How to convert chrono::seconds to string in HH:MM:SS format in C++?

Question

I have a function which accepts second as argument and returns a string in HH:MM:SS format. Without std::chrono, I can implement it like this:

string myclass::ElapsedTime(long secs) {
  uint32_t hh = secs / 3600;
  uint32_t mm = (secs % 3600) / 60;
  uint32_t ss = (secs % 3600) % 60;
  char timestring[9];
  sprintf(timestring, "%02d:%02d:%02d", hh,mm,ss);
  return string(timestring);
}

Using std::chrono, I can convert the argument to std::chrono::seconds sec {seconds};.

But the how can I convert it to string with the format? I saw the good video tutorial from Howard Hinnant in https://youtu.be/P32hvk8b13M. Unfortunately, there is no example for this case.

Answer 1

Using Howard Hinnant's header-only date.h library it looks like ths:

#include "date/date.h"
#include <string>

std::string
ElapsedTime(std::chrono::seconds secs)
{
    return date::format("%T", secs);
}

If you want to write it yourself, then it looks more like:

#include <chrono>
#include <string>

std::string
ElapsedTime(std::chrono::seconds secs)
{
    using namespace std;
    using namespace std::chrono;
    bool neg = secs < 0s;
    if (neg)
        secs = -secs;
    auto h = duration_cast<hours>(secs);
    secs -= h;
    auto m = duration_cast<minutes>(secs);
    secs -= m;
    std::string result;
    if (neg)
        result.push_back('-');
    if (h < 10h)
        result.push_back('0');
    result += to_string(h/1h);
    result += ':';
    if (m < 10min)
        result.push_back('0');
    result += to_string(m/1min);
    result += ':';
    if (secs < 10s)
        result.push_back('0');
    result += to_string(secs/1s);
    return result;
}

In C++20, you'll be able to say:

std::string
ElapsedTime(std::chrono::seconds secs)
{
    return std::format("{:%T}", secs);
}