Obtain time c++

Question

I am trying to convert a Windows c++ function to a portable one. The objective of the function is to obtain a reference cpu time in seconds. The Windows function uses QueryPerformanceCounter and QueryPerformanceFrequency, which are not portable to mac, so I have tried to use std::chrono::high_resolution_clock, but I do not understand how it works.

The function that I already have is the following:

double GetSeconds(void)
{
  double sec;
  LARGE_INTEGER Frequency, PerformanceCount;

  QueryPerformanceFrequency( &Frequency );
  QueryPerformanceCounter( &PerformanceCount );

  sec = (double)PerformanceCount.QuadPart /(double)Frequency.QuadPart;

  return(sec);
}

With high_resolution_clock I have the following code working (it returns the time that printing "Hello World" takes):

std::chrono::steady_clock::time_point start = std::chrono::steady_clock::now();
std::cout << "Hello World\n";
std::chrono::steady_clock::time_point end = std::chrono::steady_clock::now();
float a = std::chrono::duration_cast<std::chrono::seconds>(end - start).count();

But I do not understand why the following line does not work (I just want to obtain the start time point):

float a = std::chrono::duration_cast<std::chrono::seconds>(start).count();    

Thank you for your help

Answer 1

Because start isn't a duration, it's a time point. Also note that std::chrono::steady_clock is not related to the wall clock, so getting its current value in itself doesn't really tell you that much, it's just a counter that steadily counts up.