用std :: chrono显示日，月和年？

Question

I read that I can show the current day, month and year with std::chrono but how I can do that?

// Example program
#include <iostream>
#include <string>
#include <chrono>

int main()
{
  using namespace std::chrono;
  cout << std::chrono::day;
}

I do this code but it doesn't work, I always receive this

 error: 'day' is not a member of 'std::chrono

What I'm doing wrong?

Answer 1

std::put_time is what you need:

#include <chrono>
#include <iostream>
#include <iomanip>
#include <ctime>    

int main()
{
    auto now = std::chrono::system_clock::to_time_t(std::chrono::system_clock::now());
    std::cout << std::put_time(std::localtime(&now), "%Y-%m-%d") << "\n";
}

Prints:

2018-05-14

Answer 2

A different approach based on std:strftime :

#include <iomanip>
#include <ctime>
#include <chrono>
#include <iostream>

int main()
{
    auto now_c = std::chrono::system_clock::to_time_t(std::chrono::system_clock::now());
    std::tm ptm;
    localtime_s(&ptm, &now_c);
    char buffer[80];
    std::strftime(buffer, sizeof(buffer), "%Y-%m-%d %H:%M:%S ", &ptm);
    std::cout << buffer;
}

Result:

2018-05-14 19:33:11

( localtime_s is outside of std namespace and uses a slightly different interface)