Multiple Inheritance
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This document discusses some of the problems that can arise with multiple inheritance in object-oriented programming, including name ambiguity when classes have methods with the same name, issues with inheritance from common ancestors, and how inner classes can provide an alternative to multiple inheritance without the same semantic problems. It provides examples and potential solutions to demonstrate these problems.
Multiple Inheritance
- 1. Introduction Multiple Inheritance Name Ambiguity Common Ancestors Inner Classes Summary References MULTIPLE INHERITANCE Muhammad Adil Raja Roaming Researchers, Inc. cbna April 20, 2015
- 2. Introduction Multiple Inheritance Name Ambiguity Common Ancestors Inner Classes Summary References OUTLINE I INTRODUCTION MULTIPLE INHERITANCE NAME AMBIGUITY COMMON ANCESTORS INNER CLASSES SUMMARY REFERENCES
- 3. Introduction Multiple Inheritance Name Ambiguity Common Ancestors Inner Classes Summary References INTRODUCTION • We will investigate some of the problems that can arize when a language allows a child class to have multiple parents. • Name ambiguity. • Impact on substitution. • The Problem of Common Ancestors.
- 4. Introduction Multiple Inheritance Name Ambiguity Common Ancestors Inner Classes Summary References THE IDEALIZATION OF IS-A RELATIONSHIP MESSAGE SYNTAX • In one sense, the process of inheritance is a form of categorization. A TextWindow is a type of Window, so class TextWindow inherits from class Window. • But in the real world, most objects can be categorized in a variety of ways. • The author of the textbook is: • North American. • Male. • Professor. • Parent. • None of these are proper subsets of the other, and we cannot make a single rooted inheritance hierarchy out of them.
- 5. Introduction Multiple Inheritance Name Ambiguity Common Ancestors Inner Classes Summary References INHERITANCE AS COMBINATION • Instead, real world objects are combinations of features from different classification schemes, each category giving some new insight into the whole: • Author is North American, and • Author is Male, and • Author is a Professor, and • Author is a Parent. • Note that we have not lost the is-a relationship; it still applies in each case.
- 6. Introduction Multiple Inheritance Name Ambiguity Common Ancestors Inner Classes Summary References CS EXAMPLE – COMPLEX NUMBERS Two abstract classifications • Magnitude – things that can be compared to each other. • Number – things that can perform arithmetic. Three specific classes • Integer – comparable and arithmetic. • Char – comparable but not arithmetic. • Complex – arithmetic but not comparable.
- 7. Introduction Multiple Inheritance Name Ambiguity Common Ancestors Inner Classes Summary References POSSIBLE SOLUTIONS 1. Make Number subclass of Magnitude, but redefine comparison operators in class complex to give error message if used. (subclassing for limitation) 2. Don’t use inheritance at all – redefine all operators in all classes. (flattening the inheritance tree). 3. Use part inheritance, but simulate others – use Number, but have each number implement all relational operators. 4. Make Number and Magnitude independent, and have Integer inherit from both. (multiple inheritance).
- 8. Introduction Multiple Inheritance Name Ambiguity Common Ancestors Inner Classes Summary References INHERITANCE AS A FORM OF COMBINATION FIGURE : Inheritance.
- 9. Introduction Multiple Inheritance Name Ambiguity Common Ancestors Inner Classes Summary References ANOTHER EXAMPLE – WALKING MENUS • A Menu is a structure charged with displaying itself when selected by the user. • A • Menu maintains a collection of MenuItems. • Each MenuItem knows how to respond when selected. • A cascading menu is both a MenuItem and a Menu.
- 10. Introduction Multiple Inheritance Name Ambiguity Common Ancestors Inner Classes Summary References PROBLEM WITH MI – NAME AMBUITY • What happens when same name is used in both parent classes. • A • CardDeck knows how to draw a Card. • A GraphicalItem knows how to draw an image on a screen. • A GraphicalCardDeck should be able to draw. which?
