Conformance with C++ standard timers

[kunaltyagi]

Sometimes, I find the need to switch from system clock to ROS clock and it's not straightforward.

I read the C++ standard (a bit lazily) and found that the following code allows me to switch from std::chrono::steady_clock to ros::my_clock.

I don't know the following details

• type of clock used by ros::Time::now()
• where would this code go
• correctness of period as std::ratio<1,1>

namespace ros {
struct my_clock : public ros::Time
{
    using rep = double;
    using period = std::ratio<1, 1>;
    using duration = ros::Duration;
    using time_point = ros::Time;
    constexpr static bool is_steady(void) noexcept { return false; }
    static time_point now(void) noexcept(noexcept(ros::Time::now()))
    {
        return ros::Time::now();
    }
};
} // namespace ros

[cwecht]

As long as ros::WallTime::now() uses a time-source, which is not std::chrono-based, your question can not be anwsered reliably. For example Linux provides about three different clocks. As you can see here supports two different clock sources. libstdc++ seems to use the same (have a look here), but I think, that we should not rely on that. libc++ (LLVM) might do it differently and on Windows/Android this is a whole different story.

I'd discourage the use of differenct clouck sources. Use either ros::Time::now() or std::chrono::steady_clock but not both. Since ROS-components usually use ros::Time::now()/ros::WallTime::now(), I'd encourage you to use ros::Time::now().

If you need any other chrono-functionality, you can write conversion functions in a straight forward manner.

In the future, we could think about using std::chrono as a portable backend of ros::Time, but this would be be a rather extensive change.

[flixr]

If you want to use the steady clock there is ros::SteadyTime.

[kunaltyagi]

I have some code which is non-ROS and am porting it to work with ROS. For that, both clocks require same interface to access the time, and since boost and c++11 have the same structure, I found it easier to create a Clock wrapper for ROS Time. I'm proposing to create the same interface in ROS as in Boost and C++ standard for chrono utilities because it allows people to use code with similar semantics.

Eg: ROS::steady_clock vs boost::chrono::steady_clock vs std::chrono::steady_clock only require changing the class (well namespace, but templating over class is easier)

Currently, ROS has Time and not Clocks. I hope to constrain the discussion solely to adding clocks in ROS so that people (hopefully others apart from just me :wink: ) can use clocks without worrying about syntax error in moving from one library to another. Having a standard back-end for ROS is an implementation detail which is a different decision.

PS: If you didn't know about Boost Thread Clock and are debugging threading issues, look into it :smile: