CS 2130

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CS 2130

• Presentation 31
• Class Object Implementation
• including Inheritence and Polymorphism

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History

• Simula 67
• Designed for simulation
• Development of class construct
• Template for instances
• Data and routines to manipulate data packed
  together
• Code executed at initialization only
• Hierarchies allowing code reuse
• Smalltalk
• Alan Kay Dissertation on Dynabook
• General use computers
• Interactive
• Graphical interface (LOGO -- Papert)
• Flex (based on Simula 67)
• Dynabook concept of top sheet in focus others
  partially covered

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History

• Smalltalk (continued)
• Kay joins PARC
• Learning Research Group founded
• Implementation of Dynabook
• Xerox Alto Hardware
• Smalltalk 72
• C
• Bjarne Soustrup - Bell Labs
• C with Classes - 1981
• Simula 67 Organization
• No performance penalty
• Used wherever C could be used
• C 1984
• Virtual functions, dynamic binding, function name
  operator overloading and reference types.

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History

• C
• Bjarne
• Backed by Bell
• Easy path from C to C
• C Initially very cheap
• C highly available

• Eiffel
• Integrated use of assertions to enforce contract
• Bertrand Meyer
• No major backer
• Difficult to migrate
• Expensive
• Compilers not available

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History

• Java
• Provides much of the power and flexibility of C
• Smaller, simpler, safer language
• Designed for embedded consumer electronics
  devices
• Reliability critical
• Useful for web programming
• Principal designer James Gosling
• Designed emacs

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Sidebar

• Clean Shaven (by choice)
• Konrad Zuse (Plankalkül)
• John Backus (Fortran, BNF)
• Fritz Bauer (Algol)
• Ole-Johan Dahl (Simula)
• Robert Kowalski (Prolog)

• Clean Shaven (not by Choice)
• Grace Hopper (Cobol)
• Adele Goldberg

• Not Clean Shaven
• Bjarne Soustrap (C)
• James Gosling (emacs, Java)
• Peter Naur (Algol, BNF)
• Dennis Ritchie (C)
• John Kemeny (Basic, Moustache)
• Nicklaus Wirth (Pascal, Modula)
• Edsger Dijkstra (Concurrency)
• John McCarthy (Lisp)
• Brian Kernighan (C)

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Question?

• Do you have to be using an OO language to write
  OO programs?
• 1. yes
• 2. no

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OO Example

• Language Java
• Class Time
• Will hold a time "quantity"
• Hours
• Minutes
• Seconds
• Constructor(s)
• Method to allow setting time
• Calculation of time in seconds

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• class Time
• private int hours, minutes, seconds
• // Constructor 1
• public Time()
• setTime(0, 0, 0)
• // Constructor 2
• public Time(int hours, int minutes, int seconds)
• setTime(hours, minutes, seconds)

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• // class Time (Continued)
• // Modifier
• public void setTime(int h, int m, int s)
• this.hours h
• this.minutes m
• this.seconds s
• // Accessor
• public int elapsedSeconds()
• return ( (this.hours 60) this.minutes )
• 60 this.seconds
• // class Time

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Oh, say can you C?

• How close can we come in C?

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Obvious start

• typedef struct
• int hours
• int minutes
• int seconds

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Do we put functions in here?

• typedef struct
• int hours
• int minutes
• int seconds

Do we want each instance to have a copy of the
function code?
Functions here?
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No!

• We'll use function pointers!

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Function Pointers in Place

• typedef struct timeobj
• int hours
• int minutes
• int seconds
• void (setTime) (int h, int m, int s,
• struct timeobj this)
• int (elapsedSeconds) (struct timeobj this)
• Time

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We still need to write the functions

• void f_setTime(int h, int m, int s,
• struct timeobj
  this)
• this-gthours h
• this-gtminutes m
• this-gtseconds s
• int f_elapsedSeconds(struct timeobj this)
• return ( (this-gthours 60) this-gtminutes )
  60
• 
  this-gtseconds

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Constructor

• // Recall the Java version
• Time t1 new Time()
• Time t2 new Time(10, 20, 30)

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Our Constructor

• Time new_Time(int h, int m, int s)
• Time newTime
• newTime malloc(sizeof(newTime))
• / Error checking omitted /
• newTime-gtsetTime f_setTime
• newTime-gtelapsedSeconds f_elapsedSeconds
• newTime-gtsetTime(h, m, s, newTime)
• return newTime
• / Use /
• Time t1 new_Time(0, 0, 0)
• t1-gtsetTime(12, 34, 56, t1)

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Inheritence
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Java Subclass

• class PTime extends Time
• void printTime()
• System.out.println(
• "Time " hours"" minutes"" seconds)

