为什么此代码在std :: chrono :: system_clock上不起作用？

Question

I was trying to create a program who tells me what day is tomorrow (starting from 01 Jan) but the code I wrote down doesn't seem to work.

This is my code:

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

int main (int argc, char** argv) {
std::chrono::system_clock::time_point today = std::chrono::system_clock::now();

std::tm timeinfo = std::tm();
timeinfo.tm_mon = 0;
timeinfo.tm_mday = 1;
std::time_t tt = std::mktime (&timeinfo);

std::chrono::system_clock::time_point tp = std::chrono::system_clock::from_time_t (tt);


std::chrono::duration<int,std::ratio<60*60*24> >one_day (1);


std::chrono::system_clock::time_point tomorrow = today + one_day;


std::time_t tv;

tt = std::chrono::system_clock::to_time_t ( today );
std::cout << "today is: " << ctime(&tv); 

tt = std::chrono::system_clock::to_time_t ( tomorrow );
std::cout << "tomorrow will be: " << ctime(&tv);

return 0;
}

I'm not getting any error when I compile my code, but when I run my program the output is:

today is: Thu Jan 01 01:00:34 1970 tomorrow will be: Thu Jan 01 01:00:34 1970

Why is it acting this way?

Thanks everybody!

Answer 1

Actually your program is correct. You just messed up the output. The variables used in ctime refer to the (non-initialized) variable tv instead of the variable tt that holds the values you compute from today and tomorrow .

tt = std::chrono::system_clock::to_time_t ( today );
std::cout << "today is: " << ctime(&tv); 

tt = std::chrono::system_clock::to_time_t ( tomorrow );
std::cout << "tomorrow will be: " << ctime(&tv);

Should be

tt = std::chrono::system_clock::to_time_t ( today );
std::cout << "today is: " << ctime(&tt); 

tt = std::chrono::system_clock::to_time_t ( tomorrow );
std::cout << "tomorrow will be: " << ctime(&tt);

instead. After correcting that, it works for me. I now get this output:

today is: Sun Jan 31 13:22:30 2016
tomorrow will be: Mon Feb  1 13:22:30 2016

Answer 2

Your variable tv is uninitialized!

See my comments annotating your source code:

std::time_t tv; // uninitialized

tt = std::chrono::system_clock::to_time_t(today);
std::cout << "today is: " << ctime(&tv); // did you mean tt?

tt = std::chrono::system_clock::to_time_t(tomorrow);
std::cout << "tomorrow will be: " << ctime(&tv); // did you mean tt?