troche bardziej rozbudowane dziedziczeniem i bledy linkera

Witam, mam problemy przy kompilacji, wyrzuca mi taki błąd:

/tmp/cclBBvDw.o: In function `circuit_element::circuit_element()':
test.cpp:(.text._ZN15circuit_elementC2Ev[circuit_element::circuit_element()]+0xb): undefined reference to `vtable for circuit_element'
/tmp/cclBBvDw.o: In function `circuit_element::~circuit_element()':
test.cpp:(.text._ZN15circuit_elementD2Ev[circuit_element::~circuit_element()]+0xb): undefined reference to `vtable for circuit_element'
/tmp/cclBBvDw.o:(.rodata._ZTI9fotodioda[typeinfo for fotodioda]+0x8): undefined reference to `typeinfo for circuit_element'
collect2: ld returned 1 exit status

Niestety nie doszedłem o co chodzi w tym błędzie, dlatego prosiłbym o pomoc, oto mój kod:

Klasa abstrakcyjna circuit_element.h:

#include <iostream>
#include <string>

using namespace std;

struct colour 
{
	int R, G, B;
};

struct point 
{
	int x, y;
};

class circuit_element
{
	protected:	

	string name;
	colour wire_colour, bg_colour;
	point center;
	point * nodes_list;
	int wire_width, width;

	public:

	string virtual elem_SVG();
	void virtual set_wire_colour(colour&); 
	void virtual set_bg_colour(colour&);
	void virtual set_wire_width(int); 
	void virtual set_width(int);
	void virtual set_center(point&);
};

Klasa pochodna fotodioda.h:

#include <iostream>
#include <string>

using namespace std;

class fotodioda:public circuit_element
{
	friend class wire;

	public:

	fotodioda(string s, colour& w, colour& bg, point& c, int w_width, int side, point& left_node, point& right_node)
	{
		name = s;
		int nodes_num = 2;
		nodes_list = new point[2];
		nodes_list[0] = left_node;
		nodes_list[1] = right_node;
		wire_colour = w;
		bg_colour = bg;
		center = c;
		wire_width = w_width;
		width = side;
	};
	~fotodioda() 
	{
		delete [] nodes_list;
	};

	string elem_SVG() {};
	void set_wire_colour(colour& w) {wire_colour = w;};
	void set_bg_colour(colour& bg) {bg_colour = bg;};
	void set_wire_width(int w) {wire_width = w;}; 
	void set_width(int w) {width = w;};
	void set_center(point& p) {center = p;};
};

test.cpp:

#include <iostream>
#include "circuit_element.h"
#include "fotodioda.h"

using namespace std;

int main()
{
	colour black = {black.R = 0, black.G = 0, black.B = 0};
	colour white = {white.R = 255, white.G = 255, white.B = 255};
	point punkt1 = {punkt1.x = 35, punkt1.y = 100};
	point punkt2 = {punkt2.x = 135, punkt2.y = 200};
	point punkt3 = {punkt3.x = 235, punkt3.y = 300};
	point& ref1 = punkt1; 
	point& ref2 = punkt2; 
	point& ref3 = punkt3; 
	cout << "black: " << black.R << " " << black.G << " " << black.B << endl; 
	cout << "white: " << white.R << " " << white.G << " " << white.B << endl; 
	fotodioda dioda("dioda", black, white, ref1, 3, 80, ref2, ref3);   // tu zdaje sie zamiast tych   ref-ów mozna wstawic bezposrednio point

}

make clean
make

Klasa abstrakcyjna circuit_element.h:

To nie jest klasa abstrakcyjna, tylko klasa z metodami wirtualnymi, które nigdzie nie zostały zdefiniowane. Zrób tak:

class circuit_element
{
	...
public:

	string virtual elem_SVG() = 0;
	void virtual set_wire_colour(colour&) = 0;
	...
	void virtual set_center(point&) = 0;
};