Time sections of code in nanoseconds

timer.cc

#include <chrono>

class Timer
{
        public:
                Timer() { start(); };
                void start() {m_start = std::chrono::high_resolution_clock::now(); };
                void stop() {m_stop = std::chrono::high_resolution_clock::now(); };
                double nanos() { std::chrono::duration<double> d = m_stop - m_start;
                        return std::chrono::duration_cast<std::chrono::nanoseconds>(d).count(); };
        private:
                std::chrono::time_point<std::chrono::high_resolution_clock>
                        m_start, m_stop;

};

void using_it()
{
  Timer tim;
  tim.start();
  // ... some code that you want to time
  tim.stop();
  cout << "Elapsed: " << tim.nanos() << endl;
  
  // more timing
  tim.start();
  // ... yet more code
  tim.stop();
  cout << "Elapsed: " << tim.nanos() << endl;
}