time.h

APIs

Time manipulation - Functions	Description
clock	Clock program (function)
difftime	Return difference between two times (function)
mktime	Convert tm structure to time_t (function)
time	Get current time (function)

Conversion - Functions	Description
asctime	Convert tm structure to string (function)
ctime	Convert time_t value to string (function)
gmtime	Convert time_t to tm as UTC time (function)
localtime	Convert time_t to tm as local time (function)
strftime	Format time as string (function)

Macro constants	Description
CLOCKS_PER_SEC	Clock ticks per second (macro)
NULL	Null pointer (macro)

types	Description
clock_t	Clock type (type)
size_t	Unsigned integral type (type)
time_t	Time type (type)
struct tm	Time structure (type)

Demos

Delay

// waiting.cpp -- using clock() in a time-delay loop
#include <iostream>
#include <ctime> // describes clock() function, clock_t type
int main()
{
    using namespace std;
    cout << "Enter the delay time, in seconds: ";
    float secs;
    cin >> secs;
    clock_t delay = secs * CLOCKS_PER_SEC;  // convert to clock ticks
    cout << "starting\a\n";
    clock_t start = clock();
    while (clock() - start < delay )        // wait until time elapses
        ;                                   // note the semicolon
    cout << "done \a\n";
    // cin.get();
    // cin.get();
    return 0; 
}

Demo：clock()，clock_t，CLOCKS_PER_SEC

发现 i++和++i 速度其实差不多（这和《C++ primer plus》说的不一样）

#include <iostream>
#include <ctime>

int main()
{
    using namespace std;
    clock_t start, end;
    double elapsed_time;

    start = clock();
    for (long i = 0; i < 100000000; ++i)
        ;
    end = clock();

    elapsed_time = static_cast<double>(end - start) / CLOCKS_PER_SEC;
    cout << "Elapsed time: " << elapsed_time << " seconds" << endl;

    start = clock();
    for (long i = 0; i < 100000000; i++)
        ;
    end = clock();

    elapsed_time = static_cast<double>(end - start) / CLOCKS_PER_SEC;
    cout << "Elapsed time: " << elapsed_time << " seconds" << endl;

    return 0;
}

(base) kimshan@MacBook-Pro output % ./"test1"
Elapsed time: 0.152691 seconds
Elapsed time: 0.136307 seconds
(base) kimshan@MacBook-Pro output % ./"test1"
Elapsed time: 0.13305 seconds
Elapsed time: 0.132354 seconds
(base) kimshan@MacBook-Pro output % ./"test1"
Elapsed time: 0.158112 seconds
^[[AElapsed time: 0.13959 seconds
(base) kimshan@MacBook-Pro output % ./"test1"
Elapsed time: 0.145602 seconds
^[[AElapsed time: 0.139031 seconds
(base) kimshan@MacBook-Pro output % ./"test1"
Elapsed time: 0.145388 seconds
^[[AElapsed time: 0.136443 seconds
(base) kimshan@MacBook-Pro output % ./"test1"
Elapsed time: 0.142237 seconds
^[[AElapsed time: 0.141559 seconds
(base) kimshan@MacBook-Pro output % ./"test1"
Elapsed time: 0.147315 seconds
^[[AElapsed time: 0.141024 seconds
(base) kimshan@MacBook-Pro output % ./"test1"
Elapsed time: 0.138636 seconds
^[[AElapsed time: 0.134697 seconds
(base) kimshan@MacBook-Pro output % ./"test1"
Elapsed time: 0.14576 seconds
^[[AElapsed time: 0.140552 seconds
(base) kimshan@MacBook-Pro output % ./"test1"
Elapsed time: 0.147056 seconds
^[[AElapsed time: 0.13832 seconds
(base) kimshan@MacBook-Pro output % ./"test1"
Elapsed time: 0.142151 seconds
^[[AElapsed time: 0.137866 seconds
(base) kimshan@MacBook-Pro output % ./"test1"
Elapsed time: 0.148099 seconds
^[[AElapsed time: 0.141851 seconds
(base) kimshan@MacBook-Pro output % ./"test1"
Elapsed time: 0.148521 seconds
^[[AElapsed time: 0.138828 seconds
(base) kimshan@MacBook-Pro output % ./"test1"
Elapsed time: 0.151287 seconds
Elapsed time: 0.134296 seconds

Demo：time()，struct tm

struct tm {
  int tm_sec;   // 秒，正常范围从 0 到 59，但允许至 61
  int tm_min;   // 分，范围从 0 到 59
  int tm_hour;  // 小时，范围从 0 到 23
  int tm_mday;  // 一月中的第几天，范围从 1 到 31
  int tm_mon;   // 月，范围从 0 到 11
  int tm_year;  // 自 1900 年起的年数
  int tm_wday;  // 一周中的第几天，范围从 0 到 6，从星期日算起
  int tm_yday;  // 一年中的第几天，范围从 0 到 365，从 1 月 1 日算起
  int tm_isdst; // 夏令时
}

#include <iostream>
#include <ctime>

int main()
{
    using namespace std;
    time_t seconds;
    time(&seconds); 
    //time_t time(time_t *t)
    struct tm *info = localtime(&seconds);
    cout << info->tm_hour << "时 " << info->tm_min << "分 " << info->tm_sec << "秒" << endl;
    return 0;
}

(base) kimshan@MacBook-Pro output % ./"test1"
12时 4分 23秒

Reference

[1] https://cplusplus.com/reference/ctime/