C++ datetime calculation api

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本文介绍了一个使用C++中的localtime与mktime函数进行日期时间加减运算的示例，包括天数、月份数及年份的增减，并展示了如何打印格式化的日期时间。

Description

A sample of how use C++ localtime/mktime to implement datetime calculation, for example plus/minus a few days, months, years, etc.

Source Code:

#include <time.h>
#include <iomanip>

#include <iostream>

void printtm(const char * prefix, tm * tp) {
    std::cout << prefix;
    std::cout << std::setw(4) << std::setfill('0') << tp->tm_year + 1900;   // year
    std::cout << "/";
    std::cout << std::setw(2) << std::setfill('0') << tp->tm_mon + 1;   // month
    std::cout << "/";
    std::cout << std::setw(2) << std::setfill('0') << tp->tm_mday;  // day
    std::cout << " ";
    std::cout << std::setw(2) << std::setfill('0') << tp->tm_hour;  // hour
    std::cout << ":";
    std::cout << std::setw(2) << std::setfill('0') << tp->tm_min;   // minute
    std::cout << ":";
    std::cout << std::setw(2) << std::setfill('0') << tp->tm_sec;   // second
    std::cout << std::endl;
}

int main(int argc, char * argv[]) {
    time_t t = time(NULL);
    tm * tp = localtime(&t);
 
    printtm("today     :", tp);
    tp->tm_mday += 2; mktime(tp);
    printtm("+2 days   :", tp);  // 2 days later
    tp->tm_mon  += 2; mktime(tp);
    printtm("+2 months :", tp);  // 2 months later
    tp->tm_year -= 2; mktime(tp);
    printtm("-2 year   :", tp);  // 2 years before
    return 0;
}

Compile and execute:

$ g++ tt.cpp 
$ ./a.out 
today     :2015/12/29 04:16:25
+2 days   :2015/12/31 04:16:25
+2 months :2016/03/02 04:16:25
-2 year   :2014/03/02 04:16:25

Limitation:

Function localtime() is not thread-unsafe, it uses an internal memory space to keep the datetime data structure, when multiply calling localtime(..) they overwrite the same memory space.

So localtime() cannot be shared among multiply threads.

The end