C++ 格式化日期和时间戳

1. 格式化日期

参考：https://blog.csdn.net/K346K346/article/details/47735791

1.1 说明

1. 一般步骤为通过localtime函数获取日期信息，通过strftime函数格式化日期
2. localtime是个线程不安全的函数，因为用到了全局变量。于是有了线程安全的localtime_s和localtime_r
3. localtime_s是适配windows平台的
4. localtime_r是适配linux平台的
5. localtime_s和localtime_r使用时，参数顺序不一样

1.2 基本用法

#include <time.h>

time_t t = time(0);
char tmp[32] = { NULL };
strftime(tmp, sizeof(tmp), "%Y-%m-%d_%H:%M:%S", localtime(&t));
string date(tmp);

date: (样例)： 2015-04-02_23:12:56

1.3 windows localtime_s

struct tm stime;
time_t now = time(0);
localtime_s(&stime, &now);

char tmp[32] = { NULL };
strftime(tmp, sizeof(tmp), "%Y-%m-%d_%H:%M:%S", &stime);
string date(tmp);

1.4 linux localtime_r

struct tm stime;
time_t now = time(0);
localtime_r(&now, &stime);

char tmp[32] = { NULL };
strftime(tmp, sizeof(tmp), "%Y-%m-%d_%H:%M:%S", &stime);
string date(tmp);

1.5 通过平台编译宏统一使用

参考： https://stackoverflow.com/questions/38034033/c-localtime-this-function-or-variable-may-be-unsafe

inline std::tm localtime_xp(std::time_t timer)
{
    std::tm bt {};
#if defined(__unix__)
    localtime_r(&timer, &bt);
#elif defined(_MSC_VER)
    localtime_s(&bt, &timer);
#else
    static std::mutex mtx;
    std::lock_guard<std::mutex> lock(mtx);
    bt = *std::localtime(&timer);
#endif
    return bt;
}

char tmp[32] = { NULL };
auto stime = localtime_xp(std::time(0));
strftime(tmp, sizeof(tmp), "%Y-%m-%d_%H:%M:%S", &stime);
string date(tmp);

2. 时间戳

2.1 秒级时间戳

#include<iostream>
#include<ctime>

int main()
{
    std::time_t t = std::time(0);  // t is an integer type
    std::cout << t << " seconds since 01-Jan-1970\n";
    return 0;
}

1554986565 seconds since 01-Jan-1970

或者

#include <chrono>

auto now = std::chrono::system_clock::now();
auto in_time_t = std::chrono::system_clock::to_time_t(now);

2.2 毫秒级时间戳

#include <time.h>
#include <chrono>

auto now = std::chrono::system_clock::now();
std::chrono::time_point<std::chrono::system_clock,std::chrono::milliseconds> tp = std::chrono::time_point_cast<std::chrono::milliseconds>(now);//获取当前时间点
std::time_t timestamp =  tp.time_since_epoch().count(); //计算距离1970-1-1,00:00的时间长度

c/c++ 标准日期和时间戳互相转化
https://blog.csdn.net/u011304615/article/details/71713179

3. 其他

目前获取日期的方法都不友好，在C++20之后估计会有改善
https://www.modernescpp.com/index.php/calendar-and-time-zone-in-c-20