Hi, I get an "undefined reference to `vtable for controlSystem'" when compiling the following code. I can't find the problem, I thought that this compiler message appeared when you forgot to implement a virtual member, but i think it's not my case.

Thanks in advance.

Code:
class controlSystem {
	protected:
		byte varNumber;
		byte varSize;
	
	public:
		controlSystem();
		byte iBL (unsigned int value);
		byte iBH (unsigned int value);
		unsigned int b2I(byte bH, byte bL);
		virtual unsigned int calculateOutput (unsigned int fullStatus[17]);
		virtual void sensorUpdate (unsigned int lastValue[16], unsigned int newValue[16]);
		virtual void saveEEPROM(unsigned int position);
		virtual void initializeEEPROM(unsigned int position);
		virtual void loadEEPROM(unsigned int position);
		};
/*fin de control System e inicio de thresholdSystem*/

class thresholdSystem : public controlSystem {
private:
  unsigned int _threshold[16];
  unsigned int _valueObj[16];
  boolean _isrising[16];
  unsigned int _active;

protected:

public:
  thresholdSystem();
   void saveEEPROM (unsigned int position);
   void initializeEEPROM (unsigned int position);
   void loadEEPROM (unsigned int position);
   unsigned int calculateOutput (unsigned int [17]);
   void sensorUpdate (unsigned int [16], unsigned int [16]);
};

---------------------------
byte controlSystem::iBL (unsigned int value) {
...
}

byte controlSystem::iBH (unsigned int value) {
...
}

unsigned int controlSystem::b2I(byte bH, byte bL) {
...
}

controlSystem::controlSystem () {
  int a=0;
}


thresholdSystem::thresholdSystem () {
  ...
}

//Cambiar código para coger dirección por parámetro

void thresholdSystem::loadEEPROM (unsigned int position) {
  ...
}

void thresholdSystem::saveEEPROM (unsigned int position) {
  ...
}

void thresholdSystem::initializeEEPROM (unsigned int position) {
  ...
}

unsigned int thresholdSystem::calculateOutput (unsigned int fullStatus[17]) {
  ...
}

void thresholdSystem::sensorUpdate (unsigned int lastValue[16], unsigned int newValue[16]){
  ...
}