如何使用date.h在C ++中获取当天的当前日期？

Question

I am using C++ 14 and am trying to get the current day of week. After some reading I am using date.h by Howard E. Hinnant.

However I am struggling to get a day of the week (encoded as 0 thru 6).

Something like this prints out Thu :

 int main(void)
 {
     date::sys_days t;
     weekday wd{t};
     cout << wd << '\n';
 }

Many answers on this site regarding getting the current day of the week use chrono .

How can I print the current weekday as a range from 0-6 according to when the program is run, using date.h ?

For example, if I run the program today (Tuesday) I would expect a value of 2.

Any suggestions would be appreciated.

---

Just to be clear about what I am trying to achieve, something similar in Java:

Calendar calendar = Calendar.getInstance(TimeZone.getTimeZone("Europe/London"));
int dayOfWeek = calendar.get(Calendar.DAY_OF_WEEK);

Answer 1

The problem with your code is that t is not being initialized to anything interesting. You just need to set it to "now". There's a slight hitch, though: now() returns a different kind of time point, that can't automagically be converted into a date::sys_days value.

So, here's the minimal change to make your program work:

#include "date.h"
#include <iostream>
#include <cstdint>

int main()
{
     auto now = std::chrono::system_clock::now();
     date::sys_days t { std::chrono::time_point_cast<date::days>(now) };
     date::weekday wd{t};
     std::cout << wd << '\n';
}

edit: Now let's do a bit more, and thanks @HowardHinnant for the informative comments.

Use more meaningful names

Replace the code for main() with

auto now = std::chrono::system_clock::now();
date::sys_days now_in_days { std::chrono::time_point_cast<date::days>(now) };
date::weekday weekday {now_in_days};
std::cout << weekday << '\n';

Get the weekday as a number

You said you wanted to get the weekday as a number. Well, we can do that:

auto weekday_index = weekday.c_encoding();

the type will be unsigned , and the value will be in the range 0...6

Get the weekday in your own timezone

Your code and mine, so far, is UTC. Which is good as a default, but may give you something surprising as the weekday. We can use zoned_time and write:

auto now = std::chrono::system_clock::now();
auto now_local = zoned_time{current_zone(), now}.get_local_time();
date::sys_days now_local_in_days { std::chrono::time_point_cast<date::days>(now_local) };
auto weekday_index = now_local_in_days.c_encoding();

Gotcha for dates before the epoch (1970)

This is super annoying, but time_point_cast() may not always do what you want! Howard says that for days before 1970 we need to use floor() instead:

date::sys_days now_local_in_days { std::chrono::floor<date::days>(now_local) };

Final program

#include "date.h"
#include <iostream>
#include <cstdint>

int main()
{
    auto now = std::chrono::system_clock::now();
    auto now_local = zoned_time{current_zone(), now}.get_local_time();
    date::sys_days now_local_in_days { std::chrono::floor<date::days>(now_local) };
    auto weekday_index = now_local_in_days.c_encoding();
    std::cout << weekday_index << '\n';
}

And if today is Tuesday, the resulting output should be 2 .

Answer 2

This would give the index of the day using date.h:

using namespace date;
using namespace std::chrono;
auto now = system_clock::now();
std::cout << "The current time is " << now << " UTC\n";
auto current_day = (year_month_weekday{floor<days>(now)}.weekday() - Sunday).count();
std::cout << "current_day " << current_day << "\n";

---

you can also use std::time_get::get_weekday

#include <iostream>       // std::cout, std::ios
#include <sstream>        // std::istringstream
#include <ctime>          // std::tm
#include <locale>         // std::locale, std::time_get, std::use_facet

int main ()
{
  std::locale loc;        // classic "C" locale

  // get time_get facet:
  const std::time_get<char>& tmget = std::use_facet <std::time_get<char> > (loc);

  std::ios::iostate state;
  std::istringstream iss ("Friday");
  std::tm when;

  tmget.get_weekday (iss, std::time_get<char>::iter_type(), iss, state, &when);

  std::cout << "weekday: " << when.tm_wday << '\n';
  return 0;
}

or:

time_t t = time(nullptr);
tm* timePtr = localtime(&t);
uint32_t y = timePtr->tm_year + 1900;
uint32_t m = timePtr->tm_mon + 1;
uint32_t d = timePtr->tm_wday;
cout<< d<<endl;