Time in C++20: New data types for time of day and calendar date
Time of day
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After the articles on the basic data types for the chrono functionality, this time I’m looking at a new data type in C++20.
Rainer Grimm has been working as a software architect, team and training manager for many years. He enjoys writing articles on the programming languages C++, Python and Haskell, but also enjoys speaking frequently at specialist conferences. On his blog Modern C++ he deals intensively with his passion C++.
std::chrono::hh_mm_ss is the duration since midnight, divided into hours, minutes, seconds and fractions of seconds. This data type is typically used as a formatting tool. The following table initially gives a brief overview of the std::chrono::hh_mm_ss instance tOfDay.
Depending on the time duration used, the static member returns the corresponding tOfDay.fractional_width. If no such value for fractional_width in range [0, 18] exists, fractional_width is equal to 6. See for example std::chrono::duration> in the following table.
Using the std::chrono::hh_mm_ss data type is quite simple.
// timeOfDay.cpp
#include
#include
int main() {
using namespace std::chrono_literals;
std::cout In (1), I create a new instance of std::chrono::hh_mm_ss: timeOfDay. Thanks to the Chrono literals from C++14, I can add a few time durations to initialize a Time of Day object. With C++20 you can output timeOfDay directly (2). The rest should be easy to read. (4) returns the components of the time since midnight in hours, minutes, seconds and fractions of a second. (5) returns the time since midnight in seconds. (6) is more interesting: the seconds given correspond to the time shown in (2). (7) returns whether the specified hour is in the morning. (8) finally returns the 12-hour equivalent of the specified hour.
Here is the output of the program:
Calendar date
A new data type of the Chrono extension in C++20 is a calendar date. C++20 supports various ways to create and interact with a calendar date. First of all, what does a calendar date look like?
A Calendar date consists of a year, a month and a day. That’s why C++20 has a special data type std::chrono::year_month_day. C++20 has much more to offer. The following table is intended to provide an overview of the calendar types before I show different use cases.
thanks to the Cute Syntax one can use std::chrono::operator / to create Gregorian calendar dates.
Calendar data types support various operations. The following table provides an overview. For readability reasons, I partially ignore the std::chrono namespace.
The increment and decrement operations ++/– are supported in the prefix and postfix versions. Adding or subtracting +/- requires objects of data type std::chrono::duration. This means that if you make the difference between two objects of the data type std::chrono::day, you get an object of the type std::chrono::days. is the new three-way comparison operator.
The following program uses the operations for the calendar types:
// calendarOperations.cpp
#include
#include
int main() {
std::cout 2021y/July: ”
2021y/July) The program performs operations on std::chrono::month (1), std::chrono::weekday (2), std::chrono::year_month (3) and std::chrono::year_month_weekday_last (4).
When adding or subtracting the time period std::chrono::months, modulo operations are automatically applied (5 and 6). Subtracting two std::chrono::month objects equals 1 month. There are 2629746 seconds in a month (7). Accordingly, one can add a time period std::chrono::days to or subtract from the calendar dates std::chrono::day (8 and 9). Subtracting two std::chrono::day objects produces a std::chrono::days object. std::chrono::year_month allows subtraction (10), difference (11) and comparison of times (12). Objects of type std::chrono::weekday_last allow the addition/subtraction of the std::chrono::months and std::chrono::years time periods. Additionally, these std::chrono::weekday_last objects can be compared with each other.
What’s next?
C++20 supports constants and literals to make the use of data types more convenient. (rme)
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