Undefined reference to `vtable for Enemy

Hi I am trying to make the game engine in the book SDL Game Development, I am at the stage of implementing polymorphism to the engine but when I compile the files I get the error : /tmp/ccEYKDYc.o: In function Enemy::Enemy()': main.cpp:(.text._ZN5EnemyC2Ev[_ZN5EnemyC5Ev]+0x1b): undefined reference tovtable for Enemy’
collect2: error: ld returned 1 exit status
I am compiling with : g++ main.cpp -fpermissive -Wall -lSDL2 -lSDL2_image

can anyone help me fix the error and explain why it is happening?

Thanks.

main.cpp


#include <SDL2/SDL.h>
#include <SDL2/SDL_image.h>
#include <iostream>
#include <string>
#include <map>
#include <vector>
#include "TextureManager.h"
#include "TextureManager.cpp"
#include "GameObject.h"
#include "GameObject.cpp"
#include "Player.h"
#include "Player.cpp"
#include "Game.h"
#include "Game.cpp"
#include "Enemy.h"
#include "Enemy.cpp"

int main(int argc, char * args[])
{
Game * g_game = new Game;

g_game->init("Chapter 1", 100,100,640,480,false);

while(g_game->running())
{
	g_game->handleEvents();
	g_game->update();
	g_game->render();
	
	SDL_Delay(10);

}
g_game->clean();

return 0;

}

enemy.h

#ifndef __Enemy__
#define __Enemy__

#include "GameObject.h"

class Enemy : public GameObject
{
public:
void load(int x, int y, int width, int height, std::string textureID);
void draw(SDL_Renderer* pRenderer);
void update();
void clean();
};

#endif // defined __Enemy_h__

enemy.cpp

#include "Enemy.h"

void Enemy::update()
{
m_y += 1;
m_x += 1;
m_currentFrame = int((SDL_GetTicks() / 100) % 6);
}

void Enemy::draw(SDL_Renderer* pRenderer) 
{
//TextureManager::Instance()->drawFrame(m_textureID, m_x, m_y, m_width, m_height, m_currentRow,
//m_currentFrame, pRenderer);
}

Game.h

#ifndef __Game__
#define __Game__

#include"GameObject.h"

class Game
{
public:
Game() {}
~Game() {}



bool init(const char* title, int xpos, int ypos, int width,
int height, bool fullscreen);
void render();
void update();
void handleEvents();
void clean();
void draw();

// a function to access the private running variable
bool running() {return m_bRunning;}

private:
std::vector<GameObject*> m_GameObjects;
GameObject* m_player;
GameObject* m_enemy1;
GameObject* m_enemy2;
GameObject* m_enemy3;
SDL_Window * m_pWindow;
SDL_Renderer * m_pRenderer;
SDL_Surface * pTempSurface;

int m_currentFrame;

bool m_bRunning;
};

GameObject * m_go;
GameObject * m_player;
#endif /* defined(__Game_h__) */

Game.cpp

#include "Game.h"
#include "Enemy.h"
#include "GameObject.h"

bool Game::init(const char* title, int xpos, int ypos, int width,
int height, bool fullscreen)
{
	m_player = new Player();
	m_enemy1 = new Enemy();
	m_enemy2 = new Enemy();
	m_enemy3 = new Enemy();

	m_GameObjects.push_back(m_player);
	m_GameObjects.push_back(m_enemy1);
	m_GameObjects.push_back(m_enemy2);
	m_GameObjects.push_back(m_enemy3);

	int flags = 0;

	if (fullscreen)
	{
	flags = SDL_WINDOW_FULLSCREEN;
	}

		// attempt to initialize SDL
		if(SDL_Init(SDL_INIT_EVERYTHING) == 0)
		{
		std::cout<<"SDL Init Success\n";
		// init the window
		m_pWindow = SDL_CreateWindow(title, xpos, 
		ypos,width,height,flags);			       
		}
		else
		{
		std::cout<<"Window Init Fail\n";
		return false; //window init fail
		}

