C/C++ 记录时间

#include <ctime>

void f() {
  using namespace std;
  clock_t begin = clock();

  code_to_time();

  clock_t end = clock();
  double elapsed_secs = double(end - begin) / CLOCKS_PER_SEC;
}

The time() function is only accurate to within a second, but there are CLOCKS_PER_SEC "clocks" within a second. This is an easy, portable measurement, even though it‘s over-simplified.

You can abstract the time measuring mechanism and have each callable‘s run time measured with minimal extra code, just by being called through a timer structure. Plus, at compile time you can parametrize the timing type (milliseconds, nanoseconds etc).

^{Thanks to the review by Loki Astari and the suggestion to use variadic templates. This is why the forwarded function call.}

#include <iostream>
#include <chrono>

template<typename TimeT = std::chrono::milliseconds>
struct measure
{
    template<typename F, typename ...Args>
    static typename TimeT::rep execution(F&& func, Args&&... args)
    {
        auto start = std::chrono::steady_clock::now();
        std::forward<decltype(func)>(func)(std::forward<Args>(args)...);
        auto duration = std::chrono::duration_cast< TimeT> 
                            (std::chrono::steady_clock::now() - start);
        return duration.count();
    }
};

int main() {
    std::cout << measure<>::execution(functor(dummy)) << std::endl;
}

Demo

According to the comment by Howard Hinnant it‘s best not to escape out of the chrono system until we have to. So the above class could give the user the choice to call count manually by providing an extra static method (shown in C++14)

template<typename F, typename ...Args>
static auto duration(F&& func, Args&&... args)
{
    auto start = std::chrono::steady_clock::now();
    std::forward<decltype(func)>(func)(std::forward<Args>(args)...);
    return std::chrono::duration_cast<TimeT>(std::chrono::steady_clock::now()-start);
} 

// call .count() manually later when needed (eg IO)
auto avg = (measure<>::duration(func) + measure<>::duration(func)) / 2.0;

and be most useful for clients that

"want to post-process a bunch of durations prior to I/O (e.g. average)"

The complete code can be found here. My attempt to build a benchmarking tool based on chrono is recorded here.

If C++17‘s std::invoke is available, the invocation of the callable in execution could be done like this :

invoke(forward<decltype(func)>(func), forward<Args>(args)...);

to provide for callables that are pointers to member functions.