Inconsistent results using strptime/strftime

Question

I have written a C++ class that represent a Date and I use strptime/strftime to write and instantiate Dates from string.

The full code is here

When I run it several times on my linux using the bash in the tab "Sample output", sometimes I got the same Date created and parsed back, sometimes I get the date with a shift of one hour (my timezone it UTC+1).

So, what is happening here, I have no clue!

#ifndef DOLIPRANE_TIMEUNIT_HPP
#define DOLIPRANE_TIMEUNIT_HPP

enum TimeUnit  {
    DAY,
    HOUR,
    MINUTE,
    SECOND
  };
#endif
#ifndef DOLIPRANE_DATE_HPP
#define DOLIPRANE_DATE_HPP

#include <ctime>
#include <string>


class Date
{
public:

  Date();
  Date(time_t epoch);

  /**
   * Expected format: dd/MM/YYYY HH:mm:[ss]
   */
  Date(const std::string &date);

  ~Date();

  void
  add(long val, TimeUnit u = SECOND);


  bool
  operator==(const Date &other) const;

  bool
  operator!=(const Date &other) const;

  bool
  operator<(const Date &other) const;

  bool
  operator<=(const Date &other) const;

  bool
  operator>(const Date &other) const;

  bool
  operator>=(const Date &other) const;

  friend std::ostream&
  operator<<(std::ostream &, const Date&);

  friend std::istream&
  operator>>(std::istream &, Date&);

private:

  static const std::string FORMAT;

  time_t m_time;


};
#endif


#include <iostream>
#include <stdexcept>
#include <ctime>

const char SEPARATOR=';';

const std::string Date::FORMAT="%d/%m/%Y %H:%M:%S";

Date::Date()
{
  m_time = time(NULL);
}

Date::Date(time_t epoch) 
  : m_time(epoch)
{}

Date::Date(const std::string &date)
{
  struct tm t;
  const char* ptr = strptime(date.c_str(), FORMAT.c_str(), &t);
  if (!ptr) {
    std::string cause = "Cannot parse date ";
    cause += date;
    throw std::invalid_argument(cause);
  }
  m_time = mktime(&t);
  if (m_time == -1) {
    std::string cause = "Cannot compute epoch from " + date;
    throw std::range_error(cause);
  }
}

Date::~Date()
{
}

void
Date::add(long val, TimeUnit u) {

  switch(u){
  case DAY:
    m_time += 86400*val;
    break;
  case HOUR:
    m_time += 3600*val;
    break;
  case MINUTE:
    m_time += 60*val;
    break;
  case SECOND:
    m_time += val;
    break;
  default:
    throw std::invalid_argument("Unknown TimeUnit specified");
  }
}

bool
Date::operator==(const Date& o) const
{
  return m_time == o.m_time;
}

bool
Date::operator!=(const Date& o) const
{
  return ! (*this==o);
}

bool
Date::operator<(const Date &other) const
{
  return m_time < other.m_time;
}

bool
Date::operator<=(const Date &other) const
{
  return m_time <= other.m_time;
}

bool
Date::operator>(const Date &other) const
{
  return m_time > other.m_time;
}

bool
Date::operator>=(const Date &other) const
{
  return m_time >= other.m_time;
}

std::ostream&
operator<<(std::ostream& out, const Date &d)
{
  struct tm* tm = localtime(&d.m_time);
  char buffer[20];
  strftime(buffer, 20, Date::FORMAT.c_str(), tm);
  out << buffer << SEPARATOR;
  return out;
}

std::istream&
operator>>(std::istream &in, Date &d)
{
  std::string buf;
  std::getline(in, buf, SEPARATOR);
  Date o(buf);
  d = o;
  return in;
}

#include <iostream>
#include <fstream>

int
main(void)
{
  Date d;
  std::cout << d << std::endl;
  std::ofstream out("tmp.txt");
  out << d;
  out.close();

  std::ifstream in("tmp.txt");
  Date d2;
  in >> d2;
  in.close();
  std::cout << d2 << std::endl;

}

And finally, how I'm testing it:

$ for i in `seq 1 10`; do echo "test $i:"; ./test; rm tmp.txt; done
test 1:
26/03/2016 00:30:31;
26/03/2016 00:30:31;
test 2:
26/03/2016 00:30:31;
26/03/2016 00:30:31;
test 3:
26/03/2016 00:30:31;
25/03/2016 23:30:31;
test 4:
26/03/2016 00:30:31;
26/03/2016 00:30:31;
test 5:
26/03/2016 00:30:31;
25/03/2016 23:30:31;
test 6:
26/03/2016 00:30:31;
26/03/2016 00:30:31;
test 7:
26/03/2016 00:30:31;
25/03/2016 23:30:31;
test 8:
26/03/2016 00:30:31;
25/03/2016 23:30:31;
test 9:
26/03/2016 00:30:31;
25/03/2016 23:30:31;
test 10:
26/03/2016 00:30:31;
26/03/2016 00:30:31;

Provide a minimal working example that exhibits the behavior and include it here, dont link to the code somewhere (once the link goes down the question becomes useless to everyone else). — Borgleader
I cannot reproduce it on OS X. He said he was running on ubuntu, but I didn't get what version. — xaxxon
I just took all the tabs from the link, cat'd them together (compiled it to make sure it worked) and edited the question with the full code (you owe me remi) — xaxxon

Ville Voutilainen Ville Voutilainen · Accepted Answer · 2016-03-26T00:18:08

The problem is
struct tm t;
in the Date constructor that takes a string, which never initializes the object, and thus its tm_isdst field has an unspecified value. Set the tm_isdst field properly and you will get consistent results over multiple runs.

Inconsistent results using strptime/strftime

2 Answers