OBJECTORIENTED PROGRAMMING

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OBJECT-ORIENTED PROGRAMMING

• Spring 2009 Haifa University

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OOP - OPerators

• Operators Intro
• Overloading operators
• Streams
• Examples

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Operators in C / 1

• Operators in programming languages are similar to
  operators in math
• C supports many operators
• Arithmetic -b (negative), ab, a-b, ab, a/b,
  ab, a, a--, a, --a, a b, a-b, ab, a/b,
  ab
• Comparison ab, altb, agtb, altb, agtb, a!b
• Logical a b, a b , !b
• Bitwise ab, ab, ab, a, agtgtb, altltb, ab,
  ab, altltb, altltb
• Others ab, a() function call, ab array
  member, new a, new a, delete a, delete a,
  ab, a.b, a-gtb, a, a, a?bc

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Operators in C / 2

• Operator classification by number of operands
• Unary -b (negative), !b, a, new a, new a,
  delete a, delete a, a, a--, a, --a, a, a,
  and etc
• Binary ab, a-b, ab, a/b, ab, a b, a-b,
  ab, a/b, ab, ab, altb, agtb, altb, agtb,
  a!b, a b, a b, ab, ab, ab, agtgtb, altltb,
  ab, ab, altltb, altltb, ab, and etc
• Ternary a?bc

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Operators Precedence / 1

• Precedence classifies all operators in to groups.
  An operator of higher precedence is evaluated
  first
• x 2 4 2
• a 2 x a 2

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Operators Precedence / 2
int main() // example 1 int a 2 //
0b0101 int b 3 // 0b0011 int c 4 //
0b0100 int d 5 // 0b0101 int ans a
b c d // ans ? // example 2 int a
5 0b0101 int b 7 0b0111 bool ans a
b 2 // true or false
if (a b2)
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Operators Associativity / 1

• Associativity determines how operators of the
  same precedence are grouped in the absence of
  parentheses
• Left-to-right, right-to-left

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Operators Associativity / 2
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Operators Associativity / 3
int main() // example 1 int a,b1 a b
3 // a ?, b ? // example 2 int
a2, b8 int c b / a 2 // c ? //
example 3 int b 2 b 3 // ? b
3 // ?
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Operators with Classes

• Which operators are defined for classes?
• new, new, delete, delete create/destruct
• a.b access class member
• a-gtb access class member from a pointer to
  class object
• a b default assignment operator
• Default behavior can be overloaded
• What about all others arithmetic, comparison,
  logical, bitwise?
• There is no default behavior
• Could be user-defined

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OOP - OPerators

• Operators Intro
• Overloading operators
• Streams
• Examples

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Why overload an operator?

• Use class objects like primitive types
• Programmer convenience
• For example, overload arithmetic operators for
  complex classes like matrix
• Reuse code that was written for the primitive
  types
• For example, overload comparison operators (lt, gt,
  , !) for sorting
• Create new behavior
• Overload an array access operator for the
  LinkedList class and manage it like a simple
  array
• Overload operator for string and use it as
  concatenator

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How to overload an operator?

• It is possible to overload operator of a specific
  class
• An operator can emulated by a function
• For example a b can be emulated in two ways
• a.operator(b) // like a member function of the
  class
• b can be anything not necessarily an object
  of the same class. For example, a1.
• operator(a,b) // a regular function defined for
  a specific class
• At least one of the operands a or b has to be
  an object. For example, 1b, a1
• Rules
• One of the objects has to be an object of the
  class
• Cant define new operators
• Cant change precedence/associativity

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Example / 1

• Overloading operator (member function)
• Overloading operator (friend function)

