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`rclcpp::Clock(RCL_ROS_TIME).now()` vs. `node_->get_clock()->now()`

asked 2021-12-02 04:11:19 -0500

galou gravatar image

Is rclcpp::Clock(RCL_ROS_TIME).now() always the same as node_->get_clock()->now()?

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answered 2022-02-23 07:28:07 -0500

galou gravatar image

updated 2022-02-23 07:29:13 -0500

Well, it looks that the answer is NO. I had some issues with this and wasn't satisfied with the answer so I wrote a small node to test these clocks. The test is carried out with Galactic. The complete code can be found at and the source is below

#include <chrono>
#include <string>

#include <rclcpp/rclcpp.hpp>

using namespace std::chrono_literals;

class TestClockNode : public rclcpp::Node

        const std::string & node_name,
        const rclcpp::NodeOptions & options = rclcpp::NodeOptions()) :
      rclcpp::Node{node_name, options}
      timer_ = create_wall_timer(1s, [this] () {timerCallback();});


    rclcpp::TimerBase::SharedPtr timer_;
    void timerCallback()
      RCLCPP_INFO_STREAM(get_logger(), "now(): " << now().seconds());
      RCLCPP_INFO_STREAM(get_logger(), "  rclcpp::Clock{}.now(): " << rclcpp::Clock{}.now().seconds());
      RCLCPP_INFO_STREAM(get_logger(), "  rclcpp::Clock{RCL_ROS_TIME}.now(): " << rclcpp::Clock{RCL_ROS_TIME}.now().seconds());
      RCLCPP_INFO_STREAM(get_logger(), "  rclcpp::Clock{RCL_SYSTEM_TIME}.now(): " << rclcpp::Clock{RCL_SYSTEM_TIME}.now().seconds());
      RCLCPP_INFO_STREAM(get_logger(), "  rclcpp::Clock{RCL_STEADY_TIME}.now(): " << rclcpp::Clock{RCL_STEADY_TIME}.now().seconds());


main(int argc, char * argv[])
  rclcpp::init(argc, argv);

  auto node = std::make_shared<TestClockNode>("test_clock");
  if (node == nullptr)
    return 1;

  auto executor = rclcpp::executors::SingleThreadedExecutor{};


  return 0;

You need a running simulated time provider (i.e. a simulator) to test and you need to launch with

ros2 run test_clock test_clock --ros-args --param use_sim_time:=true --

Example of output:

[INFO] [1645622804.092181382] [test_clock]: now(): 2216.74
[INFO] [1645622804.092376229] [test_clock]:   rclcpp::Clock{}.now(): 1.64562e+09
[INFO] [1645622804.092445844] [test_clock]:   rclcpp::Clock{RCL_ROS_TIME}.now(): 1.64562e+09
[INFO] [1645622804.092512516] [test_clock]:   rclcpp::Clock{RCL_SYSTEM_TIME}.now(): 1.64562e+09
[INFO] [1645622804.092567862] [test_clock]:   rclcpp::Clock{RCL_STEADY_TIME}.now(): 199532
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I see the results, but looking at the source I'm really not sure why there would be a difference between the two. rclcpp::Node defaults to a clock of type RCL_ROS_TIME, and the node_->now() function simply delegates to the clock's now() function instead.

aprotyas gravatar image aprotyas  ( 2022-02-27 03:02:01 -0500 )edit

answered 2021-12-04 15:13:30 -0500

aprotyas gravatar image

Well, if the rclcpp::Clock object on which you'll call now() refers to the clock belonging to the rclcpp::Node object, then yes, they are the same. These can semantically mean different things if you're talking about different clocks.

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answered 2021-12-02 20:50:55 -0500

osilva gravatar image

Looking at the source code for both, I believe they are the same:

For rcrclcpp::Clock(RCL_ROS_TIME).now()source code: https://docs.ros2.y org/foxy/api/rclcpp/clock_8hpp_source.html

And for node_->get_clock()->now()source code:

Both are declared the same way:


The only difference I see is the first one is more direct method .

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Asked: 2021-12-02 04:11:19 -0500

Seen: 94 times

Last updated: Feb 23