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Operator Overloading Fundamentals Methods (class members & friend functions) Binary operators Stream operators Unary operators Pre & post increment & decrement Fundamentals Operator overloading = using the same operator for different data types

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operator overloading
Operator Overloading
  • Fundamentals
  • Methods (class members & friend functions)
  • Binary operators
  • Stream operators
  • Unary operators
  • Pre & post increment & decrement
fundamentals
Fundamentals
  • Operator overloading = using the same operator for different data types
    • For example, “+” is used to add int, float, double, etc.
  • Function definition is written the same as any other function
    • Except function name becomes “operator” followed by the symbol that you are overloading
      • operator+
built in operators
Built-in Operators
  • A few operators don’t have to be explicitly overloaded (done automatically)
    • Assignment operator (=)
      • Memberwise assignment of class data members
    • Address operator (&)
      • Returns the address of the object in memory
    • Both can be explicitly overloaded
restrictions
Restrictions
  • For a list of operators that can be overloaded
    • See D & D, p. 670
  • Five operators cannot be overloaded
    • ., .*, ::, ?:, sizeof
  • These properties cannot be changed
    • Precedence: * has higher precedence than +
    • Associativity: (a + b) + c = a + (b + c)
    • Arity: number of operands that the operator takes (unary & binary operators)
restrictions5
Restrictions
  • Only existing operators can be overloaded
  • Overloading an assignment operator and an addition operator will not automatically overload the += operator

F1 = F2 + F3;

F1 += F2;

  • Misuse
    • Overloading the “+” operator to subtract
method 1
Method #1
  • Class Members: overloading an operator by adding another method (member function) to the class itself
    • Must use this method for overloading (), [], -> or any of the assignment operators
    • Can use this method when the leftmost operand is a class object
      • This works fine with f1 = f2 + 2; which converts “2” to a fraction (via a conversion constructor) and adds it to f2
      • But will not work with f1 = 2 + f2; since “2” is not of class fraction
method 17
Method #1

void main(){

Fraction a(1,3);

Fraction b(2,3);

Fraction c;

c = a + b;

c.print();

}

class Fraction {

int num, den;

public:

Fraction(int n=0, int d=1)

:num(n),den(d){}

Fraction operator+

(const Fraction &f){

Fraction temp;

temp.num=num*f.den+f.num*den;

temp.den=den*f.den;

return temp;}

void print() const{

cout<

}; Note: c=a+b is interpreted as

(see method1.txt) c.operator=(a.operator+(b))

method 2
Method #2
  • Friend Functions: overloading an operator as a friend function (non-member function) of the class
    • Can use this method when the leftmost or rightmost operand is a class object
      • This will fix our problem when we have a different class object on the left side of an operator
    • Must use this method when the leftmost operand is NOT a class object (for example, cout<
method 29
Method #2

void main(){

Fraction a(1,3);

Fraction b(2,3);

Fraction c;

c = a + b;

c.print();

}

class Fraction {

int num, den;

public:

Fraction(int n=0, int d=1)

:num(n),den(d){}

friend Fraction operator+

(const Fraction &a, const Fraction &b){

Fraction temp;

temp.num=a.num*b.den+b.num*a.den;

temp.den=a.den*b.den;

return temp;}

void print() const{

cout<

}; Note: c=a+b is interpreted as

(See method2.txt) c.operator=(operator+(a, b))

why does this program run
Why Does This Program Run?

void main(){

Fraction a, b;

a = a + 1;

b = 2.5 + b;

a.print(); //1/1

b.print(); //2/1

}

(See run.cpp)

class Fraction {

int num, den;

public:

Fraction(int n=0, int d=1)

:num(n),den(d){}

friend Fraction operator+

(const Fraction &a,

const Fraction &b){

Fraction temp;

temp.num=a.num*b.den+b.num*a.den;

temp.den=a.den*b.den;

return temp;}

void print() const{

cout<

};

because
Because…
  • C++ will try and convert types to perform operations requested
  • Asks: Can I convert an “int” to a Fraction?
    • Yes, we have a constructor that takes and integer and returns the equivalent Fraction
    • System invokes constructor to build Fraction from int
  • What about “Fraction = double + Fraction?
    • Affirmative. Can convert double to int (truncate)
    • Can then convert int to Fraction (constructor)
conversion constructor
Conversion Constructor
  • A constructor that transforms objects of one type into objects of another type
  • Write a constructor that will convert a double into a fraction (exercise1.txt)
overloading stream operators
Overloading Stream Operators
  • cin object
    • An instance of the istream class
    • operator>>
    • So cin>>a>>b; becomes
      • operator>>(cin,a); operator>>(cin,b);
  • cout object
    • An instance of the ostream class
    • operator<<
    • So cout<
      • operator<<(cout,a); operator<<(cout,b);
stream operators
Stream Operators

class Fraction{

. . .

friend istream & operator>>(istream &in, Fraction &f){

char ch;

in>>f.num>>ch>>f.den;

return in;}

friend ostream & operator<<(ostream &out,

const Fraction & f){

out<

return out;}

};

int main(){

Fraction a,b;

cin>>a>>b;

cout<

return 0;

} (See stream.txt)

overloading unary operators
Overloading Unary Operators
  • Can be overloaded as a class member or as a friend function
    • For example, the logical not operator (!)
      • Will change true to false or false to true
      • Returns either true (non-zero) or false (zero)
      • Use bool (boolean) data type
overloading unary operators16
Overloading Unary Operators
  • As a class member

bool operator!() const;

bool Fraction::operator!()const{

if(num) return false;

return true;

}

  • As a friend function

friend bool operator!(const Fraction &);

bool operator!(const Fraction &f){

if(f.num) return false;

return true;

} (See unary.txt)

overloading
Overloading ++ & --
  • Preincrement
    • Original code: ++f1;
    • Compiler will generate: f1.operator++();
    • Function prototype: Fraction &operator++();
      • Returns a reference to itself
overloading18
Overloading ++ & --
  • Postincrement
    • Original code: f1++;
    • Compiler will generate: f1.operator++(0);
      • This is a “dummy value” to distinguish it from preincrement
    • Function prototype: Fraction operator++(int);
      • Value return, not a reference return
unary operators
Unary Operators

class Fraction{

. . .

Fraction &operator++(){//preincrement

num+=den;

return *this;}

Fraction operator++(int){//postincrement

Fraction temp = *this;

num+=den;

return temp;}

};

void main(){

Fraction a,b,c;

b = ++a; //b.operator=(a.operator++())

c = a++; //c.operator=(a.operator++(0))

cout<

} (see prepost.txt)

use of const
Use of const
  • Why use const at the beginning of a function?
    • When that function returns a reference, the user may have direct access to the object’s data members
    • See const.txt
class exercise
Class Exercise
  • What is the output of the following program (exercise2.txt)?
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