Question

I was reading this article about parallel programming and I came across a return statement that I don’t quite understand. I’ve read about namespaces and Boost::Chrono::steady_clock, though I’ve never used either in practice, I understand their purposes.

This is the line of code found in the run_tests function which puzzles me:

return boost::chrono::duration <double, boost::milli> (end - start).count();

What’s going on here exactly? Shouldn’t an object name come before .count()? Is there some overloading of the - operator in Chrono?

The full code can be found here.

Answer 1

       boost::chrono::duration

is the name of a class template.

       boost::chrono::duration <double, boost::milli>

Is an instantiation of a class template, that is, a class.

       boost::chrono::duration <double, boost::milli> (end - start)

Creates a temporary object of that type, initialized with the value of the expression end-start.

       boost::chrono::duration <double, boost::milli> (end - start).count()

Invokes the .count() method of the temporary object.

return boost::chrono::duration <double, boost::milli> (end - start).count();

Returns the result of the .count() method.