Topic 3: Polymorphism and Overloading

Synopsis

• Polymorphism^¤
   
• Polymorphism^¤ and Inheritance
   
• Polymorphism^¤ and Abstraction^¤
   
• Polymorphism^¤ and Typing^¤
   
• Polymorphic (or Dynamic) Dispatch^¤
   
• Overloading
   
• Overloaded operations
   
• Overloaded functions^¤
   
• Dispatching overloaded functions^¤
   
• Reading
   

Polymorphism^¤

• Ever been in a singles' bar?
   
• A standard stimulus (message) provokes a unique response
   
• Depending on the class^¤ of the object you stimulate
   
• This kind of behaviour^¤ is called polymorphism.
   

 

Polymorphism^¤ and Inheritance

A belated note on terminology In the diagram below, the class Container is said to be the base of class List (and also of class DoublyLinkedList and OrderedList). In addition, Container may be called the superclass and List a subclass. The class List may also be said to have been derived from class Container. These are all ways of saying pretty much the same thing but you'll need to be familiar with the terminology.

The reason we can send a particular message to an object is because it belongs to a class^¤ for which that operation (receiving that message) is well-defined
 

Once a message is well-defined for a given class^¤ it is automatically well-defined for any subclasses^¤
 

• However, we may wish to change (override) the way in which a specific subclass^¤ responds to a given message
   
• If we are accessing an object via a pointer to its superclass^¤, that pointer could legitimately be pointing to an object of any subclass^¤
   

Polymorphism^¤ and Abstraction^¤

• Can view polymorphism^¤ as a form of abstraction^¤
   
• Abstraction^¤ of function
   

Polymorphism^¤ and Typing^¤

• Polymorphism^¤ is an example of type enforcement: if an object is of type T it must respond to a given message as a T would
   
• Polymorphism^¤ is also an example of dynamic typing^¤¤ (late binding^¤), because the actual type enforcement isn't done until the message is actually sent (i.e. you don't know how the object will respond until you send the message)
   

Polymorphic (or Dynamic) Dispatch^¤

• Normally in an OO program, when we specify that a message is to be sent to an object the response is determined by the compiler (by looking at the types of the objects and messages involved)
   
• The compiler then translates that request into the equivalent assembler-level subroutine call
   
• This is called static dispatch
   
• However, with polymorphism^¤, the code to dispatch^¤ the message to the right handler (i.e. assembler subroutine) can't be determined by the compiler
   
• This is because the necessary information (namely the actual type of the object) isn't available at compile time
   
• Hence the compiler must instead insert code that will determine and invoke the appropriate assembler subroutine at the point when the message is actually sent
   
• This is called dynamic dispatch
   

Overloading

• Polymorphism^¤ can also apply to functions and operations
   
• For example: what does a/b do?
   

Overloaded operations

• An operation is overloaded if its behaviour^¤ depends on the type of one or more of its arguments
   
• Note that this is (usually) statically determined
   
• Some languages (including C++) allow the programmer to define additional overloadings for various operators
   
• Why?
   

Overloaded functions^¤

• Another way to look at overloaded operators^¤ is to treat (say) integer division and floating-point division as entirely separate operations which just happen to have the same name
   
• Likewise we can envisage two or more functions with different behaviour^¤ but the same name
   
• For example: sizeof(), abs(), print(), etc.
   

Dispatching overloaded functions^¤

• How can the compiler tell which overloaded function^¤ to call?
   
• How does it tell which overloaded operator^¤ to call?
   
• According to the types of the arguments provided
   
• Hence functions and operators may have the same name but must have a unique signature
   

Reading

• Lippman & Lajoie

Revision: Chapters 4 and 5.

New: Sections 9.1 and 15.1.


 

• Stroustrup

Revision: Chapter 6.

New: Sections 7.4 to 7.7.


 

• Sommerville:

Chapter 13