Boost, how to parse following string to date/time

Question

I have the following milli/micro second accuracy string to parse into some sort of boost datetime.

std::string cell ="20091201 00:00:04.437";

I have seen documentation regarding facets. Something like this

date_input_facet* f = new date_input_facet();
f->format("%Y%m%d %F *");

but I don't know how to use them.

I tried this program with code scavenged from StackOverflow, but I can't get the millis to show:

#include <string>
#include <iostream>
#include <sstream>
#include <fstream>
#include <map>

#include <boost/algorithm/string.hpp>
#include <boost/date_time/posix_time/posix_time.hpp>
#include <boost/date_time.hpp>

namespace bt = boost::posix_time;

const std::locale formats[] =
{
    std::locale(std::locale::classic(),new bt::time_input_facet("%Y%m%d %H:%M:%S.f")),
    std::locale(std::locale::classic(),new bt::time_input_facet("%Y-%m-%d %H:%M:%S")),
    std::locale(std::locale::classic(),new bt::time_input_facet("%Y/%m/%d %H:%M:%S")),
    std::locale(std::locale::classic(),new bt::time_input_facet("%d.%m.%Y %H:%M:%S")),
    std::locale(std::locale::classic(),new bt::time_input_facet("%Y-%m-%d"))
};

const size_t formats_n = sizeof(formats) / sizeof(formats[0]);

std::time_t pt_to_time_t(const bt::ptime& pt)
{
    bt::ptime timet_start(boost::gregorian::date(1970,1,1));
    bt::time_duration diff = pt - timet_start;

    return diff.ticks()/bt::time_duration::rep_type::ticks_per_second;

}

void seconds_from_epoch(const std::string& s)
{
    bt::ptime pt;
    for(size_t i = 0; i < formats_n; ++i)
    {
        std::istringstream is(s);
        is.imbue(formats[i]);
        is >> pt;
        if(pt != bt::ptime()) break;
    }

    bt::time_duration td = pt.time_of_day();
    long fs = td.fractional_seconds();

    std::cout << " ptime is " << pt << '\n';
    std::cout << " seconds from epoch are " << pt_to_time_t(pt) << " " << fs << '\n';
}

int main(int, char *argv[])
{
    std::string cell ="20091201 00:00:04.437";

    seconds_from_epoch(cell);

    int enterAnumber;
    std::

    cin >> enterAnumber;
}

Answer 1

boost::posix_time::time_from_string is very rigid when it comes to parsing formats.

You are looking for a different way to create a boost::posix_time::ptime from an std::string. You want to imbue a stringstream with the format, as such:

const std::string cell = "20091201 00:00:04.437";
const std::locale loc = std::locale(std::locale::classic(), new boost::posix_time::time_input_facet("%Y%m%d %H:%M:%S%f"));
std::istringstream is(cell);
is.imbue(loc);

boost::posix_time::ptime t;
is >> t;

Then

std::cout << t << std::endl;

gives

2009-Dec-01 00:00:04.437000