Outputting Date and Time in C++ using std:: chrono

In C++, the std::chrono library provides powerful tools for date and time processing. However, std::chrono itself focuses on measuring time points (time_point) and durations (duration), rather than directly handling the formatted output of calendar dates and clock times. Starting from C++20, we can utilize new features in std::chrono for formatting date and time output. Prior to C++20, we typically combine std::chrono with other libraries, such as , to output dates and times.

The following is an example demonstrating how to use std::chrono and in C++11 to obtain and output the current date and time:

cpp
#include <iostream>
#include <chrono>
#include <ctime>

int main() {
    // Get the current time point
    std::chrono::system_clock::time_point now = std::chrono::system_clock::now();
    
    // Convert to time_t to facilitate conversion to a human-readable format
    std::time_t now_time_t = std::chrono::system_clock::to_time_t(now);

    // Convert to local time
    std::tm* now_tm = std::localtime(&now_time_t);

    // Output the current date and time
    std::cout << "Current date and time: "
              << 1900 + now_tm->tm_year << '-'  // Year
              << 1 + now_tm->tm_mon << '-'      // Month
              << now_tm->tm_mday << ' '         // Day
              << now_tm->tm_hour << ':'         // Hour
              << now_tm->tm_min << ':'          // Minute
              << now_tm->tm_sec << std::endl;   // Second

    return 0;
}

In this example, we first use std::chrono::system_clock::now() to obtain the current time point. Then, we use std::chrono::system_clock::to_time_t() to convert the time point to time_t, which is easier to convert to a human-readable format. Using std::localtime() converts time_t to std::tm structure, which contains detailed breakdown of calendar dates and times.

Output example:

shell
Current date and time: 2023-9-24 14:35:22

This method combines the high-precision time point measurement of std::chrono with traditional C library time formatting capabilities. For those requiring C++20 features, we can use std::format to directly format std::chrono types, which makes the code more concise and straightforward.