C++20: Basic Chrono Terminology with Time Duration and Time Point

Today, I continue my journey through the basic types with time durations and time points.

Time Durations

C++14 introduced helper types such as std::chrono::seconds for time durations and corresponding time literals such as 5s. C++20 added new helper types. The following table shows all for completeness.

Often the time duration std::chrono::days and the calendar date std::chrono::day are mixed up. The same holds for the time duration std::chrono::years and the calendar date std::chrono::year.

Distinguish between the time durations `std::chrono::days`, `std::chrono::years`, and the calendar types `std::chrono::day`, `std::chrono::year`

C++20 added two new literals for new calendar types std::chrono::day and std::chrono::year. The literals d and y refer to a std::chrono::day and std::chrono::year.

The day literal represents a day of the month and is unspecified if outside the range [0, 255].
The year literal represents a year in the Gregorian calendar and is unspecified if outside the range [-32767, 32767].

The following program emphasizes the difference between std::chrono::days and std::chrono::day and, accordingly, std::chrono::years and std::chrono::year.

// dayDays.cpp

#include <iostream>
#include <chrono>

int main() {

    std::cout << '\n';

    using namespace std::chrono_literals; 

    std::chrono::days days1 = std::chrono::day(30) - std::chrono::day(25);          // (1)
    std::chrono::days days2 = 30d - 25d;                                            // (3)
    if ( days1 == days2 && 
         days1 == std::chrono::days(5)) std::cout << "Five days\n";

    std::chrono::years years1 = std::chrono::year(2021) - std::chrono::year(1998);  // (2)
    std::chrono::years years2= 2021y - 1998y;                                       // (4)
    if ( years1 == years2 && 
         years1 == std::chrono::years(23)) std::cout << "Twenty-three years\n";

    std::cout << '\n';

}

When you subtract two objects of type std::chrono::day (line 1), you get an object of type std::chrono::days. The same holds for the std::chrono::year (lines 2) and std::chrono::years. Thanks to the using declaration using namespace std::chrono_literals, I can directly specify the time literals for std::chrono::day and std::chrono::year (lines 3 and 4).

Include Literals

There are various ways to include the literals.

using namespace std::literals;  
using namespace std::chrono;
using namespace std::chrono_literals;
using namespace std::literals::chrono_literals;

using namespace std::literals: includes all C++ literals
using namespace std::chrono: includes the entire namespace std::chrono
using namespace std::chrono_literals: includes all time literals
using namespace std::literals::chrono_literals: includes all time literals

The program literals.cpp shows the use of different using declarations.

// literals.cpp

#include <chrono>
#include <string>

int main() {

    {
        using namespace std::literals;

        std::string cppString = "C++ string literal"s;     // (1)
        auto aMinute = 60s;                                // (2)
        // duration aHour = 0.25h + 15min + 1800s;
    }

    {
        using namespace std::chrono;

        // std::string cppString = "C++ string literal"s;
        auto aMinute = 60s;
        duration aHour = 0.25h + 15min + 1800s;            // (3)
    }

    {
        using namespace std::chrono_literals;

        // std::string cppString = "C++ String literal"s;
        auto aMinute = 60s;
        // duration aHour = 0.25h + 15min + 1800s;
    }

}

The using namespace std::literals declarations enable it to use all built-in literals such as string literal ("C++ string literal"s in line 1) or the time literal (60s in line 2). `std::chrono::duration`cannot be used unqualified. On the contrary, the using declaration using namespace std::chrono allows it to use the time literals and the type std::chrono::duration (line 3) unqualified: duration aHour = 0.25h + 15min + 1800s. Thanks to the using declaration using namespace::std::chrono::literals, all time literals are available.

Time Points

Besides the clock and the time duration, the third fundamental type in C++11 was std::chrono::time_point.

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template<typename Clock, typename Duration = typename Clock::duration> 
class time_point;

A std::chrono::time_point depends on the clock and the time duration. C++20 provides aliases for additional time points.

With the exception of std::chrono::steady_clock, you can define a time point with the specified time duration. All but not the clock std::chrono::file_clock enables it to specify it for seconds. Additionally, std::chrono::local_t and std::chrono::system_clock enables to specify it for days.

What’s Next?

std::chrono::hh_mm_ss is the time duration since midnight, split into hours, minutes, seconds, and fractional seconds. This new data type in C++20 stands for the time of day.

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