Chrono I/O

Chrono I/O consists of reading and writing chrono types. The various chrono types support unformatted writing and formatted one with the new formatting library.

This post is the ninth in my detailed journey through the chrono extension in C++20:


Most chrono types, such as time duration, time points, and calendar dates, support direct writing without format specification.


The following tables show the default output format. Let’s start with time durations.

Time Durations

The program displays values for each time duration.

// timeDurationsOutput.cpp

#include <chrono>
#include <iostream>

int main() {

    std::cout << '\n';

    using namespace std::chrono_literals;

    std::cout << "5ns: " << 5ns << '\n';
    std::cout << "std::chrono::nanoseconds(5): " 
              << std::chrono::nanoseconds(5) << '\n';

    std::cout << '\n';

    std::cout << "5ms: " << 5ms << '\n';
    std::cout << "std::chrono::microseconds(5): " 
              << std::chrono::microseconds(5) << '\n';

    std::cout << '\n';

    std::cout << "5us: " << 5us << '\n';
    std::cout << "std::chrono::milliseconds(5): " 
              << std::chrono::milliseconds(5) << '\n';

    std::cout << '\n';

    std::cout << "5s: " << 5s << '\n';
    std::cout << "std::chrono::seconds(5): " << std::chrono::seconds(5) << '\n';

    std::cout << '\n';

    std::cout << "5min: " << 5min << '\n';
    std::cout << "std::chrono::minutes(5): " << std::chrono::minutes(5) << '\n';

    std::cout << '\n';

    std::cout << "5h: " << 5h << '\n';
    std::cout << "std::chrono::hours(5): " << std::chrono::hours(5) << '\n';

    std::cout << '\n';

    std::cout << "std::chrono::days(5): " << std::chrono::days(5) << '\n';

    std::cout << '\n';

    std::cout << "std::chrono::weeks(5): " << std::chrono::weeks(5) << '\n';

    std::cout << '\n';

    std::cout << "std::chrono::months(5): " << std::chrono::months(5) << '\n';

    std::cout << '\n';

    std::cout << "std::chrono::years(5): " << std::chrono::years(5) << '\n';  

    std::cout << '\n';


The natural numbers in the square braces of std::chrono::weeks, std::chrono::months, and std::chrono::years represent the number of seconds.

Time Points

When you use the C++20 clocks static member function now, you get the date and the time in the following format.

year-month-day hours:minutes:seconds

The following program shows the current time using all C++20 clocks.

// timePointsOutput.cpp

#include <chrono>
#include <iostream>

int main() {

    std::cout << '\n';

    auto nowSystemClock = std::chrono::system_clock::now();
    std::cout << "nowSystemClock: " << nowSystemClock << '\n';

    auto nowSteadyClock = std::chrono::steady_clock::now();
    // std::cout << "nowSteadyClock: " << nowSteadyClock << '\n';    ERROR
    auto nowFileClock = std::chrono::file_clock::now();
    std::cout << "nowFileClock:   " << nowFileClock << '\n';

    auto nowGPSClock = std::chrono::gps_clock::now();
    std::cout << "nowGPSClock:    " << nowGPSClock << '\n';

    // auto nowlocal_tClock = std::chrono::local_t::now();           ERROR

    auto nowTAIClock = std::chrono::tai_clock::now();
    std::cout << "nowTAIClock:    " << nowTAIClock << '\n';

    auto nowUTCClock = std::chrono::utc_clock::now();
    std::cout << "nowUTCClock:    " << nowUTCClock << '\n';

    std::cout << '\n';


The program shows two interesting facts. First, the current time given by the std::chrono::steady_clock::now() cannot be displayed. Second, the pseudo clock std::chrono::local_t has no static member function now().

The GPS time is 18 seconds ahead of the UTC time. The TAI time is 37 seconds ahead of the UTC time and 19 seconds ahead of the GPS time.


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    Thanks to the C++17 function std::chrono::floor, you can display the time point in different granularitiy. In this case, the time point has to be of type std::chrono::local_time.

    // timePointsOutputGranularity.cpp
    #include <chrono>
    #include <iostream>
    int main() {
        std::cout << '\n';
        auto now = std::chrono::system_clock::now();
        auto zonedTime = std::chrono::zoned_time(std::chrono::current_zone(), now); 
        auto localTime = zonedTime.get_local_time();
        std::cout << "local_time:                                               " 
                  << localTime << '\n';
        std::cout << "std::chrono::floor<std::chrono::microseconds>(localTime): " 
                  << std::chrono::floor<std::chrono::microseconds>(localTime) << '\n'; 
        std::cout << "std::chrono::floor<std::chrono::milliseconds>(localTime): "
                  << std::chrono::floor<std::chrono::milliseconds>(localTime) << '\n';
        std::cout << "std::chrono::floor<std::chrono::seconds>(localTime):      "
                  << std::chrono::floor<std::chrono::seconds>(localTime) << '\n';
        std::cout << "std::chrono::floor<std::chrono::minutes>(localTime):      "
                  << std::chrono::floor<std::chrono::minutes>(localTime) << '\n';
        std::cout << "std::chrono::floor<std::chrono::hours>(localTime):        " 
                  << std::chrono::floor<std::chrono::hours>(localTime) << '\n';
        std::cout << "std::chrono::floor<std::chrono::days>(localTime):         "
                  << std::chrono::floor<std::chrono::days>(localTime) << '\n';
        std::cout << "std::chrono::floor<std::chrono::weeks>(localTime):        "
                  << std::chrono::floor<std::chrono::weeks>(localTime) << '\n';
        // std::cout << std::chrono::floor<std::chrono::months>(localTime) << '\n';    ERROR
        // std::cout << std::chrono::floor<std::chrono::years>(localTime) << '\n';     ERROR
        std::cout << '\n';

    The program displays localTime in different accuracies, starting with the time duration std::chrono::microseconds and ending with std::chrono::weeks. Curiously, the time durations for std::chrono::months, and std::chrono::years cannot be displayed, but this will be fixed with C++23.

    What’s Next?

    Additionally, you can display Calendar Dates unformatted.

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