X2_exercises.md

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16. Classes and Structs

• Exercise X2

Exercise X2

Refactor out GhostState from GameState

In this exercise we will create a class that abstracts away common functionality.

Background: Visibility

The main difference between class and struct is the default visibility. All members of a struct are publicly accessible by default. A class on the other hand is private by default, so some sort of public: access is needed to give users access to things like constructors and functions.

Everything can be made private in a C++ class, even constructors.

Background: Default behavior

If possible, we want to rely on the default behavior of the member variables of a class. This simplifies the design of your class and reduces the things you need to worry about.

struct Position {
  double x = 0;
  double y = 0;
};

struct GridPosition {
  size_t x;
  size_t y;
  GridPosition(size_t x, size_t y)
    : x(x), y(y)
  {}
};

When someone creates an instance of Position the x and y variables are always set to 0 and there is no other way to create the class.

When someone wants to create an instance of GridPosition, they need to give the constructor the x and y values. You could set them to 0, but they still need to be given. Trying to create an instance of GridPosition with no values given will not compile.

Background: Game State

GameState is a class that holds the instances of all the game objects (ghosts, pacman, pellets, etc). Roughly every 16 milliseconds the step() function inside of GameState will be called.

GameState will call each member's update() function with the current delta time, this allows each object to know that it should update and how much time has passed since last it updated. The class also does some checks like if any pellets have been eaten and if any of the member objects are colliding.

Exercise

The ghost functions within GameState are usually grouped together. So lets abstract the ghost updating code into its own class.

1. Create files called GhostState.hpp and GhostState.cpp and run CMake so they are visible within the project.

2. Create a class called GhostState within the .hpp file.

3. Move all of the ghosts from the GameState class into the GhostState class. Then create a member variable within GameState that is of type GhostState. You do not need to move the checkColision function.

4. Move all of the ghost functionality from GameState into the GhostState class. Create functions for this behavior and call those functions from their respective locations within GameState (update, reset, frighten). You will also need to update the Canvas class.

5. Compile and run the game, check if everything is working correctly.