C++较精确的测试代码运行时间

#include <chrono>
using namespace chrono;
int main()
{
    auto t0 = system_clock::now();
    //测试代码
    auto t1 = system_clock::now();
    auto d=duration_cast<nanoseconds>
	(t1-t0);
	cout<<d.count()<<endl;
	return 0;
}