弄清std::chrono::system_clock::time_point，自定义时间工具类

time_t 是32位的整数，用来存放格林威治时间1970年01月01日00时00分00秒(北京时间1970年01月01日08时00分00秒)起至现在的总秒数，不能表示毫秒，约能表示135年

time_t timep(0);
printf("%d \n", timep); //0
time(&timep); /*获得time_t结构的时间，UTC时间*/
printf("%d \n", timep); //1550036706 到现在为止的秒数

//#include<time.h>
struct tm
{
    int tm_sec; /* 秒–取值区间为[0,59] */
    int tm_min; /* 分 - 取值区间为[0,59] */
    int tm_hour; /* 时 - 取值区间为[0,23] */
    int tm_mday; /* 一个月中的日期 - 取值区间为[1,31] */
    int tm_mon; /* 月份（从一月开始，0代表一月） - 取值区间为[0,11] */
    int tm_year; /* 年份，其值从1900开始 */
    int tm_wday; /* 星期–取值区间为[0,6]，其中0代表星期天，1代表星期一，以此类推 */
};

1、需要的头文件

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

2、std::chrono::system_clock::time_point的使用   

std::chrono::system_clock::time_point start_time;   //默认值为 1970-01-01 08:00:00

std::chrono::system_clock::time_point t(std::chrono::seconds(2)) ;//初始化2秒  1970-01-01 08:00:02
std::cout << t.time_since_epoch().count() <<std::endl;//纳秒  2000000000
std::cout << std::chrono::duration_cast<std::chrono::milliseconds>(t.time_since_epoch()).count() 
          <<std::endl;//毫秒  2000
std::cout << std::chrono::system_clock::now().time_since_epoch().count() 
          <<std::endl; //纳秒  1536115134721690145
uint64_t sec= std::chrono::duration_cast<std::chrono::duration<int>>
              (std::chrono::system_clock::now() - t).count();//秒 1536115132
std::cout << sec <<std::endl;
sec= std::chrono::duration_cast<std::chrono::seconds>//秒 1536115132
        (std::chrono::system_clock::now() - t).count();
std::cout << sec <<std::endl;
sec= std::chrono::duration_cast<std::chrono::milliseconds>//毫秒 1536115132721
        (std::chrono::system_clock::now() - t).count();
std::cout << sec <<std::endl;

3、自定义的时间工具类

#ifndef TOOL_TIME_H
#define TOOL_TIME_H

#include<iostream>
#include<chrono>
#include <time.h>
#include <sys/time.h>

class tool_time
{
public:
    static uint32_t elapse_seconds(std::chrono::system_clock::time_point &start)
    {
        return std::chrono::duration_cast<std::chrono::seconds>
                (std::chrono::system_clock::now() - start).count();
    }

    static uint64_t elapse_ms(std::chrono::system_clock::time_point &start)
    {
        return std::chrono::duration_cast<std::chrono::milliseconds>
                (std::chrono::system_clock::now() - start).count();
    }

    static uint32_t now_to_seconds()
    {
        return std::chrono::duration_cast<std::chrono::seconds>
                (std::chrono::system_clock::now().time_since_epoch()).count();
    }

    static uint64_t now_to_ms()
    {
        return std::chrono::duration_cast<std::chrono::milliseconds>
                (std::chrono::system_clock::now().time_since_epoch()).count();
    }

    static uint64_t now_to_us()
    {
        return std::chrono::duration_cast<std::chrono::microseconds>
                (std::chrono::system_clock::now().time_since_epoch()).count();
    }

    static uint64_t to_ms(const std::chrono::system_clock::time_point &time)
    {
        return std::chrono::duration_cast<std::chrono::milliseconds>
                (time.time_since_epoch()).count();
    }

