为什么在调用std :: async后时间成本为0秒？

Question

See the following code.

#include <future>
#include <iostream>
#include <ctime>

int main() 
{
    std::future<int> future = std::async(std::launch::deferred, [](){ 
            std::this_thread::sleep_for(std::chrono::seconds(5));
            return 100;  
        }); 
    std::cout << "waiting...\n";

    clock_t start = clock();
    std::future_status status = future.wait_for(std::chrono::seconds(20));
    std::cout << "result is " << future.get() << std::endl;
    clock_t end = clock();
    std::cout<<"Time Cost : "<< (double)(end-start)/CLOCKS_PER_SEC <<" seconds."<< std::endl;
}

It's very confusing about the execution result. Yep, the main thread will wait for only 5 seconds around and then print "100". But why "Time Cost" shows 0? The test environment is Cygwin with g++ 4.9.3. Then I tested it in VS2013. The result is 25 seocnds. Strange!

Answer 1

It doesn't show 0 on my machine but a very small value : 0;000156s. But as it measures processor time and your main thread does not consume any cpu (wait is not an active loop), the result is almost 0.

Answer 2

clock() returns processor time spent. It doesn't have any guarantee of advancement whatsoever. If your CPU sleeps, the value returned by it will not be advanced. To measure intervals properly, use clocks from std::chrono , for example, std::chrono::steady_clock .