在做项目中，我们经常需要获取系统的当前时间，那么如何获取呢，参见下面的代码：
/* asctime example */ #include <stdio.h> /* printf */ #include <time.h> /* time_t, struct tm, time, localtime, asctime */ int main () { time_t rawtime; struct tm * timeinfo; time ( &rawtime ); timeinfo = localtime ( &rawtime ); printf ( "The current date/time is: %s", asctime (timeinfo) ); return 0; }
输出结果为：
The current date/time is:: Thu Feb 26 11:40:19 2015
但是，有些情况下我们想分别获得年月日时分秒的值，而不是像这样得到一个整个的字符串，那么既然时间信息都存在了一个结构体timeinfo中，那么我们就先来看看这个结构体的定义吧：
struct tm { int tm_sec; // seconds of minutes from 0 to 61 int tm_min; // minutes of hour from 0 to 59 int tm_hour; // hours of day from 0 to 24 int tm_mday; // day of month from 1 to 31 int tm_mon; // month of year from 0 to 11 int tm_year; // year since 1900 int tm_wday; // days since sunday int tm_yday; // days since January 1st int tm_isdst; // hours of daylight savings time }
好了，看完了结构体的定义，我们再来看看asctime()函数是如何取出想要的数据，并加以转换的：
char* asctime(const struct tm *timeptr) { static const char wday_name[][4] = { "Sun", "Mon", "Tue", "Wed", "Thu", "Fri", "Sat" }; static const char mon_name[][4] = { "Jan", "Feb", "Mar", "Apr", "May", "Jun", "Jul", "Aug", "Sep", "Oct", "Nov", "Dec" }; static char result[26]; sprintf(result, "%.3s %.3s%3d %.2d:%.2d:%.2d %d\n", wday_name[timeptr->tm_wday], mon_name[timeptr->tm_mon], timeptr->tm_mday, timeptr->tm_hour, timeptr->tm_min, timeptr->tm_sec, 1900 + timeptr->tm_year); return result; }
如果我们直接将timeinfo结构体中的年月日时分秒打印出来，会得到：
115 1 26 11 46 37
而我们需要的是：
2015 2 26 11 46 37
仔细观察asctime()函数，我们可以发现年的保存方式是自1900年之后，所以要+1900，月是把名称简写存入了一个数组，数组从0开始，所以要得到月的数字，需要+1，后面的日时分秒没有变化，可以直接使用。
若想获得类似20150226114637这样的年月日时分秒的字符串，可以用下面的函数：
std::string getTimeStamp() { time_t rawtime; struct tm * timeinfo; time( &rawtime ); timeinfo = localtime( &rawtime ); char year[5], mon[3], mday[3], hour[3], minute[3], sec[3]; sprintf(year, "%d", timeinfo->tm_year + 1900); sprintf(mon, "%d", timeinfo->tm_mon + 1); sprintf(mday, "%d", timeinfo->tm_mday); sprintf(hour, "%d", timeinfo->tm_hour); sprintf(minute, "%d", timeinfo->tm_min); sprintf(sec, "%d", timeinfo->tm_sec); std::string yearStr = std::string(year); std::string monStr = std::string(mon); if (monStr.size() == 1) monStr = "0" + monStr; std::string mdayStr = std::string(mday); if (mdayStr.size() == 1) mdayStr = "0" + mdayStr; std::string hourStr = std::string(hour); if (hourStr.size() == 1) hourStr = "0" + hourStr; std::string minuteStr = std::string(minute); if (minuteStr.size() == 1) minuteStr = "0" + minuteStr; std::string secStr = std::string(sec); if (secStr.size() == 1) secStr = "0" + secStr; return yearStr + monStr + mdayStr +\ hourStr + minuteStr + secStr; }
另外，如果编译器支持C++11的话，也可以使用std::chrono来操作时间，可参见下列实例代码：
#include <iostream> #include <chrono> #include <ctime> long fibonacci(int n) { if (n < 3) return 1; return fibonacci(n-1) + fibonacci(n-2); } int main() { std::chrono::time_point<std::chrono::system_clock> start, end; start = std::chrono::system_clock::now(); std::cout << "f(42) = " << fibonacci(42) << '\n'; end = std::chrono::system_clock::now(); std::chrono::duration<double> elapsed_seconds = end-start; std::time_t end_time = std::chrono::system_clock::to_time_t(end); std::cout << "finished computation at " << std::ctime(&end_time) << "elapsed time: " << elapsed_seconds.count() << "s\n"; }
本文转自博客园Grandyang的博客，原文链接：获取当前时间C++ Get Current Time ，如需转载请自行联系原博主。
