The Singleton

• A design pattern is a reusable solution to a commonly occurring problem.
  • Think of these as object oriented program structures.
  • Chapter 33 discusses a number of these.
  • Wesley will give a great presentation on these in a few weeks.
• The singleton pattern is probably the easiest to understand and use/misuse.
  • I have something like a game board or a map that I need everywhere in the program.
  • I could pass it to everything, but that would become bothersome.
  • I could make it global, but I don't want to fail the class.
• The singleton solves the following problems:
  • Make sure that only one copy exists.
  • Provide easy access to that object.
  • Provide easy, careful construction
  • Provide possible delayed construction.
  • Provide encapsulation and all of the "class" things you normally want.
• We are looking at this right now because it is a direct application of the static member functions.
• The big idea is
  • We disallow public construction in any way.
  • This means that the class will make sure only one copy exists.
  • This is done through a virtual method (GetInstance)

  • public:
        static BoardT & GetInstance(){
            if (instance == nullptr) {
               instance = new BoardT;
            }
        
            return * instance;
        }
    
    private:
        BoardT(): ctor-init:{ code};
        static inline BoardT * instance{nullptr}; 

  • Note, if instance is nullptr, then no instance has been created, so make one.
  • In any case, return it.
• The singleton will contain other access methods for the individual implementation.
  • Or not, see Wesley's presentation.
• In my case, I implemented a BoardT.h
  • I have additional static methods to
    • Get Rows and Columns.
    • Set default rows and columns before the initialization.
  • I had methods to Move from one location to another.
  • Get the reference to a location.
  • I used periodic boundary conditions so I don't need to worry about out of bounds.
    • But I almost built a boundary utility function in as a member function..
• I used this to implement the beginning of a memory game. (StringGame.cpp.
• And I implemented Conway's Game of Life in conway.cpp.