How to properly initialize time with localtime in c++?

Question

I am getting the beginning of Epoch, where I should get current time:

date.hpp:

#ifndef DATE_HPP
#define DATE_HPP

#include <time.h>
#include <iostream>
#include <sstream>

class Date
{
    std::stringstream format;
    struct tm *date_tm;
    time_t date;

public:
    Date() : date(time(NULL)), date_tm(localtime(&date)) {}
    Date(std::istream &in);
    Date(std::string str);

    const std::string getDate();
};

#endif //DATE_HPP

date.cpp:

#include "date.hpp"
#include <iostream>
#include <sstream>
#include <iomanip>

Date::Date(std::istream &in)
{
    std::cout << "enter date [mm/dd/yy]: ";
    format.basic_ios::rdbuf(in.rdbuf());
    format >> std::get_time(date_tm, "%m/%d/%y");
}

Date::Date(std::string str)
{
    format << str;
    format >> std::get_time(date_tm, "%m/%d/%y");
}

const std::string Date::getDate()
{
    format << std::put_time(date_tm, "%m/%d/%y");
    return format.str();
}

main.cpp:

#include "date.hpp"
#include <iostream>

int main()
{
    Date now;
    std::cout << now.getDate() << std::endl;
}

When I run it ./a.out, I get 01/01/70. Well obviously, I would expect current time, since the time(NULL) which is used in localtime(now), should contain seconds from Epoch till now. So what could went wrong?

Turn on your compiler warnings. In addition, your class will break if multiple instances are created because `localtime` essentially gives back a pointer to a global. — chris, Nov 17 '20 at 20:11
Consider `struct tm date_tm;` (no `*`) and `*date_tm(localtime(&date)` (Added `*`) — chux - Reinstate Monica, Nov 17 '20 at 20:17
C++ has [`chrono`](https://en.cppreference.com/w/cpp/chrono) which is way, way better than `time(NULL)`. — tadman, Nov 17 '20 at 20:17
Perhaps `date(time(NULL)), date_tm(localtime(&date))` is not sequentially evaluated? — chux - Reinstate Monica, Nov 17 '20 at 20:20
And isn't compiler required to evaluate to the default ctor sequentially? — milanHrabos, Nov 17 '20 at 20:26
Side note: you really don't want the default constructor of Date to give you the now moment, it's a hidden performance issue. Better to make it a static method — JVApen, Nov 17 '20 at 21:36
For example if you use it as a value in std::map with operator[] it will call this default constructor before assigning your new value. (Oh, and it's a pain to fix this later, I have experience with that) — JVApen, Nov 17 '20 at 22:10

score 1 · Answer 1 · answered Nov 17 '20 at 20:19

1

Aside from the defects mentioned in the comments (localtime() returns a pointer to a global) you're running into a construction order issue.

In your class, date_tm is constructed/initialized before date, regardless of their order in the initialization list.

Change your class definition to see the results you expect:

class Date
{
    std::stringstream format;
    time_t date;        // ensure date is constructed/initialized before date_tm
    struct tm *date_tm;

    // ...
}

(see: https://ideone.com/FRJrdB)

As stated in the comments, the compiler should warn you about this if you have the proper warning levels enabled.

answered Nov 17 '20 at 20:19

Chad

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what is "pointer to global", how it differs to "pointer to local"? – milanHrabos Nov 17 '20 at 20:24
I thought, having pointer declared *inside* of class declaration, would not make it global – milanHrabos Nov 17 '20 at 20:24
@milanHrabos, The global thing is the object being pointed to by the return value of `localtime`, and hence pointed to by your class member as well. Calling `localtime` twice (or a few other functions that share that state) can overwrite the object while you're still pointing to it and return the same pointer instead of returning a pointer to a new object. If it helps, you can think of the `localtime` implementation as something like `tm g_state; tm* localtime(const time_t* t) { g_state = calculate_local_time(t); return &g_state; }` – chris Nov 17 '20 at 21:25

How to properly initialize time with localtime in c++?

1 Answers1