Multiple Inheritance

1
Multiple Inheritance

• Class Structure Directed Acyclic Graph

2
Examples in System Modeling

• Multiple inheritance facilitates combination
  of (orthogonal) abstractions.
• Company plane
• Airplane (Pilots perspective)
• (queries) passenger_count, altitude, position,
  speed, etc.
• (commands) take_off, land, set_speed, etc.
• Asset (Accountants perspective)
• (features) purchase_price, resale_value,
  depreciate, etc.
• Dining car
• Train car (coach)
• Restaurant
• Watch calculator

3

• Multiple inheritance facilitates representation
  of various roles of an object.
• Sofa bed, Mobile home, ...
• Knowledge Representation
• Clinton is a father.
• Clinton is the ex-President of USA.
• Nixon was a Republican.
• Nixon was a Quaker.
• Dolphins are aquatic creatures.
• Dolphins are mammals.

4
Examples from Software Systems

• Window
• (Hierarchical Structure) Tree
• (features) listOfSubwindows, parentWindow,
  addWindow, removeWindow, etc.
• (Graphical Object) Rectangle
• (features) height, width, position, display,
  hide, etc.

5
Linked list (leftmost child right sibling)

• Tree
• List
• (features) countChildren, leftMostChild,
  addChild, removeChild, etc.
• Cell
• (features) parent, nextSibling, etc.

6
Graphics Example

• class Composite_Figure extends
• Figure, LinkedListFigure ...
• void display()
• forEachDo item.display()
• void translate()
• ...
• Illustrates multiple inheritance, polymorphic
• data structure, dynamic binding, recursion, etc.

7
Design Pattern

• Describing composite structures using multiple
  inheritance with a container class as a parent
• More Examples
• Menus with Submenus
• Compound Commands
• Graphical Object Groups

8
Eiffels approach to Multiple Inheritance
9
Name Clashes and Feature Renaming

• Eiffel bans intra-class overloading.
• Name clashes resulting from multiple inheritance
  can be resolved in the child class using feature
  renaming mechanism.
• Redeclaration/redefinition changes the feature,
  but keeps the name. Renaming changes the name,
  but keeps the feature.
• Redeclaration is semantic renaming is syntactic.
• Renaming also enables class designers to give
  locally appropriate names to features.

10
Repeated Inheritance
A
B
C
D

• Class D is a descendant of class A in more than
  one way. E.g.,
• A Driver (age, address, birthday(), etc.)
• B French_Driver
• C US_Driver
• D US_French_Driver
• Share age
• Replicate address

11
Multiple Inheritance of Features

• A class that inherits different but identically
  named features from different parents is invalid.
  
• Feature renaming may be used to avoid name clash.
• Note that this case excludes repeated
  inheritance.
• An attribute coming from repeated ancestor is
  shared, by default. Feature renaming may be
  used to replicate it.
• In C, all the fields of a repeated ancestor
  are either shared (virtual) or duplicated.

12
Transcontinental Drivers in Eiffel

• class French_US_Driver inherit
• French_Driver
• rename
• address as french_address,
• violations as french_violations,
• pay_fee as french_pay_fee
• end
• US_Driver
• rename
• address as us_address,
• violations as us_violations,
• pay_fee as us_pay_fee
• end
• feature . . . // shared age, name, ...
• end

13
Conflicting Redefinitions

• What if a feature f inherited by class D from
  repeated ancestor A has been redefined in B
  and/or C?
• Conflicts under sharing (two impl., one name)
• Problem Name Clash
• Eiffel provides rename, redefine, and
  undefine to enable a programmer to disambiguate.
  

14
(contd)

• Conflicts under replication (two impl., two
  names)
• Problem Ambiguity for Dynamic binding
• A a new D() a.f()
• Eiffel provides select to let the programmer pick
  the appropriate impl. to run for f() on a D via
  a A-entity.

15
Windows in Eiffel

• class Window
• feature display is end
• class Window_with_Border inherit Window
• feature display is end
• class Window_with_Menu inherit Window
• feature display is end
• class Window_with_Border_and_Menu inherit
• Window_with_Border
• Window_with_Menu
• feature display is
• Window_with_Border Precursor
• Window_with_Menu Precursor
• end
• Problem Indirectly invokes Window.display
  twice!!

16
(Contd)

• class Window_with_Border_and_Menu inherit
• Window
• rename id as window_id
• select window_id
• redefine display end
• Window_with_Border
• rename id as border_id
• redefine display end
• Window_with_Menu
• rename id as menu_id
• redefine display end
• feature
• display is
• Window Precursor
• draw_border draw_menu
• end
• end