Advice on converting timestamp string in "HH:MM:SS.microseconds" format - c++

Question

I'm given a list of timestamps (suppose we have a ready-made std::vector<std::string>) in a string format of a kind std::vector<std::string> = {"12:27:37.740002", "19:37:17.314002", "20:00:07.140902",...}. No dates, no timezones. What would be a preferable way to parse these strings to some kind of C++ type (std::chrono::time_point ?) to be able to perform some comparisons and sorting later.
For example: compare value, which was parsed from "20:00:07.140902" and value, was parsed from "20:00:07.000000".
C++17 is ok, but I can't use any third-party library (Boost, Date etc).
Keeping microseconds precision essential.

Answer 1

You can build this functionality completly with C++ standard library functionality.
For parsing the string use std::regex.
For time related datatypes use std::chrono
Example :
#include <stdexcept>
#include <regex>
#include <chrono>
#include <iostream>
auto parse_to_timepoint(const std::string& input)
{
// setup a regular expression to parse the input string
// https://regex101.com/
// each part between () is a group and will end up in the match
// [0-2] will match any character from 0 to 2 etc..
// [0-9]{6} will match exactly 6 digits
static const std::regex rx{ "([0-2][0-9]):([0-5][0-9]):([0-5][0-9])\\.([0-9]{6})" };
std::smatch match;
if (!std::regex_search(input, match, rx))
{
throw std::invalid_argument("input string is not a valid time string");
}
// convert each matched group to the corresponding value
// note match[0] is the complete matched string by the regular expression
// we only need the groups which start at index 1
const auto& hours = std::stoul(match[1]);
const auto& minutes = std::stoul(match[2]);
const auto& seconds = std::stoul(match[3]);
const auto& microseconds = std::stoul(match[4]);
// build up a duration
std::chrono::high_resolution_clock::duration duration{};
duration += std::chrono::hours(hours);
duration += std::chrono::minutes(minutes);
duration += std::chrono::seconds(seconds);
duration += std::chrono::microseconds(microseconds);
// then return a time_point (note this will not help you with correctly handling day boundaries)
// since there is no date in the input string
return std::chrono::high_resolution_clock::time_point{ duration };
}
int main()
{
std::string input1{ "20:00:07.140902" };
std::string input2{ "20:00:07.000000" };
auto tp1 = parse_to_timepoint(input1);
auto tp2 = parse_to_timepoint(input2);
std::cout << "start time = " << ((tp1 < tp2) ? input1 : input2) << "\n";
std::cout << "end time = " << ((tp1 >= tp2) ? input1 : input2) << "\n";
return 0;
}

Answer 2

I don't see why this shouldn't work. Using std::chrono::from_stream to parse the string into a time point, then just compare the two time points.
However, I've been trying it now with Visual Studio 2022 17.0.2 (Community Edition) and it fails to parse the string into a tp.
There is this answer from Ted Lyngmo's talking about a bug (fixed in VS2022 17.0.3) when parsing seconds with subseconds. I have to say though that his solution didn't work for me either in my VS2022.
Anyway, you may want to give it a try.
#include <chrono>
#include <iomanip> // boolalpha
#include <iostream> // cout
#include <sstream> // istringstream
#include <string>
auto parse_string_to_tp(const std::string& str)
{
std::istringstream iss{ str };
std::chrono::sys_time<std::chrono::microseconds> tp{};
std::chrono::from_stream(iss, "%H:%M:%S", tp); // or simply "%T"
return tp;
}
int main()
{
const std::string str1{ "12:27:37.740002" };
const std::string str2{ "13:00:00.500000" };
auto tp1{ parse_string_to_tp(str1) };
auto tp2{ parse_string_to_tp(str2) };
std::cout << "tp1 < tp2: " << std::boolalpha << (tp1 < tp2) << "\n";
std::cout << "tp2 < tp1: " << std::boolalpha << (tp2 < tp1) << "\n";
}
EDIT: it works if you just use durations instead of time points:
#include <chrono>
#include <iomanip> // boolalpha
#include <iostream> // cout
#include <sstream> // istringstream
#include <string>
auto parse_string_to_duration(const std::string& str)
{
std::istringstream iss{ str };
std::chrono::microseconds d{};
std::chrono::from_stream(iss, "%T", d);
return d;
}
int main()
{
const std::string str1{ "12:27:37.740002" };
const std::string str2{ "23:39:48.500000" };
auto d1{ parse_string_to_duration(str1) };
auto d2{ parse_string_to_duration(str2) };
std::cout << "d1 < d2: " << std::boolalpha << (d1 < d2) << "\n";
std::cout << "d2 < d1: " << std::boolalpha << (d2 < d1) << "\n";
}