std::chrono::system_clock:  依據(jù)系統(tǒng)的當(dāng)前時間 (不穩(wěn)定)
 std::chrono::steady_clock:  以統(tǒng)一的速率運(yùn)行(不能被調(diào)整)
 std::chrono::high_resolution_clock: 提供最高精度的計時周期(可能是steady_clock或者system_clock的typedef)

#include<iostream>
#include<vector>
#include<string>
#include<ctime>//將時間格式的數(shù)據(jù)轉(zhuǎn)換成字符串
#include<chrono>
using namespace std::chrono;
using namespace std;
int main()
{
    //獲取系統(tǒng)的當(dāng)前時間
    auto t = system_clock::now();
    //將獲取的時間轉(zhuǎn)換成time_t類型
    auto tNow = system_clock::to_time_t(t);
    //ctime()函數(shù)將time_t類型的時間轉(zhuǎn)化成字符串格式,這個字符串自帶換行符
    string str_time = std::ctime(&tNow);
    cout<<str_time;
    return 0;
}

#include<iostream>
#include<string.h>
#include<chrono>
using namespace std::chrono;
using namespace std;
int main()
{
    auto start = std::chrono::steady_clock::now();
    for(int i=0;i<100;i++)
        cout<<"nice"<<endl;
    auto end = std::chrono::steady_clock::now();
    auto tt = std::chrono::duration_cast<microseconds>(end - start);
    cout<<"程序用時="<<tt.count()<<"微秒"<<endl;
    return 0;
}

//設(shè)置一個高精度時間點(diǎn)
    std::time_point<high_resolution_clock> high_resolution_clock::now();

//設(shè)置系統(tǒng)時鐘
	std::chrono::time_point<std::chrono::system_clock> now=std::chrono::system_clock::now();

