Advanced Topics on Inheritance

1
Advanced Topics on Inheritance

• Multiple inheritance allows a derived class to
  inherit from more than one direct base class
• class Bear public ZooAnimal /.../
• class Panda public Bear, public Endangered
  /.../
• Virtual Inheritance allows the derived class to
  share the base class, as a single virtual base
  class
• Regardless how often a base class shows up on
  different inheritance paths, only one base class
  will be included

2
Single Inheritance

• class A
• public
• A(int num) aInt(num)
• cout ltlt "Aconstructor"ltltendl
• A()
• cout ltlt"Adestructor"ltltendl
• protected
• int aInt
• class B public A
• public
• B(int numa, int numb)
• A(numa), bInt(numb)
• cout ltlt "Bconstructor"ltltendl
• B()
• cout ltlt "Bdestructor"ltltendl
• protected
• int bInt

• int main (int, char )
• B b(2, 3)
• return 0
• Output
• Aconstructor
• Bconstructor
• Bdestructor
• Adestructor
• Order of execution
• constructors base class first then derived class
• destructors reverse of constructors

A
B
3
Single Inheritance, continued

• // A and B are as declared
• // in the previous slide
• class C public B
• public
• C(int numa, int numb, string s )
• B(numa, numb), cStr(s)
• cout ltlt "Cconstructor"ltltendl
• C()
• cout ltlt "Cdestructor"ltltendl
• protected
• string cStr
• int main (int, char )
• C c(4, 7, "hello")
• return 0

• Output
• Aconstructor
• Bconstructor
• Cconstructor
• Cdestructor
• Bdestructor
• Adestructor

A
B
C
4
Multiple Inheritance

• class X
• public
• X() xChar('D') cout ltlt "Xdefault
  constructor"ltltendl
• X(char c) xChar(c) cout ltlt "Xconstructor"ltlten
  dl
• X()
• cout ltlt "Xdestructor"ltltendl
• protected
• char xChar
• class Y
• public
• Y(char c) yChar(c)
• cout ltlt "Yconstructor"ltltendl
• Y() cout ltlt "Ydestructor"ltltendl
• protected
• char yChar

• class Z public X, public Y
• public
• Z(char xC,char yC, int num)
• X(xC), Y(yC), zInt(num)
• cout ltlt "Zconstructor"ltltendl
• Z()
• cout ltlt "Zdestructor"ltltendl
• protected
• int zInt
• int main (int, char ))
• Z zObj('z', 'b', 8)
• return 0
• Output
• Xconstructor
• Yconstructor
• Zconstructor
• Zdestructor

Y
X
Z
5
Multiple Inheritance, continued

• class MI public C, public Z
• public
• MI(int numa, int numb, string s, char xC, char
  yC, int numZ )
• C(numa, numb, s), Z(xC, yC, numZ) cout ltlt
  "MIconstructor"ltltendl
• MI() cout ltlt "MIdestructor"ltltendl
• protected
• string cStr

• int main (int, char )
• MI mi(2,4,"eve", 'r', 's', 26)
• return 0
• Output
• Aconstructor
• Bconstructor
• Cconstructor
• Xconstructor
• Yconstructor
• Zconstructor
• MIconstructor
• MIdestructor
• Zdestructor
• Ydestructor
• Xdestructor
• Cdestructor

A
Y
X
B
Z
C
MI
6
All Base Class Constructors are Called

• // X and Y as declared previously
• class R public X, public Y
• public
• R(char xC, char yC, int numx) Y(yC), X(xC),
  rInt(numx) cout ltlt
• "Rconstructor" ltlt endl
• R()
• cout ltlt "Rdestructor" ltltendl
• protected
• int rInt
• class S public Y, public X
• public
• S(char yC, int numx) Y(yC), sInt(numx) cout ltlt
  "Sconstructor"ltltendl
• S() cout ltlt "Sdestructor"ltltendl
• protected
• int sInt

• int main (int, char )
• R r ('x', 'y', 8)
• return 0
• Output
• Xconstructor
• Yconstructor
• Rconstructor
• Rdestructor
• Ydestructor
• Xdestructor
• int main (int, char )
• S s('y', 10)
• return 0
• Output
• Yconstructor
• Xdefault constructor

7
Base Pointer/Reference Type Restricts Interface

• // Based on LLM Ch. 18.3 and
• // todays studio exercises
• Bear bear_ptr
• new Panda ("bao_bao")
• bear_ptr -gtprint() // ok
• bear_ptr -gttoes() // ok
• bear_ptr -gtcuddle() // not ok
• bear_ptr -gtgrowl() // not ok
• delete bear_ptr // ok
• Endangered endangered_ptr
• new Grizzly
• endangered_ptr-gtprint() // ok
• endangered_ptr-gttoes() // not ok
• endangered_ptr -gtcuddle()// not ok
• endangered_ptr -gtgrowl() // not ok
• delete endangered_ptr // ok

Method Classes Declaring It
print Animal Bear Endangered Panda Grizzly
toes Bear Panda Grizzly
growl Grizzly
cuddle Panda
(virtual) destructor Animal Bear Endangered Panda Grizzly
8
Member Inheritance from Multiple Base Classes

• When a class has multiple base classes, a derived
  class can inherit a member with the same name
  from two or more base classes
• Unqualified uses of that name are ambiguous
• // Loosely based on LLM Ch. 18.3
• double Gryphonmax_weight() const
• return stdmax (Eaglemax_weight_, //
  scoping necessary
• Lionmax_weight_) //
  scoping necessary
• This in turn motivates the use of virtual base
  classes
• // Single Animal instance shared by Eagle and
  Lion parts of Gryphon
• class Eagle virtual public Animal /.../
• class Lion virtual public Animal /.../
• class Gryphon public Eagle, public Lion
  /.../

9
More About Virtual Base Classes

• Still Polymorphic
• Can convert between uses as Derived vs. Base
• Members of virtual Base class normally can be
  uniquely identified
• base class is instantiated only once
• if the variable is in both base and derived
  class, then derived class has higher precedence
• If the member is in 2 derived classes, then it is
  still ambiguous
• move members up to avoid that, e.g.,
  Animalmax_weight_
• The most derived class controls the
  initialization of the shared virtual base class

10
Virtual Base Class Constructor/Destructor Order

• Constructors
• All virtual base classes
• up each branch of the inheritance lattice,
  according to order in which immediate base
  classes were declared
• Non-virtual base classes
• up each branch of the inheritance lattice,
  according to order in which immediate base
  classes were declared
• Most derived class last
• Destructors (in reverse order of constructors)
• Most derived class first
• Then each non-virtual base class, moving up
  lattice
• Then each virtual base class, moving up lattice