Add rclcpp::Time::seconds() (#526)

* Get seconds since epoch as double

rclcpp/include/rclcpp/time.hpp

@@ -102,6 +102,13 @@ public:
   rcl_time_point_value_t
   nanoseconds() const;
 
+  /// \return the seconds since epoch as a floating point number.
+  /// \warning Depending on sizeof(double) there could be significant precision loss.
+  /// When an exact time is required use nanoseconds() instead.
+  RCLCPP_PUBLIC
+  double
+  seconds() const;
+
   RCLCPP_PUBLIC
   rcl_clock_type_t
   get_clock_type() const;

rclcpp/src/rclcpp/time.cpp

@@ -227,6 +227,12 @@ Time::nanoseconds() const
   return rcl_time_.nanoseconds;
 }
 
+double
+Time::seconds() const
+{
+  return std::chrono::duration<double>(std::chrono::nanoseconds(rcl_time_.nanoseconds)).count();
+}
+
 rcl_clock_type_t
 Time::get_clock_type() const
 {

rclcpp/test/test_time.cpp

@@ -244,3 +244,9 @@ TEST(TestTime, overflows) {
   rclcpp::Time two_time(2);
   EXPECT_NO_THROW(one_time - two_time);
 }
+
+TEST(TestTime, seconds) {
+  EXPECT_DOUBLE_EQ(0.0, rclcpp::Time(0, 0).seconds());
+  EXPECT_DOUBLE_EQ(4.5, rclcpp::Time(4, 500000000).seconds());
+  EXPECT_DOUBLE_EQ(2.5, rclcpp::Time(0, 2500000000).seconds());
+}