Implementing finite state machines

What we really need is the ability to define our states outside the game class, and have the state itself take care of what it needs to load, render, and update. For this we can create what is known as an FSM. The definition of FSM, as we will use it, is a machine that can exist in a finite number of states, can exist in only one state at a time (known as the current state), and can change from one state to another (known as a transition).

A base class for game states

Let's start our implementation by creating a base class for all of our states; create a header file called GameState.h:

#include<string>
class GameState
{
public:
  virtual void update() = 0;
  virtual void render() = 0;

  virtual bool onEnter() = 0;
  virtual bool onExit() = 0;

  virtual std::string getStateID() const = 0;
};

Just like our GameObject class, this is an abstract base class; we aren't actually putting any functionality into it, we just want all of our derived classes to...