C++ class for accurate time measurement

timer.h

#ifndef TIMER_H
#define TIMER_H

#include <ctime>

//link with rt (-lrt)

class timer {
public:
    timer() {
        clock_gettime(CLOCK_THREAD_CPUTIME_ID, &start_time_);
    }
    void   restart() {
        clock_gettime(CLOCK_THREAD_CPUTIME_ID, &start_time_);
    }

    // return elapsed time in seconds
    double elapsed() const {
        timespec curr_time;
        clock_gettime(CLOCK_THREAD_CPUTIME_ID, &curr_time);
        return  double(curr_time.tv_nsec - start_time_.tv_nsec) / 1.0e9 +
                double(curr_time.tv_sec - start_time_.tv_sec);
    }

 private:
  timespec start_time_;
}; // timer


#endif // TIMER_H