abstract classes C++ - c++

Question

So it is my understanding that to make a class abstract in c++ you have to create one, just one, pure virtual method in that class. In my code i create an abstract GameObject class, which is inherited by my Player class, but the problem is i get errors in my Player.cpp saying error LNK2001: unresolved extrenal symbol "public:virtual void__thiscall GameObject::Load(void)" (?Load#GameObject##UAEXXZ) for every method except the initialise, and this gets fixed when i set them all = 0, im just wondering why
// Abstract class to provide derived classes with common attributes
#include <SDL.h>
#include "AnimationManager.h"
#include "Debug.h"
#include "InputHandler.h"
class GameObject
{
public:
virtual void Initialise() = 0;
virtual void Load();
virtual void HandleEvents();
virtual void Update();
virtual void Draw();
Vector2D* position;
int currantFrame;
SDL_Renderer* renderer;
float speed;
bool alive;
};
#include "GameObject.h"
class Player : public GameObject
{
public:
virtual void Initialise();
virtual void Load();
virtual void HandleEvents();
virtual void Update();
virtual void Draw();
Player(SDL_Renderer* r);
~Player();
};
#include "Player.h"
Player::Player(SDL_Renderer* r)
{
renderer = r;
}
Player::~Player()
{
}
void Player::Initialise()
{
position = new Vector2D(10, 10);
currantFrame = 0;
}
void Player::Load()
{
TheAnimationManager::Instance()->load("Assets/circle.png", "player", renderer);
}
void Player::HandleEvents()
{
SDL_Event event;
if (SDL_PollEvent(&event))
{
switch(event.type)
{
case SDL_KEYDOWN:
switch(event.key.keysym.sym)
{
case SDLK_a:
DEBUG_MSG("A Pressed");
position->m_x -= 10;
break;
case SDLK_d:
DEBUG_MSG("D Pressed");
position->m_x += 10;
break;
}
break;
}
}
}
void Player::Update()
{
Vector2D* p = TheInputHandler::Instance()->GetMousePosition();
DEBUG_MSG(p->GetY());
DEBUG_MSG(p->GetX());
currantFrame = int(((SDL_GetTicks() / 100) % 4));
}
void Player::Draw()
{
TheAnimationManager::Instance()->Animate("player", (int)position->GetX(), (int)position->GetY(), 90, 82, 0, currantFrame, renderer, SDL_FLIP_NONE);
}

Answer 1

Every virtual method which might be called through a base class pointer that isn't pure virtual needs to have an implementation.
You seem to have a backwards idea of how abstract/virtual works. You don't choose to make a class abstract, why would you? You choose to make a member function pure virtual, because there is no sensible implementation for it. Then, as a consequence of having a pure virtual function, the class becomes abstract.
The class becoming abstract does not mean that every one of its functions suddenly becomes pure virtual.
Your error is a linker error. The linker was looking for the implementation of the GameObject::Load function, which you didn't provide. If you mark the function as pure virtual, the linker won't look for an implementation.

Answer 2

Some compilers require that for each signature the virtual keyword would specified only once, in the base class. Try this:
class GameObject
{
public:
virtual void Initialise() = 0;
virtual void Load() = 0;
virtual void HandleEvents() = 0;
virtual void Update() = 0;
virtual void Draw() = 0;
...
};
class Player : public GameObject
{
public:
void Initialise();
void Load();
void HandleEvents();
void Update();
void Draw();
...
};

Answer 3

if you don't need the public GameObject::Initialise method be pure virtual, at least give it an empty body, like this
class GameObject
{
public:
void Initialise() {}
...
};

Answer 4

Another conjecture: do you have, by any chance, any instantiations of GameObject in your code? Do you have something like this:
GameObject go;
or something like this:
void foo( GameObject go );
or something like this:
GameObject bar();
or something like this:
vector<GameObject> all_my_objects;