- 11. Introduction Multiple Inheritance Name Ambiguity Common Ancestors Inner Classes Summary References ONE SOLUTION – REDEFINITION One solution is to redefine one or the other operation in the child class. REDEFINITION class GraphicalCardDeck : public CardDeck , public GraphicalObject { public : virtual void draw ( ) { return CardDeck : : draw ( ) ; } virtual void paint ( ) { GraphicalObject : : draw ( ) ; } } GraphicalCardDeck gcd ; gcd−>draw ( ) ; / / selects CardDeck draw gcd−>paint ( ) ; / / selects GraphicalObject draw
- 12. Introduction Multiple Inheritance Name Ambiguity Common Ancestors Inner Classes Summary References PROBLEM WITH REDEFINITION SOLUTION • The redefinition solution is not without cost, however. • Now what happens when we run up against the principle of substitution? REDEFINITION GraphicalObject ∗ g = new GraphicalCardDeck ( ) ; g−>draw ( ) ; / / opps , doing wrong method ! This problem can be mitigated, but the solution is complex and not perfect.
- 13. Introduction Multiple Inheritance Name Ambiguity Common Ancestors Inner Classes Summary References OTHER APPROACHES TO NAME AMBIGUITY • Other languages use different approaches to solving the problem of ambiguous names. • Eiffel uses the ability to rename features from the parent class. • A polymorphic variable accessing through the parents name will access the renamed feature in the child. • CLOS and Python resolve ambiguous names by the order in which the parent classes are listed. • The first occurrence of the name found in a systematic search is the one selected.
- 14. Introduction Multiple Inheritance Name Ambiguity Common Ancestors Inner Classes Summary References MULTIPLE INHERITANCE OF INTERFACES • Multiple inheritance of interfaces does not present the same problem of name ambiguity as does multiple inheritance of classes. • Either the ambiguous methods in the parent classes have different type signatures, in which case there is no problem, or • The ambiguous methods in the parent classes have the same signature. Still no problem, since what is inherited is only a specification, not an implementation. • This is why Java permits multiple inheritance of interfaces, not of classes. Nevertheless, C# does not permit the same method name to be inherited from two parent interfaces.
- 15. Introduction Multiple Inheritance Name Ambiguity Common Ancestors Inner Classes Summary References INHERITANCE FROM COMMON ANCESTORS • Another problem with MI occurs when parent classes have a common root ancestor. • Does the new object have one or two instances of the ancestor? FIGURE : Inheritance from common ancestors.
- 16. Introduction Multiple Inheritance Name Ambiguity Common Ancestors Inner Classes Summary References DATA FIELD IN COMMON ANCESTOR • Imagine that the common ancestor declares a data member. Should the child class have one copy of this data field, or two? • Both answers can be justified with examples. • C++ gets around this by introducing the idea of a virtual parent class. • If your parent is virtual there is one copy, and if not there is two. • Not a perfect solution, and makes the language complicated.
- 17. Introduction Multiple Inheritance Name Ambiguity Common Ancestors Inner Classes Summary References INNER CLASSES • The ability to next classes in C++ and Java provides a mechanism that is nearly equivalent to multiple inheritance, without the semantic problems. INNER CLASS class Child extends ParentOne { . . . class InnerChild extends ParentTwo { . . . / / can access both parents } }
- 18. Introduction Multiple Inheritance Name Ambiguity Common Ancestors Inner Classes Summary References SUMMARY • In this chapter we have explored some of the problems that arise of the concept of multiple inheritance. • Name ambiguity. • Impact on substitution. • The Problem of Common Ancestors.
- 19. Introduction Multiple Inheritance Name Ambiguity Common Ancestors Inner Classes Summary References REFERENCES • Images and content for developing these slides have been taken from the follwoing book with the permission of the author. • An Introduction to Object Oriented Programming, Timothy Budd. • This presentation is developed using Beamer: • Singapore, wolverine.