What is the best way to time how long functions take in C++? [duplicate]

Answer 1

High-precision timers are platform-specific and so aren't specified by the C++ standard, but there are libraries available. See this question for a discussion.

Answer 2

I use my own version of Python's time_it function. The advantage of this function is that it repeats a computation as many times as necessary to obtain meaningful results. If the computation is very fast, it will be repeated many times. In the end you obtain the average time of all the repetitions. It does not use any non-standard functionality:

#include <ctime>

double clock_diff_to_sec(long clock_diff)
{
    return double(clock_diff) / CLOCKS_PER_SEC;
}

template<class Proc>
double time_it(Proc proc, int N=1) // returns time in microseconds
{   
    std::clock_t const start = std::clock();
    for(int i = 0; i < N; ++i)
        proc();
    std::clock_t const end = std::clock(); 
    if(clock_diff_to_sec(end - start) < .2) 
        return time_it(proc, N * 5); 
    return clock_diff_to_sec(end - start) * (1e6 / N);
}

The following example uses the time_it function to measure the performance of different STL containers:

void dummy_op(int i)
{
    if(i == -1)
        std::cout << i << "\n";
}

template<class Container>
void test(Container const & c)
{
    std::for_each(c.begin(), c.end(), &dummy_op);
}

template<class OutIt>
void init(OutIt it)
{
    for(int i = 0; i < 1000; ++i)
        *it = i;
}
    
int main( int argc, char ** argv )
{
    {
        std::vector<int> c;
        init(std::back_inserter(c));
        std::cout << "vector: " 
                  << time_it(boost::bind(&test<std::vector<int> >, c)) << "\n";
    }      
    {
        std::list<int> c;
        init(std::back_inserter(c));
        std::cout << "list: "
                  << time_it(boost::bind(&test<std::list<int> >, c)) << "\n";
    }
    {
        std::deque<int> c;
        init(std::back_inserter(c));
        std::cout << "deque: " 
                  << time_it(boost::bind(&test<std::deque<int> >, c)) << "\n";
    }
    {
        std::set<int> c;
        init(std::inserter(c, c.begin()));
        std::cout << "set: " 
                  << time_it(boost::bind(&test<std::set<int> >, c)) << "\n";
    }
    {
        std::tr1::unordered_set<int> c;
        init(std::inserter(c, c.begin()));
        std::cout << "unordered_set: " 
           << time_it(boost::bind(&test<std::tr1::unordered_set<int> >, c)) << "\n";
    }    
}

In case anyone is curious here is the output I get (compiled with VS2008 in release mode):

vector: 8.7168

list: 27.776

deque: 91.52

set: 103.04

unordered_set: 29.76

Answer 3

#include <time.h>

clock_t start, end;
start = clock();
//Do stuff
end = clock();

printf("Took: %f\n", (float)((end - start) / (float)CLOCKS_PER_SEC));

Answer 4

I used boost::timer for measuring the duration of an operation. It provides a very easy way to do the measurement, and at the same time being platform independent. Here is an example:

boost::timer myTimer;
doOperation();
std::cout << myTimer.elapsed();

P.S. To overcome precision errors, it would be great to measure operations that take a few seconds. Especially when you are trying to compare several alternatives. If you want to measure something that takes very little time, try putting it into a loop. For example run the operation 1000 times, and then divide the total time by 1000.

Answer 5

I've implemented a timer for situations like this before: I actually ended up with a class with two different implemations, one for Windows and one for POSIX.

The reason was that Windows has the QueryPerformanceCounter() function which gives you access to a very accurate clock which is ideal for such timings.

On POSIX however this isn't available so I just used boost.datetime's classes to store the start and end times then calculated the duration from those. It offers a "high resolution" timer but the resolution is undefined and varies from platform to platform.

Answer 6

You can find useful this class.

Using RAII idiom, it prints the text given in construction when destructor is called, filling elapsed time placeholder with the proper value.

Example of use:

int main()
{
   trace_elapsed_time t("Elapsed time: %ts.\n");
   usleep(1.005 * 1e6);
} 

Output:

Elapsed time: 1.00509s.