Annoying Error

[xwolf9100]

So I keep getting a million unresolved external symbol errors when I try to compile my program, I'll post all my code and the errors.

tower.h:

#ifndef TOWER_H
#define TOWER_H

#include <string>
#include "player.h"

class tower {
public:
	static int towerid;
protected:
	bool isPlaced(int towerid);
	virtual void upgrade(player& cplayer);
	virtual void sell(int towerid,player& cplayer);
	virtual void sprite(int x, int y);
	virtual void buy(player& cplayer);
	virtual void selecttower(bool towerselected);
	virtual bool gettowerselected();
	int cost;
	int damage;
	int upgradecost;
	double speed;
	int upgrades;
	int upgradepath;
	bool towerselected;
};

#endif 

arrowtower.h:

#ifndef ARROWTOWER_H
#define ARROWTOWER_H

#include "tower.h"
#include "player.h"
#include "darkGDK.h"

class arrowtower : public tower {
public:
	arrowtower();
	void upgrade(player &cplayer);
	void sell(int towerid,player& cplayer);
	void sprite(int x,int y);
	void buy(player& cplayer);
	void selecttower(bool towerselected);
	bool gettowerselected();
};

#endif 

player.h:

#ifndef PLAYER_H
#define PLAYER_H

class player {
public:
	player();
	int getGold();
	void addGold(int amount);
private:
	int gold;
};

#endif 

arrowtower.cpp:

#include "arrowtower.h"

arrowtower::arrowtower(){
	cost = 100;
	damage = 4;
	upgradecost = 250;
	speed = 1.0;
	upgrades = 0;
	towerselected = false;
	dbLoadImage("arrowtower.png",1);
}
void arrowtower::upgrade(player &cplayer){
	if (cplayer.getGold() >= upgradecost && upgrades == 0){
		damage = 8;
		upgradecost = 500;
		upgrades++;
	}
	else if (cplayer.getGold() >= upgradecost && upgrades == 1 && upgradepath == 1){
		damage = 15;
		speed = 4.0;
	}
	else if (cplayer.getGold() >= upgradecost && upgrades == 1 && upgradepath == 2){
		damage = 10;
		speed = 0.5;
	}
	else {
		return;
	}
}
void arrowtower::sell(int towerid,player& cplayer){
	dbDeleteSprite(towerid);
	cplayer.addGold(cost/2);
}
void arrowtower::sprite(int x,int y){
	dbSprite(tower::towerid,x,y,1);
}
void arrowtower::buy(player& cplayer){
	if (cplayer.getGold() >= cost){
		cplayer.addGold(-cost);
	}
	else {
		return;
	}
}
void arrowtower::selecttower(bool towerselected){
	if (towerselected)
		towerselected = true;
	else if (!towerselected)
		towerselected = false;
	else {
		return;
	}
}
bool arrowtower::gettowerselected(){
	return towerselected;
}

player.cpp:

#include "player.h"

player::player(){
	gold = 500;
}
int player::getGold(){
	return gold;
}
void player::addGold(int amount){
	gold += amount;
}

main.cpp:

#include "darkGDK.h"
#include "player.h"
#include "arrowtower.h"

void DarkGDK(void){
	
	int sprites = 0;
	arrowtower mainarrow;
	player player1;
	dbSyncOn();
	dbSyncRate(60);
	dbDisableEscapeKey();
	dbLoadImage("graphics/background.png",1);
	dbSprite(1,0,0,1);
	dbDrawSpritesFirst();

	while (LoopGDK()){
		if (dbMouseX() > 153 && dbMouseX() < 227 && dbMouseY() > 14 && dbMouseY() < 93){
			if (dbMouseClick() == 1){
				mainarrow.buy(player1);
				mainarrow.sprite(dbMouseX() + 37, dbMouseY() + 37);
				mainarrow.selecttower(true);
			}
		}
		if (mainarrow.gettowerselected()){
			mainarrow.sprite(dbMouseX() + 37, dbMouseY() + 37);
		}
		if (dbEscapeKey() == 1){
			break;
		}
		dbSync();
	}

	for (int i = 0; i < sprites; i++){
		dbDeleteImage(i);
		dbDeleteSprite(i);
	}

	return;
}

errors:

1>LINK : H:\Documents and Settings\Rensel\Desktop\C++ Projects\DeathToGoblins\Debug\DeathToGoblins.exe not found or not built by the last incremental link; performing full link
1>LINK : warning LNK4098: defaultlib 'LIBCMT' conflicts with use of other libs; use /NODEFAULTLIB:library
1>arrowtower.obj : error LNK2001: unresolved external symbol "protected: virtual void __thiscall tower::upgrade(class player &)" (?upgrade@tower@@MAEXAAVplayer@@@Z)
1>arrowtower.obj : error LNK2001: unresolved external symbol "protected: virtual void __thiscall tower::sell(int,class player &)" (?sell@tower@@MAEXHAAVplayer@@@Z)
1>arrowtower.obj : error LNK2001: unresolved external symbol "protected: virtual void __thiscall tower::sprite(int,int)" (?sprite@tower@@MAEXHH@Z)
1>arrowtower.obj : error LNK2001: unresolved external symbol "protected: virtual void __thiscall tower::buy(class player &)" (?buy@tower@@MAEXAAVplayer@@@Z)
1>arrowtower.obj : error LNK2001: unresolved external symbol "protected: virtual void __thiscall tower::selecttower(bool)" (?selecttower@tower@@MAEX_N@Z)
1>arrowtower.obj : error LNK2001: unresolved external symbol "protected: virtual bool __thiscall tower::gettowerselected(void)" (?gettowerselected@tower@@MAE_NXZ)
1>arrowtower.obj : error LNK2001: unresolved external symbol "public: static int tower::towerid" (?towerid@tower@@2HA)

[Peter87]

You have not implemented the functions in tower.

[vlad from moscow]

Maybe you should define your class tower as abstract. In this case its virtual functions can be declared as pure virtual the following way (for example)

bool isPlaced(int towerid) = 0;