		if(m_pWindow != 0) //window init success
		{
		std::cout<<"Window Creation Success\n";
		m_pRenderer = SDL_CreateRenderer(m_pWindow, -1, 0);

			if(m_pRenderer !=0) // render init success
			{
			std::cout<<"Renderer Creation Success\n";
			SDL_SetRenderDrawColor(m_pRenderer, 255,0,0,255);
			}	
			else
			{
			std::cout<<"Renderer Init Fail\n";
			return false; // renderer init fail
			}



	}

	else
	{
	std::cout<<"SDL Init Fail\n";
	return false; // SDL init fail
	}

	
	
		//to load					
		if(!TheTextureManager::Instance()->load("assets/animate-alpha.png","animate", m_pRenderer))
		{
		return false;
		}

m_go = new GameObject();
m_player = new Player();

m_go->load(100,100,128,82, "animate");
m_player->load(300,300,128,82, "animate");
	
m_GameObjects.push_back(m_go);
m_GameObjects.push_back(m_player);

GameObject* m_enemy;

m_enemy = new Enemy();
m_enemy ->load(0,0,128,82,"animate");
m_GameObjects.push_back(m_enemy);

	std::cout<<"Init success\n";
	m_bRunning = true; // everything inited successfully
	// start the main loop

	return true;

}

void Game::render()
{

SDL_RenderClear(m_pRenderer);

// loop through our objects and draw them

for(std::vector<GameObject*>::size_type i =0; i!=m_GameObjects.size(); i++)
{
m_GameObjects[i]->draw(m_pRenderer);
}

//to draw
//TheTextureManager::Instance()->draw("animate",0,0, 128, 82, m_pRenderer);

//to animate
//TheTextureManager::Instance()->drawFrame("animate",100,100,128,82,1,m_currentFrame, m_pRenderer);

//m_go.draw(m_pRenderer);

//m_player.draw(m_pRenderer);

SDL_RenderPresent(m_pRenderer);

}

void Game::update()
{
//m_currentFrame = int(((SDL_GetTicks() / 100) % 6));
//m_go.update();
//m_player.update();

for(std::vector<GameObject*>::size_type i = 0; i != m_GameObjects.size(); i++)
{
m_GameObjects[i]->update();
}

}

void Game::clean()
{
std::cout<<"Cleaning game\n";
SDL_DestroyWindow(m_pWindow);
SDL_DestroyRenderer(m_pRenderer);
SDL_Quit();
}




void Game::handleEvents()
{
SDL_Event event;
	if(SDL_PollEvent(&event))
	{
		switch (event.type)
		{
		case SDL_QUIT:
 		m_bRunning = false;
		break;
		}
	}
}

void Game::draw()
{
for(std::vector<GameObject*>::size_type i = 0; i != m_GameObjects.size(); i++)
{
m_GameObjects[i]->draw(m_pRenderer);
}
}

GameObject.h

#ifndef __GameObject__
#define __GameObject__

//#include <SDL_image.h>
#include "TextureManager.h"

class GameObject
{
public:
virtual void load(int x, int y, int width, int height, std::string textureID);
virtual void draw(SDL_Renderer* pRenderer);
virtual void update();
virtual void clean();


protected:

std::string m_textureID;
int m_currentFrame;
int m_currentRow;
int m_x;
int m_y;
int width;
int height;
int m_width;
int m_height;
};

#endif /* defined(__GameObject_h__) */

GameObject.cpp

#include "GameObject.h"

void GameObject::load(int x, int y, int width, int height, std::string textureID)
{
m_x = x;
m_y = y;
m_width = width;
m_height = height;
m_textureID = textureID;

m_currentRow = 1;
m_currentFrame = 1;
}

void GameObject::draw(SDL_Renderer* pRenderer)

