Classes in C

1
Classes in C

• C originally called "C with classes"
• Swedish connection Bjarne Stoustrup borrowed
  from Simula (67)
• Simulating classes of real world objects
• C continues to evolve
• Version 1.0 released by ATT in 1986
• Version 2.0 in 1990 (added multiple inheritance)
• Version 3.0 in 1992 (templates)
• ANSI standard in 1996 (exception handling, run
  time type info)
• C became dominant OOPL in early 90's
• Now Java and C challenge
• A class extends the C type system
• class Account //creates a new type
• //Note you need this semi-colon in C
• Account a1,a2 //define variables of type Account

2
C structs

• A data structure for dates in C
• struct Date
• int month, day, year
• int set(struct Date, int m, int d, int y)
• int print(struct Date)
• Whats the problem?
• No information hiding
• No way to control access to data (obscure side
  effects)
• No way to prevent assigning an illegal value to
  month
• Changing representation of Date breaks all client
  code

3
C structs

• C provides for closer coupling of data and
  functions
• struct Date
• int month, day, year
• void set(int m, int d, int y)
• void print() //implement elsewhere
• Invoke functions with variable .
  memberFunction()
• Date today //In C, structs automatically
  define types
• today.set(9,29,1953) today.print()
• Now we have data abstraction
• Procedural abstraction hides details of code in
  functions
• Data abstraction couples data structure and
  functions
• Still, no information hiding today.set(50,-10,0)

4
C adds class

• C adds new keywords to support information
  hiding
• class Date
• int month, day, year
• public
• void set(int m, int d, int y)
• void print() //implement elsewhere
• Members after public are visible to clients
• Date today today.print()
• today.month 50 //is this legal?
• Members after class are by default private

5
Member functions

• What can we add to Date to allow access to month?
• class Date
• int month, day, year
• public
• void set(int m, int d, int y)
• void print() //implement elsewhere
• int getMonth() return month
• cout ltlt today.getMonth() //outside class
  Date
• Pros and cons of this approach?
• inline function is efficient, though it tends to
  break information hiding
• Lets define set() and protects the month data
• void Date set(int m, int d, int y)
• assert(m gt 1 m lt 31) //include
  ltassert.hgt
• month m
• How can we ensure that month data is set to valid
  values?

6
Inheritance (class derivation)

• class Account generalizes many kinds of bank
  accounts checking, savings, etc.
• C class derivation captures this generation
• class Checking public Account
• public
• Checking(float balance)
• Checking() //default constructor
• denotes derivationChecking inherits from
  Account
• public derivation denotes subtype inheritance
• Accounts public methods accessible to instances
  of Checking
• Checking myChecking(200) //Checking constructor
• myChecking.getBalance() //Account function

7
OOP Data abstraction inheritance dynamic
binding

• Polymorphism a function can mean different
  things at runtime
• Dynamic binding defer function binding to a
  subtype until runtime
• Suppose we want to draw a heterogeneous
  collection of shapes?
• class Point ... //a Point has x and
  y coordinates
• class Shape
• protected //accessible to subclasses but
  otherwise private
• Point center //all Shapes have a center Point
• public
• Point where() return center
• virtual void move(Point to) //virtual can be
  overridden
• center to draw() // by derived
  classes
• virtual void draw()0 //a "pure" virtual
  function
• //draw() must be implemented by derived
  classes
• //...

8
Subclasses override virtual functions

• class Triangle public Shape
• Point sw, se, top //Three points define
  triangle
• public
• Triangle(Point a, Point b, Point c) sw(a),
  se(b), top(c)
• draw() //implementing pure virtual function
• put_line(sw,top) //draw line from sw to
  top
• put_line(top,se) //draw line from
  top to se
• put_line(se,sw) //draw line from
  se to sw
• class Circle public Shape
• int radius
• public
• Circle(Point a, int r) center(a), radius(r)
  
• draw() //draw a circle using center and
  radius

9
Calling a virtual function

• //Construct some shapes
• Shape aShape //illegal--why?
• Circle c(Point(20,30),7) //legal--what does it
  do?
• //Create an array of various shapes
• Shape shapes10 //Why is this legal?
• shapes0 new Circle(Point(20,30),7)
  //assign a Circle
• shapes1 new Triangle(Point(50,50),Point(30,30
  ),Point(40,40))
• //... maybe assign other shapes, Rectangles,
  Squares, etc.
• for (int i0 i lt 10 i) //draw all the
  shapes
• shapesi-gtdraw() //each shape draws
  itself!
• Why do we say that elements of shapes array are
  polymorphic?
• How does polymorphic design support Open-Closed
  principle?

10
Why dynamic binding?

• What kind of code does dynamic binding avoid?
• Avoids lots of switch statements, e.g.
• switch (shapesi-gtisa)
• //each Shape derived class has an isa data
  member
• case(triangle) Triangledraw() //test
  enumeration
• case(circle) Circledraw() //run
  specific draw()
• // ...
• Why is the dynamic binding version better for
  big, growing programs?