Chapter 66: Date and time using <chrono> header

Section 66.1: Measuring time using <chrono>

The system_clock can be used to measure the time elapsed during some part of a program's execution.

Version = c++11

#include <iostream> #include <chrono> #include <thread>

int main() {

auto start = std::chrono::system_clock::now(); // This and "end"'s type is std::chrono::time_point

{ // The code to test std::this_thread::sleep_for(std::chrono::seconds(2));

}

auto end = std::chrono::system_clock::now();

std::chrono::duration<double> elapsed = end - start; std::cout << "Elapsed time: " << elapsed.count() << "s";

}

In this example, sleep_for was used to make the active thread sleep for a time period measured in

std::chrono::seconds, but the code between braces could be any function call that takes some time to execute.

Section 66.2: Find number of days between two dates

This example shows how to find number of days between two dates. A date is specified by year/month/day of month, and additionally hour/minute/second.

Program calculates number of days in years since 2000.

#include <iostream> #include <string> #include <chrono> #include <ctime>

/***

*Creates a std::tm structure from raw date.

*\param year (must be 1900 or greater)

*\param month months since January – [1, 12]

*\param day day of the month – [1, 31]

*\param minutes minutes after the hour – [0, 59]

*\param seconds seconds after the minute – [0, 61](until C++11) / [0, 60] (since C++11)

*Based on http://en.cppreference.com/w/cpp/chrono/c/tm

*/

std::tm CreateTmStruct(int year, int month, int day, int hour, int minutes, int seconds) { struct tm tm_ret = {0};

tm_ret.tm_sec = seconds; tm_ret.tm_min = minutes; tm_ret.tm_hour = hour; tm_ret.tm_mday = day;

tm_ret.tm_mon = month - 1; tm_ret.tm_year = year - 1900;

return tm_ret;

}

int get_days_in_year(int year) {

using namespace std;

using namespace std::chrono;

// We want results to be in days

typedef duration<int, ratio_multiply<hours::period, ratio<24> >::type> days;

// Create start time span

std::tm tm_start = CreateTmStruct(year, 1, 1, 0, 0, 0);

auto tms = system_clock::from_time_t(std::mktime(&tm_start));

// Create end time span

std::tm tm_end = CreateTmStruct(year + 1, 1, 1, 0, 0, 0); auto tme = system_clock::from_time_t(std::mktime(&tm_end));

// Calculate time duration between those two dates

auto diff_in_days = std::chrono::duration_cast<days>(tme - tms);

return diff_in_days.count();

}

int main()

{

for ( int year = 2000; year <= 2016; ++year )

std::cout << "There are " << get_days_in_year(year) << " days in " << year << "\n";

}