CSC 143 O 1

1
CSC 143

• Inheritance and Object Oriented Design

2
Public Inheritance means "isa"

• If class D publicly inherits from class B, every
  object of type D is also an object of type B (not
  vice-versa)

class Person class Student public Person
void dance(const Person p) // anyone can
dance void study(const Student s) // only
students study
Person p Student s // p is a Person, s a
Student dance(p) // fine, p is a
Person dance(s) // fine, s is a Student,
therefore isa Person study(s) // OK study(p) //
error! p is not a Student
3
IsA Design (1)
class Bird public virtual void fly() // birds
can fly class Penguin public Bird //
penguins are birds

• Trouble penguins can fly!
• C is stricter than the English language. In
  C, "Birds can fly" means "All birds can fly"
• Solution?

4
IsA Design (2)

• Refine the hierarchy

5
IsA Design (3)
class Bird //no fly function class
FlyingBird public Bird virtual void
fly() class NonFlyingBird //no fly
function class Penguin public NonFlyingBird
// no fly function

• Could also do Penguins can fly, but it is an
  error for them to try to do so. How?

6
Interface ? Implementation

• Interface function declaration
• Implementation code for the function
• Member function interfaces are always inherited.
  If a function applies to a class, it must also
  apply to its subclasses.
• If you want the subclasses to inherit
• the interface only, make the member function pure
  virtual (virtual 0)
• the interface and a default implementation, make
  the member function virtual
• Non virtual means inherit the interface as well
  as a mandatory implementation

7
Example A Shape Class

• class Shape
• public
• virtual void draw() const 0 // Abstract
  class
• virtual void error(const string msg)
• int ObjectID() const
• class Rectangle public Shape
• class Ellipse public Shape
• Rectangle and Ellipse must provide a draw
  function
• Every class must support an error function (but
  you don't have to write your own if you don't
  want to)
• Every object has an ID. That ID is determined by
  the definition of ShapeobjectID. Don't change
  it!

8
Virtual Destructor

• Make sure that base classes have a virtual
  destructor
• If D is derived from B, what happens in the
  following?
• B pB new D
• delete pB
• If you try to delete a derived class object
  through a base pointer and the base class doesn't
  have a virtual destructor, the result is
  undefined.