• class Matrix
• public
• Matrix (int m, int n)
• Matrix (const Matrix other)
• Matrix()
• int get_cell(int m, int n) const
• void set_cell(int m, int n, int val)
• int getM() const return m
• int getN() const return n
• Matrix operator(const Matrix other)
• friend Matrix operator(const Matrix a,
  const Matrix b)
• private
• int m,n
• int vals

matrix.h
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Example / 2

• include matrix.h
• Matrix Matrixoperator(const Matrix other)
• Matrix temp Matrix(m,n)
• for(int i0iltmi)
• for(int j0jltnj)
• temp-gtvalsij valsij
• other.valsij
• return temp
• Matrix operator(const Matrix a, const Matrix
  b)
• Matrix temp Matrix(a.m,b.n)
• for(int i0ilta.mi)
• for(int j0jltb.nj)
• temp-gtvalsij 0
• for(int l0llta.nl)
• temp-gtvalsij
• a.valsilb.valslj
• return temp

matrix.cpp
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A Friend Access Modifier

• A friend access modifier is used in
  object-oriented programming to allow access to
  private or protected data in a class from outside
  the class
• C supports friend class and friend function

class A class B public friend void foo(B
b) friend class A
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Encapsulation Clarification

• Friend modifier breaks the encapsulation
  principle
• Per-class protection vs Per-object protection
• In per-class protection class methods can access
  any member of any object of that class and not
  just the receiver (C)
• MyClass a,b
• a.foo(b) can access any fields/methods of b
• In per-object protection class methods can access
  any member of the receiver object, but only
  public members of other objects of that class
  (Java)
• MyClass a,b
• a.foo(b) can access only public fields/methods
  of b

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Can we manage without friend?

• It is possible in classes where every private
  member have a selecting function (a getter)

• include matrix.h
• // remove this function declaration from the
  header file
• Matrix operator(const Matrix a, const Matrix
  b)
• Matrix temp Matrix(a.getM(),b.getN())
• for(int i0ilta.getM()i)
• for(int j0jltb.getN()j)
• temp-gtset_cell(i,j,0)
• for(int l0llta.getN()l)
• temp-gtset_cell(i,j(temp-gtget_cell(i
  ,j)
• a.get_cell(i,l)b.set_cell(l,j)
• return temp

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What can we overload?
- / ! lt gt
- / ltlt gtgt ltlt gtgt ! lt gt
-- , -gt -gt () what not ? .
. ? sizeof
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Overloading Operator / 1

• Default behavior is a bit-by-bit copy, like in
  copy-constructor
• Has the same problem as the default
  copy-constructor

• Generally, if there are pointers in class (data
  members), when all constructor, copy-constructor,
  destructor and operator should be overloaded

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Overloading Operator / 2

• class Student
• public
• Student(long id 0)
• Student(char id)
• Student(const Student other)
• Student()
• Student operator(const Student other)
• long get_id()
• void set_id (long id 0)
• void set_id(char id)
• char get_name()
• void set_name(char fname)
• private
• long id
• char fname

student.h
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Overloading Operator / 3

• include Student.h
• include ltstring.hgt
• Student Studentoperator(const Student other)
• id other.id
• fname new charstrlen(other.fname)1
• strcpy(other.fname, fname)
• return this
• int main()
• Student s1(12345)
• Student s2
• s2 s1

s2 s2.operator(s1)
student.cpp
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Overloading unary operators

• Unary operator
• -a (negative), !a, a, new a, new a, delete a,
  delete a, a, a--, a, --a, a, a, and etc
• Member function
• MyClass operator (void)
• Non-member (friend)
• MyClass operator (const MyClass )

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Overloading and --

• Pre-increment and pre-decrement
• a, --a
• MyClass operator()
• MyClass operator--()
• Post-increment and post-decrement
• a a--
• Should be also const MyClass operator()
• Should be also const MyClass operator--()
• How to resolve the ambiguity
• Post-increment and post-decrement use dummy
  parameter
• const MyClass operator(int)
• const MyClass operator--(int)

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OOP - OPerators

• Operators Intro
• Overloading operators
• Streams
• Examples

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Streams

• C provides a new library for I/O called
  ltiostreamgt
• ltiostreamgt provides the following classes
• ostream (output stream)
• write //block of data
• put // char
• flush
• operatorltlt
• istream (input stream)
• get, getline
• read
• operatorgtgt
• ltiostreamgt defines standard streams supported by
  OS
• ostream cout (standard output)
• ostream cerr (standard error)
• istream cin (standard input)

