Category: Tips

5 years ago I’ve shown how to use clock_gettime to do basic high-resolution profiling. The approach there is very useful, but unfortunately, not cross-platform. It works only on POSIX-compliant systems (especially not Windows).

Luckily, the not-so-new C++11 provides, among other things, an interface to high-precision clocks in a portable way. It’s still not a perfect solution, as it only provides wall time (clock_gettime can give per-process and per-thread actual CPU time as well). However, it’s still nice.

#include <iostream>
#include <chrono>
using namespace std;
 
int main()
{
    cout << chrono::high_resolution_clock::period::den << endl;
    auto start_time = chrono::high_resolution_clock::now();
    int temp;
    for (int i = 0; i< 242000000; i++)
        temp+=temp;
    auto end_time = chrono::high_resolution_clock::now();
    cout << chrono::duration_cast<chrono::seconds>(end_time - start_time).count() << ":";
    cout << chrono::duration_cast<chrono::microseconds>(end_time - start_time).count() << ":";
    return 0;
}

I’ll explain the code a bit. chrono is the new header file that provides various time- and clock-related functionality in the new standard library. high_resolution_clock should be, according to the standard, the clock with the highest precision.

cout << chrono::high_resolution_clock::period::den << endl;

Note that there isn’t a guarantee of how many ticks per second it has, only that it’s the highest available. Hence, the first thing we do is get the precision by printing how many times a second the clock ticks. My system provides 1000000 ticks per second, which is microsecond precision.

Getting the current time using now() is self-explanatory. The possibly tricky part is

cout << chrono::duration_cast<chrono::seconds>(end_time - start_time).count() << ":";

(end_time - start_time) is a duration (a newly defined type), and the count() method returns the number of ticks it represents. As we said, the number of ticks per second may change from system to system, so in order to get the number of seconds we use duration_cast. The same goes for microseconds in the next line.

The standard also provides other useful time units, such as nanoseconds, milliseconds, minutes, and even hours.