#define _CRT_SECURE_NO_WARNINGS //localtime()需要這個宏
#include<iostream>
#include<chrono>
#include<vector>
#include<string>
#include<algorithm>
//#include<stdio.h>
#include<iomanip> //put_time需要的頭文件
#include<sstream>
    // template <class _Rep>
    // struct treat_as_floating_point : is_floating_point<_Rep> {}; // tests for floating-point type
    // template <class _Rep>
    // _INLINE_VAR constexpr bool treat_as_floating_point_v = treat_as_floating_point<_Rep>::value;
    // // STRUCT TEMPLATE duration_values
    // template <class _Rep>
    // struct duration_values { // gets arithmetic properties of a type
    //     _NODISCARD static constexpr _Rep zero() noexcept {
    //         // get zero value
    //         return _Rep(0);
    //     }
    //     _NODISCARD static constexpr _Rep(min)() noexcept {
    //         // get smallest value
    //         return numeric_limits<_Rep>::lowest();
    //     }
    //     _NODISCARD static constexpr _Rep(max)() noexcept {
    //         // get largest value
    //         return (numeric_limits<_Rep>::max)();
    //     }
    // };
//時間長度
void Func1(){
    //chrono重載了各種運(yùn)算符
    std::chrono::hours             c1(1);                //1小時
    std::chrono::minutes           c2(60);               //60分鐘
    std::chrono::seconds           c3(60*60);            //60*60s
    std::chrono::milliseconds      c4(60*60*1000);       //60*60*1000毫秒
    std::chrono::microseconds      c5(60*60*1000*1000);  //微秒 溢出
    std::chrono::nanoseconds       c6(60*1000*1000*1000);//納秒 溢出
    if(c1==c2){
        std::cout<<"c1==c2"<<std::endl;
    }
    if(c1==c3){
        std::cout<<"c1==c3"<<std::endl;
    }
    if(c2==c3){
        std::cout<<"c2==c3"<<std::endl;
    }
    //獲取時鐘周期的值，返回的是int整數(shù)
    std::cout<<"c1= "<<c1.count()<<std::endl;
    std::cout<<"c2= "<<c2.count()<<std::endl;
    std::cout<<"c3= "<<c3.count()<<std::endl;
    std::cout<<"c4= "<<c4.count()<<std::endl;
    std::chrono::seconds             c7(1);               //1秒
    std::chrono::milliseconds        c8(1*1000);          //1000毫秒;1s
    std::chrono::microseconds        c9(1*1000*1000);     //1000*1000微秒 1s
    std::chrono::nanoseconds         c10(1*1000*1000*1000);//1000*1000*1000納秒 1s;
    std::cout<<c7.count()<<std::endl;
    std::cout<<c8.count()<<std::endl;
    std::cout<<c9.count()<<std::endl;
    std::cout<<c10.count()<<std::endl;
}
//系統(tǒng)時間
/**
 * @brief 
 * 
 * system_clock 類支持了對系統(tǒng)時鐘的訪問， 提供了三個靜態(tài)成員函數(shù)：
返回當(dāng)前時間的時間點(diǎn)。
static std::chrono::time_point<std::chrono::system_clock> now() noexcept;
將時間點(diǎn) time_point 類型轉(zhuǎn)換為 std::time_t 類型。
static std::time_t to_time_t( const time_point& t ) noexcept;
將 std::time_t 類型轉(zhuǎn)換為時間點(diǎn) time_point 類型。
static std::chrono::system_clock::time_point from_time_t( std::time_t t ) noexcept;
 */
void Func2(){
    //靜態(tài)成員函數(shù) static std::chrono::time_point<std::chrono::system_clock> now() noexcept
    //這些都可以簡寫用auto now=std::chrono::system_clock()::now()接收
    //1、靜態(tài)成員函數(shù) static std::chrono::system_clock::now()用來獲取系統(tǒng)時間，C++時間
    std::chrono::time_point<std::chrono::system_clock> now=std::chrono::system_clock::now();
    //2、靜態(tài)成員函數(shù) static std::chrono::system_clock::to_time_t()把C++系統(tǒng)時間轉(zhuǎn)換為time_t （utc時間)
    time_t t_now=std::chrono::system_clock::to_time_t(now);
    //3、std::localtime()函數(shù)把time_t時間轉(zhuǎn)換為本地時間(北京時間)
    // std::localtime()不是線程安全的，VS用localtime_t()代替，linux用local_time_r()代替
    //tm結(jié)構(gòu)體
    tm* tm_now=localtime(&t_now);
    //格式化輸出tm結(jié)構(gòu)體中的成員
    std::cout<<std::put_time(tm_now,"%Y-%m-%d %H:%M:%S")<<std::endl;
    std::cout<<std::put_time(tm_now,"%Y-%m-%d")<<std::endl;
    std::cout<<std::put_time(tm_now,"%H:%M:%S")<<std::endl;
    //但是通常C++一般不打印出時間，而是把時間存儲到一個字符串中
    std::stringstream ss;  //創(chuàng)建stringstream對象 ss,需要包含<sstream>頭文件
    ss<<std::put_time(tm_now,"%Y-%m-%d %H:%M:%S");
    std::string time_str=ss.str();
    std::cout<<time_str<<std::endl;
}   
// struct steady_clock { // wraps QueryPerformanceCounter
//         using rep                       = long long;
//         using period                    = nano;
//         using duration                  = nanoseconds;
//         using time_point                = chrono::time_point<steady_clock>;
//         static constexpr bool is_steady = true;
//         _NODISCARD static time_point now() noexcept { // get current time
//             const long long _Freq = _Query_perf_frequency(); // doesn't change after system boot
//             const long long _Ctr  = _Query_perf_counter();
//             static_assert(period::num == 1, "This assumes period::num == 1.");
//             // Instead of just having "(_Ctr * period::den) / _Freq",
//             // the algorithm below prevents overflow when _Ctr is sufficiently large.
//             // It assumes that _Freq * period::den does not overflow, which is currently true for nano period.
//             // It is not realistic for _Ctr to accumulate to large values from zero with this assumption,
//             // but the initial value of _Ctr could be large.
//             const long long _Whole = (_Ctr / _Freq) * period::den;
//             const long long _Part  = (_Ctr % _Freq) * period::den / _Freq;
//             return time_point(duration(_Whole + _Part));
//         }
//     };
//     using high_resolution_clock = steady_clock;
// } // namespace chrono
//計時器 steady_clock 類相當(dāng)于秒表，操作系統(tǒng)只要啟動就會進(jìn)行時間的累加，常用于耗時的統(tǒng)計（精確到納秒） 。
void Func3(){
    //靜態(tài)成員函數(shù)std::chrono::steady_clock::now()獲取時間的開始點(diǎn)
    std::chrono::time_point<std::chrono::steady_clock> start=std::chrono::steady_clock::now();
    //auto start=std::chrono::steady_clock::now();
    //執(zhí)行一些代碼，消耗時間
    std::vector<std::string> vec1{"banana","apple","pear"};
    std::for_each(vec1.begin(),vec1.end(),[&vec1](std::string str){
        std::cout<<str<<" ";
    });
    std::cout<<std::endl;
    //靜態(tài)成員函數(shù)std::chrono::steady_clock::now()獲取時間的結(jié)束點(diǎn)
    auto end=std::chrono::steady_clock::now();
    //計算消耗的時間，單位是納秒
    auto dt=end-start;
    std::cout<<"耗時: "<<dt.count()<<"納秒 ("<<(double)dt.count()/(1000*1000*1000)<<"秒) "<<std::endl;
}
int main(int argc,char* argv[]){
    Func1();
    Func2();
    Func3();
    return 0;
}