CIS 403503 Accelerated DataFile Structures

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CIS 403/503Accelerated Data-File Structures

• Lecture 6
• C Operator Overloading

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Objectives

• In this lecture, you will learn
• Abstract methods and classes
• Slicing and Type Conversion
• Basics of operator overloading
• Overloading I/O
• Global vs. Class Operators
• What can be overloaded?
• A Rational Number Class

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Abstract Methods and Classes
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Abstract Methods and Classes

• In C, a method is abstract if
• It is declared virtual
• The declaration is followed by 0
• A class is abstract if it has at least one
  abstract method an abstract class cannot be
  instantiated

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Example

• class Shape
• public
• Shape(const string s)shapeType(s)
• virtual Shape()
• const string getType() const
• return shapeType
• virtual double getArea() const 0
• private
• string shapeType

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Example

• Class Circle public Shape
• public
• Circle(double r)Shape(Circle), radius( r)
• double getArea() const
• return 3.14 radius radius
• private
• double radius

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Slicing
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Slicing

• Circle c(3, 100, 80)
• Point p(10, 20)
• P c
• p.print()

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Type Conversion

• Cast down the inheritance hierarchy
• Point p new Circle(3, 100, 80)
• ((Circle ) p)-gtradius
• What happens if p is actually pointing at a Point
  at runtime?

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Dynamic Cast

• Runtime checked cast in an inheritance hierarchy
• Finding the beginning of an object
• Circle c dynamic_castltCircle gt (p)

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Static Cast

• Only allowed for conversions that the compiler
  can check
• Cannot cast a pointer to non-pointer
• Cannot cast to remove const-ness
• Cannot cast from a virtual base class

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Basics of Operator Overloading
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Operator Overloading

• Redefine the existing operators to work with new
  class types
• For example, overloading
• ltlt, gtgt

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Example 1

• class Person
• public
• Person(int s, const string n )ssn(s),
  name(n)
• const string getName() const
• return name
• int getSsn() const
• return ssn
• bool equals(const Person rhs) const
• return ssn rhs.ssn
• private
• const int ssn
• string name

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Example 2

• class Person
• public
• Person(int s, const string n )ssn(s),
  name(n)
• const string getName() const
• return name
• int getSsn() const
• return ssn
• bool equals(const Person rhs) const
• return ssn rhs.ssn
• bool operator (const Person rhs) const
• return equals(rhs)
• private
• const int ssn
• string name

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Overloading I/O
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Example 1

• class Person
• public
• Person(int s, const string n )ssn(s),
  name(n)
• const string getName() const
• return name
• int getSsn() const
• return ssn
• void print(ostream out cout) const
• out ltlt ltlt ssn ltlt, ltlt name ltlt
• private
• const int ssn
• string name

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Overload operatorltlt

• If defined in the Person class,
• operatorltlt(ostream out) const
• Must be invoked as p.operatorltlt(cout) instead of
  cout ltlt p
• Change the ostream class?
• Define non-class function operator ltlt

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Example 2

• class Person
• public
• Person(int s, const string n )ssn(s),
  name(n)
• const string getName() const
• return name
• int getSsn() const
• return ssn
• void print (ostream out) const
• out ltlt ltlt ssn ltlt , ltlt name ltlt
• private
• const int ssn
• string name
• ostream operatorltlt (ostream out, const Person
  p)
• p.print(out)
• return out

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Global vs. Class Operators
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Discussion

• An operator can be overloaded either as a member
  function or as a non-member function
• If its overloaded as a member function, then the
  first operand (lhs) must be of the class type
  (this is lhs)

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Discussion

• Disadvantage of overloading operator as
  non-member
• Cannot access the private data member of the
  class unless it is declared a friend
• ostream operator ltlt (ostream out, const
  Person p)
• out ltlt ltlt p.ssn ltlt , ltlt p.name ltlt
• return out
• Will it compile?

