functor: add the identity functor

functor.hpp

@@ -13,6 +13,20 @@
 
 
 namespace cruft::functor {
+    /// Returns the supplied argument unchanged
+    ///
+    /// CXX#20: Revisit when C++20 support is available. This is just a
+    /// trivial implementation of std::identity.
+    struct identity {
+        template <typename ValueT>
+        ValueT&&
+        operator() (ValueT &&val)
+        {
+            return std::forward<ValueT> (val);
+        }
+    };
+
+
     /// A trivial functor that wraps std::begin without any static typing.
     struct begin {
         template <typename valuet>