From eff6500aca1dbc54584a4bfe4ac64b997484ea60 Mon Sep 17 00:00:00 2001
From: Esteve Fernandez <esteve@osrfoundation.org>
Date: Thu, 19 Nov 2015 17:17:01 -0800
Subject: [PATCH] Added support for deducing the return type of a functor

---
 rclcpp/include/rclcpp/function_traits.hpp |  4 ++
 rclcpp/test/test_function_traits.cpp      | 85 ++++++++++++++++++++++-
 2 files changed, 88 insertions(+), 1 deletion(-)

diff --git a/rclcpp/include/rclcpp/function_traits.hpp b/rclcpp/include/rclcpp/function_traits.hpp
index 0f8ef75..5e3b498 100644
--- a/rclcpp/include/rclcpp/function_traits.hpp
+++ b/rclcpp/include/rclcpp/function_traits.hpp
@@ -54,6 +54,8 @@ struct function_traits
 
   template<std::size_t N>
   using argument_type = typename std::tuple_element<N, arguments>::type;
+
+  using return_type = typename function_traits<decltype( & FunctionT::operator())>::return_type;
 };
 
 // Free functions
@@ -66,6 +68,8 @@ struct function_traits<ReturnTypeT(Args ...)>
 
   template<std::size_t N>
   using argument_type = typename std::tuple_element<N, arguments>::type;
+
+  using return_type = ReturnTypeT;
 };
 
 // Function pointers
diff --git a/rclcpp/test/test_function_traits.cpp b/rclcpp/test/test_function_traits.cpp
index 5475060..363e9d1 100644
--- a/rclcpp/test/test_function_traits.cpp
+++ b/rclcpp/test/test_function_traits.cpp
@@ -14,7 +14,9 @@
 
 #include <gtest/gtest.h>
 
-#include <rclcpp/function_traits.hpp>
+#include <string>
+
+#include "rclcpp/function_traits.hpp"
 
 int func_no_args()
 {
@@ -118,6 +120,42 @@ int func_accept_callback(FunctorT callback)
   return callback(a, b);
 }
 
+template<
+  typename FunctorT,
+  std::size_t Arity = 0,
+  typename std::enable_if<
+    rclcpp::function_traits::arity_comparator<Arity, FunctorT>::value
+  >::type * = nullptr,
+  typename std::enable_if<
+    std::is_same<
+      typename rclcpp::function_traits::function_traits<FunctorT>::return_type,
+      double
+    >::value
+  >::type * = nullptr
+>
+double func_accept_callback_return_type(FunctorT callback)
+{
+  return callback();
+}
+
+template<
+  typename FunctorT,
+  std::size_t Arity = 0,
+  typename std::enable_if<
+    rclcpp::function_traits::arity_comparator<Arity, FunctorT>::value
+  >::type * = nullptr,
+  typename std::enable_if<
+    std::is_same<
+      typename rclcpp::function_traits::function_traits<FunctorT>::return_type,
+      std::string
+    >::value
+  >::type * = nullptr
+>
+std::string func_accept_callback_return_type(FunctorT callback)
+{
+  return callback();
+}
+
 /*
    Tests that funcion_traits calculates arity of several functors.
  */
@@ -452,6 +490,39 @@ TEST(TestFunctionTraits, same_arguments) {
     "Functor and lambda have the same arguments");
 }
 
+TEST(TestFunctionTraits, return_type) {
+  // Test regular function
+  static_assert(
+    std::is_same<
+      rclcpp::function_traits::function_traits<decltype(func_no_args)>::return_type,
+      int
+    >::value,
+    "Functor return ints");
+
+  // Test lambda
+  auto lambda_one_int_return_double = [](int one) -> double {
+      (void)one;
+      return 1.0;
+    };
+
+  static_assert(
+    std::is_same<
+      rclcpp::function_traits::function_traits<
+        decltype(lambda_one_int_return_double)
+      >::return_type,
+      double
+    >::value,
+    "Lambda returns a double");
+
+  // Test objects that have a call operator
+  static_assert(
+    std::is_same<
+      rclcpp::function_traits::function_traits<FunctionObjectNoArgs>::return_type,
+      int
+    >::value,
+    "Functor return ints");
+}
+
 /*
    Tests that functions are matched via SFINAE.
  */
@@ -500,4 +571,16 @@ TEST(TestFunctionTraits, sfinae_match) {
   EXPECT_EQ(2, func_accept_callback(FunctionObjectTwoInts()));
 
   EXPECT_EQ(3, func_accept_callback(FunctionObjectOneIntOneChar()));
+
+  auto lambda_no_args_double = []() -> double {
+      return 123.45;
+    };
+
+  auto lambda_no_args_string = []() -> std::string {
+      return std::string("foo");
+    };
+
+  EXPECT_EQ(123.45, func_accept_callback_return_type(lambda_no_args_double));
+
+  EXPECT_EQ("foo", func_accept_callback_return_type(lambda_no_args_string));
 }