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Inheritance

• Make a new structure.
• Include in new structure the parent structure
• typedef struct ptimeobj
• Time super
• void (printTime) (struct ptimeobj this)
• PTime

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Diagram
Time
hours
minutes
seconds
setTime
elapsedSeconds
PTime
super
printTime
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Inheritance

• The function
• void f_printTime(struct ptimeobj this)
• printf("Time d02d02d\n",
• this-gtsuper-gthours,
• this-gtsuper-gtminutes,
• this-gtsuper-gtseconds)

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Inheritance

• The constructor
• PTime new_PTime(int h, int m, int s)
• PTime newPTime
• newPTime malloc(sizeof(newPTime))
• / Error checking omitted /
• newPTime-gtsuper new_Time(h, m, s)
• newPTime-gtprintTime f_printTime
• return newPTime

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Inheritance

• Test main
• int main()
• Time t1 new_Time(0, 0, 0)
• PTime t2 new_PTime(2, 42, 13)
• t1-gtsetTime(12, 34, 56, t1)
• t2-gtprintTime(t2)
• printf("t1 elapsed time d\n",
• t1-gtelapsedSeconds(t
  1))
• return EXIT_SUCCESS

Time 24213 t1 elapsed time 45296
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Polymorphism
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What is polymorphism?

• Webster's Dictionary poly-mor-phic able to
  assume different forms.

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What is polymorphism?

• Appel A function is polymorphic (from the Greek
  manyshape) if it can operate on arguments of
  different types. There are two main kinds of
  polymorphism
• Parametric polymorphism. A function is
  parametrically polymorphic if it follows the same
  algorithm regardless of the type of its argument.
  (The Ada or Modula-3 generic mechanism , C
  templates, or ML type schemes are examples)
• Overloading. A function identifier is overloaded
  if it stands for different algorithms depending
  on the type of its argument. (Example sign same
  for floatsints)

Modern Compiler Implementation in C
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What is polymorphism?

• Loudon A language is polymorphic if it allows
  language constructs to have more than one type.
• Monomorphism names expressions have unique
  types
• Overloading Form of polymorphism where separate
  declarations are allowed for the same name
• swap(anytype a, anytype b)
• anytype t
• t a
• a b
• b t

int p, q float x,y ... swap(p,q) swap(x,y) swa
p(p,y)
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What is polymorphism?

• Sebesta "The third characteristic of
  object-oriented programming languages is a kind
  of polymorphism provided by the dynamic binding
  of messages to method definitions."
• Allows one to define polymorphic variables of the
  type of the parent class that are also able to
  reference objects of any of the subclasses of
  that class.
• The parent and child class can both have a method
  with the same name. At run time the appropriate
  method is called depending on the type of the
  object.

Concepts of Programming Languages 4th Edition
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Example

• Polymorphism in C
• define MAX(a,b) ((a) gt (b) ? (a) (b))
• void qsort(void elems,
• int numElems,
• int elemWidth,
• int (compareFunc)
• (void e1, void e2))

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Example

• Polymorphism in C
• typedef struct
• MusicianType type
• union
• Trumpeter tData
• Flautist fData
• / ... /
• u
• Musician

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Example

• Polymorphism in C
• void play(Musician m)
• switch(m-gttype)
• case TRUMPETER
• printf("trumpeter plays toot\n")
• break
• case FLAUTIST
• printf("flautist plays tweet\n")
• break
• / ... /

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Previous Example

• Polymorphism in C
• Assume that we have our previously defined
• Time and PTime types.
• The Java polymorphic type Time
• Time t1 new Time(0,0,0)
• Time t2 new PTime(1,2,3)
• typedef struct
• TimeType type
• void timeptr
• TimeRef

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Previous Example

• Java
• Time t1 new Time(0,0,0)
• Time t2
• new PTime(1,2,3)

• C
• TimeRef t1
• t1.type TIME
• t1.timeptr new_Time
• (0, 0, 0, t1.timeptr)
• TimeRef t2
• t2.type PTIME
• t1.timeptr new_PTime
• (1, 2, 3, t2.timeptr)

Note that an array of TimeRef's would be able to
hold objects of different types!
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Previous Example

• Java
• t1.setTime(4,5,6)

• C
• switch(t1.type)
• case(TIME)
• ((Time)(t1.timeptr))-gt
• setTime(4, 5, 6,
• ((Time)(t1.timeptr)))
• break
• case(PTIME)
• ((PTime)(t1.timeptr))-gt
• super-gtsetTime(4, 5, 6,
• ((PTime)
• (t1.timeptr)-gtsuper))
• break

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Questions?
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(No Transcript)