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

Question

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

Answer 1

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)
{
     // ...
}