Multiple Inheritance

1
Multiple Inheritance

• Fall 2005 OOPD
• John Anthony

2
The Problem
Male
North American
Parent
Professor
Author
3
The (Conceptual) Solution
By breaking down the hierarchy, each abstract
type can be combined to create valid is-a
Relationships.
4
Multiple Inheritance

• An Author is-a Male and is-a North American
  and.
• An Author can be viewed as-a Male and as-a
  North American and.

5
Consider
Professor
Student
getDepartment ()
getDepartment ()
Name Ambiguity Which getDepartment() is Student
going to inherit?
TeacherAssistant
getDepartment ()
6
Resolving Name Ambiguity
Force the client of the child class to resolve
the ambiguity

• TeacherAssistant assistant
• assistant-gtProfessorgetDepartment()
• assistant-gtStudentgetDepartment()

7
Resolving Name Ambiguity
If the signatures are different (and overloading
supported) then child class can simply include
both methods.
8
Resolving Name Ambiguity
If signatures are similar or if overloading not
allowed, then programmer may override one method
and rename another.

• Class TeachingAssistant public Professor,
  public Student
• public
• virtual Department getTeachingDepartment()
• return ProfessorgetDepartment()
• virtual Department getDepartment()
• return StudentgetDepartment()

What about polymorphism?
Professor p new TeachAssistant() p-gtgetDepart
ment() //returns the Department the TA is
studying in rather than teaching in.
9
Resolving Name Ambiguity
A C idiom to addressing name ambiguity.
Professor
Student
getDepartment ()
getDepartment ()
ProfessorParent
StudentParent
getDepartment ()
getDepartment ()
getMajorDepartment()
getTeachingDepartment()
TeacherAssistant
getTeachingDepartment()
getMajorDepartment()
10
Idiom Cont.

• Class ProfessorParent public Professor
• public
• virtual Department getDepartment()
• return getTeachingDepartment()
• virtual Department getTeachingDepartment()
• return Professor getDepartment()

11
Resolving Name Ambiguity

• TeachingAssistant ta new TeachingAssistant()
• Professor prof ta
• Student student ta
• prof-gtgetDepartment() //OK, will execute
  getTeachingDepartment
• Student-gtgetDepartment() //OK, will execute
  getMajorDepartment
• ta-gtgetDepartment() //compiler error, ambiguous
  invocation

12
Redefinition in Eiffel

• cClass TeachingAssistant
• inherit
• Professor
• rename
• getDepartment as getTeachingDepartment
• end
• Student
• rename
• getDepartment as getMajorDepartment
• end

13
Common Ancestors
A
A
A
B
C
B
C
D
D
14
Common Ancestors
Person
String title
the diamond of death
Professor
Student
String title
String title
TeacherAssistant
Should there be two titles or one?