HC_SR04 ofxgpio (untested)

main.cpp

#include "ofMain.h"
#include "ofAppNoWindow.h"
#include "ofxGPIO.h"

class HC_SR04{
  public:
   HC_SR04();
   ~HC_SR04();

   long micros() {
      struct timeval current_time;
      gettimeofday(&current_time, NULL);
      return current_time.tv_sec * (int)1e6 + current_time.tv_usec;
   }

   void setup(int trigger, int echo) {
      this->trigger = trigger;
      this->echo    = echo;
      pio[0].setup(trigger, OUT, LOW);
      pio[1].setup(echo,    IN,  LOW);
      usleep(5000);
   }

   double distance(int timeout) {
      usleep(100);

      pio[0].set(HIGH);
      usleep(10);
      pio[0].set(LOW);

      now = micros();
      while (pio[1].get() == LOW && (micros()-now) < timeout);
      update();
      travel_time_usec = end_time_usec - start_time_usec;
      distance_meters = 100*((travel_time_usec/1000000.0)*340.29)/2;

      return distance_meters;
   }

   void exit(){
      pio[0].close();
      pio[1].close();
   }
  private:
   void update() {
      start_time_usec = micros();
      while (pio[1].get() == HIGH );
      end_time_usec   = micros();
   }

   GPIO pio[2];
   int trigger;
   int echo;
   volatile long start_time_usec;
   volatile long end_time_usec;
   double distance_meters;
   long travel_time_usec;
   long now;
};


class no_window : public ofBaseApp{
public:
   HC_SR04 hc;
   int trigger = 0;    // change this value con pin number for trigger
   int eco     = 0;    // change this value con pin number for echo

   void setup() {
      hc.setup(trigger,eco);
   }

   void update() {
      std::cerr << hc.distance(30000) << std::endl;
   }

   void exit() {
      hc.exit();
   }
};

int main( ){
   ofAppNoWindow window;
   ofSetupOpenGL(&window, 0,0, OF_WINDOW);
   ofRunApp( new no_window() );
}

Maybe this ?

public:
   HC_SR04(){}
   ~HC_SR04(){}