CS 2130

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CS 2130

• Presentation 32
• 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

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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))

Parametric polymorphism or Overloading?
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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
• / ... /

Parametric polymorphism or Overloading?
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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
• setTime(4, 5, 6,
• ((PTime)(t1.timeptr)))
• break

Parametric polymorphism or Overloading?
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Where else have you seen polymorphism?
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Scheme?

• gt (cons 'a '(1 2 3))
• (a 1 2 3)
• gt (cons 1 '(1 2 3))
• (1 1 2 3)
• gt (cons '(1 2 3) '(1 2 3))
• ((1 2 3) 1 2 3)

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Representation of Polymorphic Variables

• Basic issue Size of data
• How to write code when size of data being passed
  into function is unknown
• Approaches
• Boxing
• Tagging
• Coercion
• Expansion
• Tag Passing

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Boxing

• Scheme/List Approach
• Put a pointer or reference to the data structure
  to be passed into a "box" so that the function
  always expects the same size item.
• cons always expects
• a pointer to something
• as the first argument to
• cons

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Tagging

• struct
• int tag
• union
• int x
• float y
• Node z

Note If the data item size is less than or equal
to the size of a pointer store the data item
directly
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Expansion

• switch(Based on the tag)
• case tag_type_1
• function for tag_type_1()
• break
• case tag_type_2
• function for tag_type_2()
• break
• ...

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Type Passing

• function(data/pointer tag)
• switch(based on tag)
• case tag1
• code
• break
• case tag2
• code
• break
• ...

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Coercion

• Function takes double
• Before call take any type (int, float, double)
• Conver to double
• Call function.

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Questions?
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