Prototype

• A fully initialized instance to be copied or cloned. Specify the kinds of objects to create using a prototypical instance, and create new objects by copying this prototype.

  

• There are two advantages of clone. 1)It can reduces complex and growing class contruction time. 2)Sometimes, you don’t know what kind of object should be created, For example, passing an abstract product obj pionter , at this time, you don’t know what kind of child class it pointed. You can use RTTI, But a better method is to use clone() directly.
  Create(absproduct * absp){ 
  // use RTTI on absp; not good 
  absp->clone(); // this a good. 
  }
• In C#, base class object include clone() method. it just use binary stream way to clone a object. In Java, you can extend IClone interface. clone is so important, it become a "build-in" feature in Java or C# language.
• In C++, you can just add clone method in your class if you want to it support clone. You don’t need to create a baseClass with virtual clone(), then inheriate you present class from it.
• But in a music note example, There are HalfNote and WholeNote classes, and dragMusicNote() need to support them all. At this time, you need to create a prototype baseClass, and make you HalfNote and WholeNote derived from it. Just like previous figure show.
  dragMusicNote(Note* p){ 
  Note* pNew = p->clone(); 
  pNew->draw(); 
  }
• MusicNote give us a good hint. Inheritance is a 1) is-a relationship, 2) You also can use it when it subclass and baseclass both support some actions. such as clone().
• Inside clone(), you can use new return a pointer, or use(*this) return a value.