Can't verify the conversion from time_point to tm and tm back to time_point

Question

I created a current time_point and converted it to structure tm and printed it's values. Now converted this tm structure to time_point. On comparing the first and second time_points, it is telling that they're different. But the values of structure are exactly same.

Can someone spot, where I'm doing wrong?

#include <iostream>
#include <ctime>
#include <chrono>

using namespace std;
using namespace std::chrono;

system_clock::time_point toTimePoint(struct tm tim)
{
    return std::chrono::system_clock::from_time_t(mktime(&tim));
}

tm toTm(system_clock::time_point tp)
{
    time_t tmt = system_clock::to_time_t(tp);
    struct tm * tim = localtime(&tmt);
    struct tm newTim(*tim);
    cout << "Info: " << tim->tm_mday << "/" << tim->tm_mon << "/" << tim->tm_year << " " << tim->tm_hour << ":" << tim->tm_min << ":" << tim->tm_sec << endl;
    cout << "Is Daylight saving: " << tim->tm_isdst << " wday: " << tim->tm_wday << " yday: " << tim->tm_yday << endl;
    return newTim;
}

int _tmain(int argc, _TCHAR* argv[])
{
    system_clock::time_point tp = system_clock::now();

    struct tm tmstruct = toTm(tp);
    system_clock::time_point newtp = toTimePoint(tmstruct);
    cout << "Time comparison: " << (tp == newtp) << endl;

    toTm(newtp);
}

Output:

Info: 8/4/115 16:26:20 Is Daylight saving: 0 wday: 5 yday: 127

Time comparison: 0

Info: 8/4/115 16:26:20 Is Daylight saving: 0 wday: 5 yday: 127

score 0 · Accepted Answer · answered May 08 '15 at 11:12

It's rounding. time_point can have much higher resolutions than time_t. time_t is simply seconds, while time_point is defined by the system. On linux libstdc++, for example, it's nanoseconds.

As an example, what you're doing is similar to the below

  float f = 4.25;
  int i = (int)f; // i is 4
  std::cout << i << std::endl;
  float f2 = i; // f2 is 4.0 
  std::cout << (f == f2) << std::endl; // false
  int i2 = (int)f2; // i2 is also 4
  std::cout << i2 << std::endl;

Can't verify the conversion from time_point to tm and tm back to time_point

1 Answers1