Comparison of Duration/Seconds in C++ using chrono doesnt work as supposed?

Question

EDIT: Works just fine, I messed up in another place of my code.

I am trying to increase an Integer once every minute, using C++11 chrono library. For some reasons, the comparison does not work as it should: It just returns true every time. Is something wrong with the cast to seconds? Shouldn't the result be an int, containing the difference of both time points in seconds?

Would really appreciate your help! Here's the Code:

std::chrono::time_point<std::chrono::system_clock> starttime = std::chrono::system_clock::now();

int timeLine = 0;

int main() {
    while (true) {
        std::chrono::time_point<std::chrono::system_clock> now = std::chrono::system_clock::now();

        int seconds = timeLine * 60;

        if ((std::chrono::duration_cast<std::chrono::seconds>(starttime - now)).count() + seconds <= 0) {
            timeLine++;
            nextConstellation();
            cout << "Timeline: " << timeLine << endl;

        }
    }
}

[This doesn't even compile](http://ideone.com/ocdngP). If I fix it by adding `` to the `time_points` it works as expected. — Hatted Rooster, Apr 07 '17 at 19:05
I am very sorry, did the definitions in another place in my code, and forgot to copy the edited my post. Thanks for telling me that it works for you, still cant figure out why it doesnt for me. — user3808217, Apr 07 '17 at 19:08
Works as expected for me, can you give us `nextConstellation()`? — Hatted Rooster, Apr 07 '17 at 19:08
Alright guys, sorry for bothering you, I messed up in my code structure and did skip the initialization of starttime. The above works, as Gill Bates told and verified. Thanks for the effort! — user3808217, Apr 07 '17 at 19:15
If you leave the question up I can help you improve the readability of the code without changing the semantics. — Howard Hinnant, Apr 07 '17 at 19:17
@HowardHinnant But there's no problem statement anymore, so I don't see the purpose. — Hatted Rooster, Apr 07 '17 at 19:18
@GillBates: The purpose is to help people learn how to properly use ``. — Howard Hinnant, Apr 07 '17 at 19:19
I hesitate to post an answer, but this should not check the time. It should instead std::this_thread::sleep_for(starttime + std::chrono::minutes(timeLine) - now); or just sleep_for(1min) — Kenny Ostrom, Apr 07 '17 at 19:44
@KennyOstrom: Yeah, I can't argue with you there. Here's a similar answer I gave yesterday and had to show it both ways: http://stackoverflow.com/a/43261119/576911 — Howard Hinnant, Apr 07 '17 at 21:43

Howard Hinnant · Answer 1 · 2017-04-07T19:24:16.533

Here is a safer, and more readable way to write this code:

std::chrono::time_point<std::chrono::system_clock> starttime = std::chrono::system_clock::now();

int timeLine = 0;

int main() {
    while (true) {
        std::chrono::time_point<std::chrono::system_clock> now = std::chrono::system_clock::now();

        std::chrono::seconds seconds = timeLine * std::chrono::minutes{1};

        if (starttime - now + seconds <= std::chrono::seconds{0}) {
            timeLine++;
            nextConstellation();
            std::cout << "Timeline: " << timeLine << std::endl;

        }
    }
}

In a nutshell, stay within the chrono-type system, and trust it to do the units conversions for you, implicitly wherever possible.

Or perhaps even more simply:

    // ...
    auto timelimit = timeLine * std::chrono::minutes{1};

    if (now  - starttime >= timelimit) {
    // ...

And if in C++14, add using namespace std::chrono_literals and:

    auto timelimit = timeLine * 1min;

Comparison of Duration/Seconds in C++ using chrono doesnt work as supposed?

1 Answers1