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Discussion

• Problem with overloading operator as member is
  lhs must match
• bool operator(const Person rhs) const
• return ssnrhs.ssn
• Person(123456789, Bob)
• cout ltlt (p1 123) ltlt endl //ok?
• cout ltlt (123 p1) ltlt endl //ok?
• What about overload operator as non-member
• bool operator(const Person lhs, const Person
  rhs)
• return lhs.equals(rhs)

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Discussion

• bool operator! (int lhs, const Person rhs)
• bool Personoperator! (const Person rhs) const

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What Can Be Overloaded?
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Discussion

• In C, only four operators cannot be overloaded
• Dot .
• .
• ?
• sizeof

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Discussion

• Although overloading and is legal, shot
  circuit evaluation feature may be lost
• Overloaded operator must have the same precedence
  and associativity
• Avity cannot be changed
• Only existing operator can be overloaded

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Rational Number Class
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Ration Number

• include Rational.h
• include ltiostreamgt
• using namespace std
• int main()
• Rational x
• Rational sum 0
• int n 0
• cout ltlt Type in rationals ltlt endl
• for (sum 0, n 0 cin gtgt x sumx, n)
• cout ltlt Read ltlt x ltlt endl
• cout ltlt Read ltlt n ltlt rationals ltlt endl
• cout ltlt Average is ltlt (sum/n) ltlt endl
• return 0

Sample input 3 4/5 7/2
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Declaration

• ifndef RATIONAL_H
• define RATIONAL_H
• include ltiostramgt
• using namespace std
• class Rational
• public
• Rational (int numerator 0) number
  (numerator), denom(1)
• Rational(int numerator, int denominator)
  number(numerator), deno(denominator)
• fixSigns() reduce()

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Declaration

• // Assignment Ops
• const Rational operator (const Rational
  rhs)
• const Rational operator -(const Rational
  rhs)
• const Rational operator /(const Rational
  rhs)
• const Rational operator (const Rational
  rhs)
• //unary Operators
• const Ratinal operator()
• Rational operator(int)
• const Ratinal operator--()
• Rational operator--(int)
• const Ratinal operator() const
• Rational operator-() const
• bool operator!() const

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Declaration

• // Name member functions
• double toDouble() const
• return static_castltdoublegt(numer) /denom
• int toInt() const
• return numergt 0? Numer/denom -(-numer/denom)
  
• bool isPositive() const return numer gt 0
• bool isNegative() constreturn numer lt0
• Bool isZero() const return numer 0
• void print(ostream out cout) const
• private
• int numer
• int denom
• void fixSigns() //ensures denom gt 0
• void reduce() //ensures lowest form

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Declaration

• // Math Binary Ops
• Rational operator(const Rational lhs, const
  Rational rhs)
• Rational operator-(const Rational lhs, const
  Rational rhs)
• Rational operator/(const Rational lhs, const
  Rational rhs)
• Rational operator(const Rational lhs, const
  Rational rhs)
• //Relational Equality Ops
• bool operatorlt (const Rational lhs, const
  Rational rhs)
• bool operatorlt (const Rational lhs, const
  Rational rhs)
• bool operatorgt (const Rational lhs, const
  Rational rhs)
• bool operatorgt (const Rational lhs, const
  Rational rhs)
• bool operator (const Rational lhs, const
  Rational rhs)
• bool operator! (const Rational lhs, const
  Rational rhs)
• //I/O
• ostream operatorltlt (ostream out, const Rational
  value)
• istream operatorgtgt (istream in, Rational
  value)
• endif

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Things to Remember

• Operator overloading redefines meanings for
  existing operators
• If an operator is overloaded as a member
  function, the first operand is this and the
  subsequent are parameters. The first operand
  must match the class type
• If an operator is overloaded as a non-member
  function, all operands are parameters, but the
  overloaded operator cannot access private members
  of any class
• The most common operators to overload include
  assignment operators, equality and relational
  operators and the input and output stream
  operators.

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Things to Remember

• Operatorltlt should be overloaded for all class
  types, typically by providing a companion print
  member function
• Operator and operator must be overloaded
  separately, and consistently. Typically operator
  simply invokes operator
• If operator is overloaded, typically both a
  prefix and a postfix version are provided
• If operator (array operator) is overloaded,
  typically both an accessor and a mutator version
  are provided

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Possible Quiz Questions

• Which operators cannot be overloaded?
• Explain the difference between overloading
  operator as a member function vs. a nonmember
  function.
• When must an operator be overloaded as a
  nonmember function?