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

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Presentation Transcript
• 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
• 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
• A few operators don’t have to be explicitly overloaded (done automatically)
• Assignment operator (=)
• Memberwise assignment of class data members
• Returns the address of the object in memory
• Both can be explicitly overloaded
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)
Restrictions
• Only existing operators can be overloaded

F1 = F2 + F3;

F1 += F2;

• Misuse
Method #1
• 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 #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<<num<<'/'<<den<<endl;}

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

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

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<<f)
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<<num<<'/'<<den<<endl;}

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

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

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<<num<<'/'<<den<<endl;}

};

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
• 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)
• 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<<a<<b; becomes
• operator<<(cout,a); operator<<(cout,b);
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<<f.num<<'/'<<f.den;

return out;}

};

int main(){

Fraction a,b;

cin>>a>>b;

cout<<a<<" "<<b<<endl;

return 0;

} (See stream.txt)

• 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
• 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)

• Preincrement
• Original code: ++f1;
• Compiler will generate: f1.operator++();
• Function prototype: Fraction &operator++();
• Returns a reference to itself
• 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

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<<a<<" "<<b<<" "<<c<<endl;

} (see prepost.txt)

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
• What is the output of the following program (exercise2.txt)?