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

Question

I have been upgrading some old code and have been trying to update to c++11 where possible. The following code is how I used to display the time and date in my program

#include <iostream> #include <string> #include <stdio.h> #include <time.h>  const std::string return_current_time_and_date() const {     time_t now = time(0);     struct tm tstruct;     char buf[80];     tstruct = *localtime(&now);     strftime(buf, sizeof(buf), "%Y-%m-%d %X", &tstruct);     return buf; } 

I would like to output the current time and date in a similar format using std::chrono(or similar) but am unsure how to go about doing so. Any help would be greatly appreciated. Thanks

Answer 1

The <chrono> library only deals with time and not dates, except for the system_clock which has the ability to convert its timepoints to time_t. So using <chrono> for dates will not improve things much. Hopefully we get something like chrono::date in the not too distant future.

That said, you can use <chrono> in the following way:

#include <chrono>  // chrono::system_clock #include <ctime>   // localtime #include <sstream> // stringstream #include <iomanip> // put_time #include <string>  // string  std::string return_current_time_and_date() {     auto now = std::chrono::system_clock::now();     auto in_time_t = std::chrono::system_clock::to_time_t(now);      std::stringstream ss;     ss << std::put_time(std::localtime(&in_time_t), "%Y-%m-%d %X");     return ss.str(); } 

Note that std::localtime may cause data races. localtime_r or similar functions may be available on your platforms.

Update:

Using a new version of Howard Hinnant's date library you can write:

#include "date.h" #include <chrono> #include <string> #include <sstream>  std::string return_current_time_and_date() {   auto now = std::chrono::system_clock::now();   auto today = date::floor<days>(now);    std::stringstream ss;   ss << today << ' ' << date::make_time(now - today) << " UTC";   return ss.str(); } 

This will print out something like "2015-07-24 05:15:34.043473124 UTC".

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On an unrelated note, returning const objects has become undesirable with C++11; const return values cannot be moved from. I also removed the trailing const because trailing const is only valid for member functions and this function has no need to be a member.

Answer 2

An example:

#include <iostream> #include <chrono> #include <ctime>  std::string getTimeStr(){     std::time_t now = std::chrono::system_clock::to_time_t(std::chrono::system_clock::now());      std::string s(30, '\0');     std::strftime(&s[0], s.size(), "%Y-%m-%d %H:%M:%S", std::localtime(&now));     return s; } int main(){      std::cout<<getTimeStr()<<std::endl;     return 0;  } 

Output as below: