How to get higher precision (fractions of a second) in a printout of current time?

Question

I've tried a couple methods to print out time from the system_clock but I can't get anything other than whole seconds:

system_clock::time_point now = system_clock::now();
std::time_t now_c = system_clock::to_time_t(now);

std::cout<<ctime(&now_c);
std::cout<<std::put_time(std::localtime(&now_c), "%T")<<" ";

Does the now() function actually hold high precision data, or is that I just can't find the function that extracts that information for printing?

Note: I am not looking to calculate a time interval. I want the current time with fractions of a second, and to print it out via cout. I just can't find a way to do this.

And I know about std::chrono::high_resolution_clock but also see no way to print out its now(). Additionally, the setprecision function has no effect on the output of put_time or ctime.

I've been getting answers that do not actually address this question.

Answer 1

You can do the following:

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

std::string GetLocalTime() {
  auto now(std::chrono::system_clock::now());
  auto seconds_since_epoch(
      std::chrono::duration_cast<std::chrono::seconds>(now.time_since_epoch()));

  // Construct time_t using 'seconds_since_epoch' rather than 'now' since it is
  // implementation-defined whether the value is rounded or truncated.
  std::time_t now_t(
      std::chrono::system_clock::to_time_t(
          std::chrono::system_clock::time_point(seconds_since_epoch)));

  char temp[10];
  if (!std::strftime(temp, 10, "%H:%M:%S.", std::localtime(&now_t)))
    return "";

  return std::string(temp) +
      std::to_string((now.time_since_epoch() - seconds_since_epoch).count());
}

int main() {
  std::cout << GetLocalTime() << '\n';
  return 0;
}

Answer 2

The existing answer is great, but it misses the leading zeros on the fractional part of the seconds, so 21:10:30.01 would be returned as 21:10:30.1

I don't have the rep to comment and my edit was rejected, so here's a fixed version:

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

std::string GetLocalTime() {
  auto now(std::chrono::system_clock::now());
  auto seconds_since_epoch(
    std::chrono::duration_cast<std::chrono::seconds>(now.time_since_epoch()));

  // Construct time_t using 'seconds_since_epoch' rather than 'now' since it is
  // implementation-defined whether the value is rounded or truncated.
  std::time_t now_t(
    std::chrono::system_clock::to_time_t(
      std::chrono::system_clock::time_point(seconds_since_epoch)));

  char temp[10];
  if (!std::strftime(temp, 10, "%H:%M:%S.", std::localtime(&now_t)))
    return "";

  std::string nanoseconds = std::to_string(
    (std::chrono::duration<long long, std::nano>(
      now.time_since_epoch() - seconds_since_epoch)).count());

  return std::string(temp) + std::string(9-nanoseconds.length(),'0') + nanoseconds;
}

int main() {
  std::cout << GetLocalTime() << '\n';
  return 0;
}