{
TextureManager::Instance()->drawFrame(m_textureID, m_x, m_y, m_width, m_height, m_currentRow,
m_currentFrame, pRenderer);
}

void GameObject::update()
{
m_x += 1;
}

void GameObject::clean() 
{
    // Do Something
}

Player.h

#ifndef __Player__
#define __Player__

#include "GameObject.h"

class Player : public GameObject
{
public:
void load(int x, int y, int width, int height, std::string textureID);

void draw(SDL_Renderer* pRenderer);

void update();

void clean()
{
GameObject::clean();
std::cout<<"clean object\n";
}

};

#endif /*defined Player.h */

Player.cpp

#include "Player.h"

void Player::load(int x, int y, int width, int height, std::string textureID)
{
GameObject::load(x,y, width, height, textureID);
}

void Player::draw(SDL_Renderer* pRenderer)
{
GameObject::draw(pRenderer);
}

void Player::update()
{
m_x -=1;
}

TextureManager.h

#ifndef __TextureManager__
#define __TextureManager__

class TextureManager
{
public:
TextureManager(){}

bool load(std::string fileName, std::string id, SDL_Renderer* pRenderer);

//draw
void draw(std::string id, int x, int y, int width, int height, SDL_Renderer* pRenderer, SDL_RendererFlip flip = SDL_FLIP_NONE);

// drawframe
void drawFrame(std::string id, int x, int y, int width, int height, int currentRow, int currentFrame, SDL_Renderer* pRenderer, SDL_RendererFlip flip = SDL_FLIP_NONE);

std::map<std::string, SDL_Texture*> m_textureMap;

static TextureManager *s_pInstance;

static TextureManager* Instance()
{
	if(s_pInstance == 0)
	{
	s_pInstance = new TextureManager();
	return s_pInstance;
	}

return s_pInstance;
}

private:
~TextureManager(){}

};

typedef TextureManager TheTextureManager;

#endif /* defined(__TextureManager_h__) */

TextureManager.cpp

#include "TextureManager.h"

TextureManager* TextureManager::s_pInstance = 0;

bool TextureManager::load(std::string fileName, std::string id, SDL_Renderer* pRenderer)
{
SDL_Surface* pTempSurface = IMG_Load(fileName.c_str());

if(pTempSurface == 0)
{
return false;
printf("IMG_Load Error %s\n", SDL_GetError());
}
else
{
std::cout <<"IMG_Load Success!\n";
}

SDL_Texture* pTexture =
SDL_CreateTextureFromSurface(pRenderer, pTempSurface);

SDL_FreeSurface(pTempSurface);

// everything went ok, add texture to our list
if(pTexture !=0)
{
m_textureMap[id] = pTexture;
return true;
}

// reaching here means something went wrong
return false;
}

void TextureManager::draw(std::string id, int x, int y, int width, int height, SDL_Renderer* pRenderer, SDL_RendererFlip flip)
{
SDL_Rect srcRect;
SDL_Rect destRect;

srcRect.x =0;
srcRect.y =0;
srcRect.w = destRect.w = width;
srcRect.h = destRect.h = height;
destRect.x =x;
destRect.y =y;

SDL_RenderCopyEx(pRenderer, m_textureMap[id], &srcRect, &destRect, 0, 0, flip);
}

void TextureManager::drawFrame(std::string id, int x, int y, int width, int height, int currentRow,
int currentFrame, SDL_Renderer *pRenderer, SDL_RendererFlip flip)
{
SDL_Rect srcRect;
SDL_Rect destRect;
srcRect.x = width * currentFrame;
srcRect.y = height * (currentRow -1);
srcRect.w = destRect.w = width;
srcRect.h = destRect.h = height;
destRect.x = x;
destRect.y = y;

SDL_RenderCopyEx(pRenderer, m_textureMap[id], &srcRect, &destRect, 0,0, flip);
}