1 / 7

Singleton

Singleton. CS 124 Reference: Gamma et al (“Gang-of-4”), Design Patterns. Singleton. Intent Ensure a class has only one instance, and provide a global point of access to it Motivation

yvonnequinn
Download Presentation

Singleton

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Singleton CS 124 Reference: Gamma et al(“Gang-of-4”), Design Patterns

  2. Singleton • Intent • Ensure a class has only one instance, and provide a global point of access to it • Motivation • Important for some classes to have exactly one instance. E.g., although there are many printers, should just have one print spooler • Ensure only one instance available and easily accessible • global variables gives access, but doesn’t keep you from instantiating many objects • Give class responsibility for keeping track of its sole instance

  3. Design Solution • Defines a getInstance() operation that lets clients access its unique instance • May be responsible for creating its own unique instance Singleton -static uniqueinstance Singleton data -Singleton() +static getInstance() Singleton methods… … return uniqueinstance;

  4. Singleton Example (Java) • Database public class Database { private static Database DB; ... private Database() { ... } public static Database getDB() { if (DB == null) DB = new Database(); return DB; } ... } Database static Database* DB instance attributes… static Database* getDB() instance methods… In application code… Database db = Database.getDB(); db.someMethod();

  5. Singleton Example (C++) class Database { private: static Database *DB; ... private Database() { ... } public: static Database *getDB() { if (DB == NULL) DB = new Database()); return DB; } ... } Database *Database::DB=NULL; In application code… Database *db = Database.getDB(); Db->someMethod();

  6. Implementation • Declare all of class’s constructors private • prevent other classes from directly creating an instance of this class • Hide the operation that creates the instance behind a class operation (getInstance) • Variation: Since creation policy is encapsulated in getInstance, it is possible to vary the creation policy

  7. Singleton Consequences • Ensures only one (e.g., Database) instance exists in the system • Can maintain a pointer (need to create object on first get call) or an actual object • Can also use this pattern to control fixed multiple instances • Much better than the alternative: global variables

More Related