what is the most precise clock(timer) in c++? [duplicate]

If i want to get time interval between two line of code.

I can call gettimeofday before and after this code, but i think gettimeofday also cost some time.

I find chrone library have a high_resolution_clock, it provide now() function, so, what is the difference between gettimeofday and now?

What is the best way to calculate the time cost?