTrait<T, hasTrait>::ValueCategory ValueCategory;
 };
 
 } // namespace