Added comments about the SFINAE voodoo

rclcpp/include/rclcpp/function_traits.hpp

@@ -25,6 +25,7 @@ namespace rclcpp
  * but unfortunately std::function's constructor on VS2015 is too greedy,
  * so we need a mechanism for checking the arity and the type of each argument
  * in a callback function.
+ * See http://blogs.msdn.com/b/vcblog/archive/2015/06/19/c-11-14-17-features-in-vs-2015-rtm.aspx
  */
 
 
@@ -58,6 +59,10 @@ struct function_traits<ReturnTypeT (ClassT::*)(Args ...) const>
   : public function_traits<ReturnTypeT(ClassT &, Args ...)>
 {};
 
+/* NOTE(esteve):
+ * VS2015 does not support expression SFINAE, so we're using this template to evaluate
+ * the arity of a function.
+ */
 template<std::size_t Arity, typename FunctorT>
 struct arity_comparator
 {

rclcpp/include/rclcpp/node.hpp

@@ -247,6 +247,16 @@ private:
     bool ignore_local_publications,
     typename message_memory_strategy::MessageMemoryStrategy<MessageT>::SharedPtr msg_mem_strat);
 
+/* NOTE(esteve):
+ * The following template machinery works around VS2015's lack of support for expression SFINAE:
+ * - We first declare the arity we want to match, i.e. 2 or 3.
+ * - Then we use the arity_comparator template to SFINAE on the arity of the passed functor.
+ * - Lastly, we SFINAE on the types of the arguments of the functor.
+ * These steps happen in different parts of the function signature because we want to stagger
+ * instantation of the templates because VS2015 can't conditionally enable templates that depend
+ * on another template.
+ * See test_function_traits.cpp for streamlined examples of how to use this pattern.
+ */
   template<
     typename ServiceT,
     typename FunctorT,