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Example / 1

• include ltiostreamgt
• include student.h
• void Studentprint() const
• cout ltlt ID ltlt id ltlt endl
• if (fname ! NULL)
• cout ltlt Full name ltlt fname ltlt endl
• else
• cout ltlt "Full name unspecified\n"
• cout ltlt Started degree in ltlt start_year
  ltlt
• 
  \n\n"

student.cpp
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Example / 2

• include ltiostreamgt
• include student.h
• int main()
• cout ltlt Enter id
• long id 0
• cin gtgt id
• cout ltlt Enter full name
• char fname new char20
• cin gtgt fname
• cout ltlt Enter first academic year
• int year
• cin gtgt year
• Student s1(id)
• s1.set_name(fname)
• s1.set_year(year)
• s1.print()
• delete fname

student.cpp
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Operator overloading with I/O

• Stream operators ltlt and gtgt work on primitive
  types
• What about classes?
• No default behaviour, but can be defined

• class Student
• public
• .....
• friend istream operatorgtgt(istream in, Student
  s)
• // replaces void print()
• friend ostream operatorltlt(ostream out, Student
  s)
• private
• .....

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Example / 3

• ostream operatorltlt(ostream out, Student s)
• outltltID ltlt s.id ltlt \n
• if (fname ! NULL) out ltlt Full name ltlt
  s.fname
• else out ltlt "Full name unspecified\n"
• out ltlt Started degree in ltlt
  s.start_year
• return out
• istream operatorgtgt(istream in, Student s)
• cout ltlt Enter id
• in gtgt s.id
• cout ltlt Enter full name
• cin gtgt s.fname
• cout ltlt Enter first academic year
• in gtgt start_year
• return in
• int main()
• Student s cin gtgt s cout ltlt s

student.cpp
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OOP - OPerators

• Operators Intro
• Overloading operators
• Streams
• Examples

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String Operators / 1

• Overload operator to concatenate strings
• Overload comparison operators

class String char chars int
length public String() String(char
value) String() String operator(const
String s) bool operatorgt(const String
s) String operator(const String s) char
getString() void setString(char value)
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String Operators / 2
bool Stringoperatorgt(const string s)
return strcmp(m_str, s.m_str) gt 0
String Stringoperator(const String
s) int new_len length s.length 1 char
v new char new_len strcpy(v,
chars) strcat(v, s.chars) delete
chars chars v length new_len return
this
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Vector2D Operators / 1

• class Vector2D
• private
• double x, y
• public
• Vector2D(double x_init0, double y_init0)
• double getX() const return x
• double getY() const return y
• friend Vector2D operator( const Vector2D
  left,
• const Vector2D
  right)
• double operator(int index) const
• friend ostream operatorltlt(ostream ostr,
• const Vector2D v)

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Vector2D Operators / 2

• friend Vector2D operator (const Vector2D left,
  
• const Vector2D
  right)
• return Vector2D(left.xright.x,left.yright.y
  )
• double operator(int index) const
• return (index 0)? xy
• friend ostream operatorltlt(ostream ostr,
• const Vector2D v)
  
• return ostr ltlt ( ltlt v.x ltlt , ltlt v.y ltlt
  )
• //usage
• Vector2D a(3,2), b(1)
• Vector2D c ab
• cout ltlt ( ltlt c.getX() ltlt , ltlt c.getY() ltlt
  )

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Member vs. Non-Member operators

• Use members
• When returning a reference
• Assignment operators
• Unary operators, usually
• Use friends for
• Symmetric operators (arithmetic, comparison,
  logical)
• Matrix m1 double arr
• m1 arr // can work as a member function
• arr m1 // cant work as a member function,
  since the first element isnt a Matrix. Use
  friend function
• Operators on classes which cannot be altered
• istream, ostream

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Danger in overloading operators

• Operator overloading changes the usual and
  expected behavior
• Could be easily misused by a programmer who is
  unaware about the change
• Operator overloading should be used with care,
  only in cases where it is needed
• When overloading operators try to keep an
  expected behavior as much as possible

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HANDS-ON ZONE