Adding the WorldState.h file

Create a new header file in the Header Files/Engine filter called WorldState.h and add the following code:

#pragma once
class WorldState
{
public:
    static const int WORLD_WIDTH = 100;
    static int WORLD_HEIGHT;
    static int SCORE;
    static int LIVES;
    static int NUM_INVADERS_AT_START;
    static int NUM_INVADERS;
    static int WAVE_NUMBER;
};

These variables are public and static. As a result, they will be accessible throughout the project and are guaranteed to have only a single instance.