The factory design pattern is used for object creation and instantiation. A factory method decides which class to instantiate and returns a generic object. There can be multiple subclasses that the factory method creates depending on the arguments passed. The factory pattern example shows a MusicFactory class that uses a switch statement based on a genre enum to return the appropriate subclass such as Rock, Pop, or Reggae.

1. Factory Design Pattern
By
Pabitra Dash
(pabitrad@yahoo.com)

2. What does a factory mean ?
• It is something which creates/manufactures
some products.
• Same is the job of factory design pattern. It is
responsible for instantiation/creation of
objects.

3. Factory Design Pattern
• Factory methods decide logically what
constructor to call.
• Instead of directly calling the constructor for a
class, we call a factory method to get the object.
• There can be multiple derived classes from a base
class. The factory method instantiates the
appropriate sub-class based on the arguments
passed to it and returns the base class type.

4. Basic structure of factory design pattern

5. Factory pattern for the class Music

6. • /*Factory Class*/
• class MusicFactory
• {
• public:
• /*Factory Method*/
• Music *getMusic(genre_e genre)
• {
• Music *music = NULL;
•
• /*Logic based on Genre*/
• switch(genre)
• {
• case ROCK:
• music = new Rock();
• break;
• case POP:
• music = new Pop();
• break;
• case REGGAE:
• music = new Reggae();
• break;
• default:
• music = NULL;
• break;
• }
• return music;
• }
• };

7. Code Sample
• int main()
• {
• /*Create factory*/
• MusicFactory *musicFactory = new MusicFactory();
•
• /*Factory instantiating an object of type ROCK*/
• Music *music = musicFactory->getMusic(ROCK);
•
• cout<<"Song: ";
• if(music)
• music->song();
• else
• cout<<"Wrong selection dude/dudette !!";
• }