Getting current time with millisecond precision using put_time in C++ - c++

Question

I am using the following code to get the current time in C++.
std::time_t t = std::time(nullptr);
std::time(&t);
std::cout << std::put_time(std::localtime(&t), "%X,");
However, this gives me time in HH::MM::SS. But for my application I also want to include time in milliseconds. Is there anyway to get something like HH::MM::SS::msecs using std::put_time?
Or what are the alternatives to get the system time with milliseconds precision inside a C++ program?

Answer 1

Here's one example using some C++11 <chrono> features. If you can use C++20, check out the new <chrono> features for more goodies or take a look at Howard Hinnants Date library.
#include <chrono>
#include <cstdint>
#include <ctime>
#include <iomanip>
#include <iostream>
#include <string>
#include <type_traits>
// A C++11 constexpr function template for counting decimals needed for
// selected precision.
template<std::size_t V, std::size_t C = 0,
typename std::enable_if<(V < 10), int>::type = 0>
constexpr std::size_t log10ish() {
return C;
}
template<std::size_t V, std::size_t C = 0,
typename std::enable_if<(V >= 10), int>::type = 0>
constexpr std::size_t log10ish() {
return log10ish<V / 10, C + 1>();
}
// A class to support using different precisions, chrono clocks and formats
template<class Precision = std::chrono::seconds,
class Clock = std::chrono::system_clock>
class log_watch {
public:
// some convenience typedefs and "decimal_width" for sub second precisions
using precision_type = Precision;
using ratio_type = typename precision_type::period;
using clock_type = Clock;
static constexpr auto decimal_width = log10ish<ratio_type{}.den>();
static_assert(ratio_type{}.num <= ratio_type{}.den,
"Only second or sub second precision supported");
static_assert(ratio_type{}.num == 1, "Unsupported precision parameter");
// default format: "%Y-%m-%dT%H:%M:%S"
log_watch(const std::string& format = "%FT%T") : m_format(format) {}
template<class P, class C>
friend std::ostream& operator<<(std::ostream&, const log_watch<P, C>&);
private:
std::string m_format;
};
template<class Precision, class Clock>
std::ostream& operator<<(std::ostream& os, const log_watch<Precision, Clock>& lw) {
// get current system clock
auto time_point = Clock::now();
// extract std::time_t from time_point
std::time_t t = Clock::to_time_t(time_point);
// output the part supported by std::tm
os << std::put_time(std::localtime(&t), lw.m_format.c_str());
// only involve chrono duration calc for displaying sub second precisions
if(lw.decimal_width) { // if constexpr( ... in C++17
// get duration since epoch
auto dur = time_point.time_since_epoch();
// extract the sub second part from the duration since epoch
auto ss =
std::chrono::duration_cast<Precision>(dur) % std::chrono::seconds{1};
// output the sub second part
os << std::setfill('0') << std::setw(lw.decimal_width) << ss.count();
}
return os;
}
int main() {
// default precision, clock and format
log_watch<> def_cp; // <= C++14
// log_watch def; // >= C++17
// alt. precision using alternative formats
log_watch<std::chrono::milliseconds> milli("%X,");
log_watch<std::chrono::microseconds> micro("%FT%T.");
// alt. precision and clock - only supported if the clock is an alias for
// system_clock
log_watch<std::chrono::nanoseconds,
std::chrono::high_resolution_clock> nano("%FT%T.");
std::cout << "def_cp: " << def_cp << "\n";
std::cout << "milli : " << milli << "\n";
std::cout << "micro : " << micro << "\n";
std::cout << "nano : " << nano << "\n";
}
Example output:
def_cp: 2019-11-21T13:44:07
milli : 13:44:07,871
micro : 2019-11-21T13:44:07.871939
nano : 2019-11-21T13:44:07.871986585

Answer 2

The accepted answer is a good one, but I would like to demonstrate doing this with an open-source, third-party, date handling library:
auto now = std::chrono::system_clock::now();
std::cout << date::format("%T", std::chrono::floor<std::chrono::milliseconds>(now));
This just output, for me: 10:01:46.654.
The fractional seconds separator is locale specific. In Sweden, where I reside, they use the comma as a separator. date::format allows supplying a std::locale, so we can force the use of the Swedish locale, for example, on my machine:
auto now = std::chrono::system_clock::now();
std::cout << date::format(std::locale("sv-SE"), "%T", std::chrono::floor<std::chrono::milliseconds>(now));
Now the ouput is: 10:02:32,169.
This formatting has been accepted in C++20, so you will get this when the vendors have implemented it :)
Of course, if the "%X" formatting is really want you want, then you cannot have decimal seconds and need to append them yourself:
auto now = std::chrono::system_clock::now();
auto ms = std::chrono::duration_cast<std::chrono::milliseconds>(now.time_since_epoch() % std::chrono::seconds{1});
std::cout << date::format("%X", std::chrono::floor<std::chrono::milliseconds>(now)) << "," << ms.count();
Note that I am using ms.count() because in C++20 streaming a duration will append the units as well, meaning that << ms would output something like 123ms.