C++: Create abstract class with abstract method and override the method in a subclass

Question

Hi,

How to create in C++ an abstract class with some abstract methods that I want to override in a subclass? How should the .h file look? Is there a .cpp, if so how should it look?

In Java it would look like this:

abstract class GameObject
{
    public abstract void update();
    public abstract void paint(Graphics g);
}

class Player extends GameObject
{
    @Override
    public void update()
    {
         // ...
    }

    @Override
    public void paint(Graphics g)
    {
         // ...
    }

}

// In my game loop:
List<GameObject> objects = new ArrayList<GameObject>();
for (int i = 0; i < objects.size(); i++)
{
    objects.get(i).update();
}
for (int i = 0; i < objects.size(); i++)
{
    objects.get(i).paint(g);
}

Translating this code to C++ is enough for me.

Edit:

I created the code but when I try to iterate over the objects I get following error:

Game.cpp:17: error: cannot allocate an object of abstract type ‘GameObject’
GameObject.h:13: note:   because the following virtual functions are pure within ‘GameObject’:
GameObject.h:18: note:         virtual void GameObject::Update()
GameObject.h:19: note:         virtual void GameObject::Render(SDL_Surface*)
Game.cpp:17: error: cannot allocate an object of abstract type ‘GameObject’
GameObject.h:13: note:   since type ‘GameObject’ has pure virtual functions
Game.cpp:17: error: cannot declare variable ‘go’ to be of abstract type ‘GameObject’
GameObject.h:13: note:   since type ‘GameObject’ has pure virtual functions

With this code:

vector<GameObject> gameObjects;

for (int i = 0; i < gameObjects.size(); i++) {
    GameObject go = (GameObject) gameObjects.at(i);
    go.Update();
}

Answer 1

The member functions need to be declared virtual in the base class. In Java, member functions are virtual by default; they are not in C++.

class GameObject
{
public:
    virtual void update() = 0;
    virtual void paint(Graphics g) = 0;
}

The virtual makes a member function virtual; the = 0 makes a member function pure virtual. This class is also abstract because it has at least one virtual member function that has no concrete final overrider.

Then in your derived class(es):

class Player : public GameObject
{
public:
    void update() { }          // overrides void GameObject::update()
    void paint(Graphics g) { } // overrides void GameObject::paint(Graphics)
}

If a member function is declared virtual in a base class, it is automatically virtual in any derived class (you can put virtual in the declaration in the derived class if you'd like, but it's optional).

Answer 2

In Java, all methods are virtual by default, unless you declare them final. In C++ it's the other way around: you need to explicitly declare your methods virtual. And to make them pure virtual, you need to "initialize" them to 0 :-) If you have a pure virtual method in your class, it automatically becomes abstract - there is no explicit keyword for it.

In C++ you should (almost) always define the destructor for your base classes virtual, to avoid tricky resource leaks. So I added that to the example below:

// GameObject.h

class GameObject
{
public:
    virtual void update() = 0;
    virtual void paint(Graphics g) = 0;
    virtual ~GameObject() {}
}

// Player.h
#include "GameObject.h"

class Player: public GameObject
{
public:
    void update();

    void paint(Graphics g);
}

// Player.cpp
#include "Player.h"

void Player::update()
{
     // ...
}

void Player::paint(Graphics g)
{
     // ...
}

Answer 3

In C++ you use the keyword virtual on your routines, and assign =0; into them. Like so:

class GameObject {
public:
    virtual void update()=0;
    virtual void paint(Graphics g)=0; 

}

Having a virtual method with a 0 assigned into it automagically makes your class abstract.