Chrono - The difference between two points in time in milliseconds?

Question

How can I get (using the std::chrono library) the difference between two points in time in milliseconds?

I could do that using this:

std::chrono::time_point<std::chrono::system_clock> now = std::chrono::system_clock::now();  std::chrono::time_point<std::chrono::system_clock> foo = now + std::chrono::milliseconds(100);  std::chrono::duration<float> difference = foo - now;  const int milliseconds = difference.count() * 1000; 

How can I get this time in milliseconds, so I can use the duration as a unsigned int, and not a float and then multiply by 1000?

Answer 1

std::chrono::duration has two template parameters, the second being exactly the unit of measure. You can invoke std::chrono::duration_cast to cast from one duration type to another. Also, there is a predefined duration type for milliseconds: std::chrono::milliseconds. Composing this together:

auto milliseconds = std::chrono::duration_cast<std::chrono::milliseconds>(foo - now); 

To get the actual number of milliseconds, use duration::count:

auto ms = milliseconds.count(); 

Its return type is duration::rep, which for standard duration types like std::chrono::milliseconds is a signed integer of unspecified size.

Answer 2

chrono::duration_cast<chrono::milliseconds>(end_time - start_time).count()