Topic 2: Typing and Inheritance

Synopsis

• Typing^¤
   
• Strong vs Weak Typing^¤¤
   
• Static vs Dynamic Typing^¤¤
   
• Inheritance
   
• Type Relationships
   
• Type Inheritance
   
• Inheritance Hierarchies (Trees)
   
• Multiple Inheritance^¤
   
• Inheritance Graphs
   
• Inheritance is Transitive
   
• Reading
   

Typing^¤

• Recall that abstraction^¤ classifies objects (creates a class^¤)
   
• Classification^¤ according to structure and/or behaviour^¤
   
• "Typing^¤" is the enforcement of class^¤ distinctions
   
• For example: assignment compatibility, correct parameter types, etc.
   

Strong vs Weak Typing^¤¤

• "Strong typing^¤¤" is the strict enforcement of type rules
   
• "Weak typing^¤¤" is the strict enforcement of type rules but with well-defined exceptions^¤ or an explicit type-violation mechanism
   
• Languages may even have "no typing^¤" (for example, assemblers)
   

Static vs Dynamic Typing^¤¤

• Another kind of duality (independent of the "strong/weak" issue)
   
• Static typing^¤¤ (or static binding, or early binding) is where the type of an object or expression is fully determined at compile time
   
• Hence C is a statically-typed language
   
• Dynamic typing^¤¤ (or late binding) is where the type of an object or expression is not known (or unable to be known) until run-time.
   

Inheritance

• Consider the kinds of class^¤ hierarchies we experience
   
• Division hierarchy (e.g. biological taxonomy) and Control hierarchy (e.g. Military Rank)
   
• Example: Zoological classification^¤
   
• Example: Martial arts moves
   
• Example: Vic Roads database (first homework exercise)
   
• More specific classes^¤ "share" features of their more abstract parent classes^¤¤
   

Type Relationships

• Suppose I have a ADT which represents a list
   
• What functions (behaviours^¤) does it have?
   
• Now suppose a define an ADT which represents an automatically ordered list
   
• What functions (behaviours^¤) does it have in common with the list ADT?
   
• What additional functions does it have?
   

Type Inheritance

• We can define the ordered list ADT in terms of the list ADT
   
• OrderedList is a List plus ordering behaviour
   
• We can represent that relationship as a UML diagram:
   

Inheritance Hierarchies (Trees)

• The "List<-OrderedList" hierarchy^¤ is the smallest possible example of inheritance.
   
• Inheritance hierarchies between classes^¤ can be much more elaborate trees:
   

Multiple Inheritance^¤

• Things can be classified (abstracted) in many different ways, simultaneously
   
• Hence it is possible for a particular class^¤ to inherit from two or more parent classes^¤¤ simultaneously:
   

Inheritance Graphs

• In the larger scheme of things, multiple inheritance^¤ produces inheritance graphs (rather than trees):
   

• Such graphs must be acyclic. Why?
   

Inheritance is Transitive

• The additional properties conferred by inheritance as passed on to classes^¤ which subsequently inherit
   
• Hence we could simplify the previous inheritance graph:
   

Reading

• Fowler

Chapters 1 to 3


 

• Lippman & Lajoie

Sections 3.1 to 3.9, 7.1 to 7.4.

Mostly revision, but some parts will be new.


 

• Stroustrup

Chapters 4, 5 and Sections 7.1 to 7.3.

Mostly revision, but some parts will be new.


 

• Summerville
  

Chapter 13.