c++ 使用chrono 统计程序的运行时间

#include <iostream>       // std::cout
#include <chrono>
using namespace std;
int main ()
{

    std::chrono::steady_clock::time_point  now = std::chrono::steady_clock::now();

    cout<<"执行main完毕！"<<endl;
    auto t2 = std::chrono::steady_clock::now();
    std::chrono::duration<double> time_span = std::chrono::duration_cast<std::chrono::duration<double>>(t2 - now);
    printf("%f\n",time_span);
    std::cout << "It took me " << time_span.count() << " seconds.";
    std::cout << std::endl;
    return 0;
}