    //"%Y-%m-%d %H:%M:%S"
    static std::string to_str(const std::chrono::system_clock::time_point &time)
    {
        char _time[25] = {0};
        time_t tt = std::chrono::system_clock::to_time_t(time);
        struct tm local_time;
        localtime_r(&tt, &local_time);
        strftime(_time, 22, "%Y-%m-%d %H:%M:%S", &local_time);

        return std::string(_time);
    }

    //"%d-%02d-%02d %02d:%02d:%02d.%03d"
    static std::string to_str_ex(const std::chrono::system_clock::time_point &time)
    {
        uint64_t mill = std::chrono::duration_cast<std::chrono::milliseconds>(time.time_since_epoch()).count()
                -std::chrono::duration_cast<std::chrono::seconds>(time.time_since_epoch()).count()*1000;

        char _time[25] = {0};
        time_t tt = std::chrono::system_clock::to_time_t(time);
        struct tm local_time;
        localtime_r(&tt, &local_time);
        //strftime(_time, 22, "%Y-%m-%d %H:%M:%S", local_time);
        sprintf(_time, "%d-%02d-%02d %02d:%02d:%02d.%03d", local_time.tm_year+1900,
                local_time.tm_mon+1, local_time.tm_mday, local_time.tm_hour,
                local_time.tm_min, local_time.tm_sec, mill);

        return std::string(_time);
    }

    static uint64_t now_to_ms_ex()
    {
        struct timeval tv;
        ::gettimeofday(&tv, 0);
        return tv.tv_sec*1000+tv.tv_usec/1000;
    }

    //"%Y-%m-%d %H:%M:%S"
    static time_t to_time(std::string str)
    {
        time_t t_;
        tm tm_;
        strptime(str.c_str(), "%Y-%m-%d %H:%M:%S", &tm_); //将字符串转换为tm时间
        t_ = mktime(&tm_); //将tm时间转换为秒时间

        return t_;
    }

    ////"%d-%02d-%02d %02d:%02d:%02d.%03d"
    static std::chrono::system_clock::time_point to_time_ex(std::string str)
    {
        int pos = str.length()-3;

        time_t t_;
        tm tm_;
        strptime(str.substr(0,pos).c_str(), "%Y-%m-%d %H:%M:%S", &tm_); //将字符串转换为tm时间
        t_ = mktime(&tm_); //将tm时间转换为秒时间

        int milli = std::stoi(str.substr(pos));

        return std::chrono::system_clock::time_point(std::chrono::milliseconds(t_*1000 + milli));
    }

    static std::string to_str_ex(uint64_t ms)
    {
        uint64_t mill = ms%1000;

        char _time[25] = {0};
        time_t tt = ms/1000;
        struct tm *local_time=localtime(&tt);
        //strftime(_time, 22, "%Y-%m-%d %H:%M:%S", local_time);
        sprintf(_time, "%d-%02d-%02d %02d:%02d:%02d.%03d", local_time->tm_year+1900,
                local_time->tm_mon+1, local_time->tm_mday, local_time->tm_hour,
                local_time->tm_min, local_time->tm_sec, mill);

        return std::string(_time);
    }

    static uint32_t elapse_seconds(time_t &start)
    {
        time_t now;
        time(&now);
        return static_cast<uint32_t>(std::difftime(now, start));
    }

    //"%Y-%m-%d %H:%M:%S"
    static std::string to_str(const std::time_t &time)
    {
        char _time[25] = {0};
        struct tm local_time;
        localtime_r(&time, &local_time);
        strftime(_time, 22, "%Y-%m-%d %H:%M:%S", &local_time);

        return std::string(_time);
    }
};

#endif // TOOL_TIME_H

测试

int main()
{
    auto now = std::chrono::system_clock::now();
    std::string tt=tool_time::to_str_ex(tool_time::now_to_ms());
    std::string tt2=tool_time::to_str_ex(tool_time::to_ms(now));

    std::string tt3=tool_time::to_str(std::chrono::system_clock::to_time_t(now));
    std::string tt4=tool_time::to_str(now);

    printf("%s\n", tt.c_str());
    printf("%s\n", tt2.c_str());

    printf("%s\n", tt3.c_str());
    printf("%s\n", tt4.c_str());

    printf("%ld\n", tool_time::to_time(tt));
    printf("%ld\n", tool_time::now_to_seconds());
    printf("%ld\n", tool_time::now_to_ms()/1000);
    printf("%ld\n", tool_time::now_to_ms_ex()/1000);

    printf("%ld\n", tool_time::now_to_ms());
    printf("%ld\n", tool_time::now_to_ms_ex());

    return 0;
}

结果：

2018-09-23 14:03:24.941
2018-09-23 14:03:24.941
2018-09-23 14:03:24
2018-09-23 14:03:24
1537682604
1537682604
1537682604
1537682604
1537682604941
1537682604941