C++ Convert a string timestamp to std::chrono::system_clock::time_point

Question

I am trying to convert a string timestamp expressed in the following format: "28.08.2017 03:59:55.0007" to a std::chrono::system_clock::time_point by preserving the microseconds precision. Is there any way to achieve this by using the standard library or boost? Thanks.

Answer 1

I'd make use of Howard Hinnant's date library https://howardhinnant.github.io/date/date.html

e.g.:

std::stringstream str( "28.08.2017 03:59:55.0007" );
str.imbue( std::locale() );
std::chrono::time_point< std::chrono::system_clock, std::chrono::microseconds > result;
date::from_stream( str, "%d.%m.%Y %H:%M:%S", result );
std::cout << result.time_since_epoch().count();

Answer 2

One implementation can be:

#include <ctime>
#include <cmath>
#include <chrono>
#include <string>
#include <cstdint>
#include <stdexcept>

std::chrono::system_clock::time_point parse_my_timestamp(std::string const& timestamp) {
    auto error = [&timestamp]() { throw std::invalid_argument("Invalid timestamp: "  + timestamp); };
    std::tm tm;
    auto fraction = ::strptime(timestamp.c_str(), "%d.%m.%Y %H:%M:%S", &tm);
    if(!fraction)
        error();
    std::chrono::nanoseconds ns(0);
    if('.' == *fraction) {
        ++fraction;
        char* fraction_end = 0;
        std::chrono::nanoseconds fraction_value(std::strtoul(fraction, &fraction_end, 10));
        if(fraction_end != timestamp.data() + timestamp.size())
            error();
        auto fraction_len = fraction_end - fraction;
        if(fraction_len > 9)
            error();
        ns = fraction_value * static_cast<std::int32_t>(std::pow(10, 9 - fraction_len));
    }
    else if(fraction != timestamp.data() + timestamp.size())
        error();
    auto seconds_since_epoch = std::mktime(&tm); // Assumes timestamp is in localtime. For UTC use timegm.
    auto timepoint_ns = std::chrono::system_clock::from_time_t(seconds_since_epoch) + ns;
    return std::chrono::time_point_cast<std::chrono::system_clock::duration>(timepoint_ns);
}

Answer 3

Thought I'd add an answer since there isn't one available that exclusively uses the standard.
Given the input: istringstream timestamp("28.08.2017 03:59:55.0007"), this could be converted to a tm via get_time, but for the fractional seconds. The fractional seconds would need to be converted manually (this could be done by constructing chrono::microseconds from the rounded remainder divided by the micro ratio.) All this could be combined into something like this:

tm tmb;
double r;

timestamp >> get_time(&tmb, "%d.%m.%Y %T") >> r;

const auto output = chrono::time_point_cast<chrono::microseconds>(chrono::system_clock::from_time_t(mktime(&tmb))) + chrono::microseconds(lround(r * micro::den));

Live Example