Operator Overloading Friend Functions & Special Forms

1. Department of Computer and Information Science,School of Science, IUPUI Operator OverloadingFriend Functions & Special Forms Dale Roberts, Lecturer Computer Science, IUPUI E-mail: droberts@cs.iupui.edu

2. Operator Overloading Using a Friend Function • Number of Parameters Accepted by an Overloaded Friend Operator Function Depend Upon the Operator Type -- One for Unary Operators and Two for Binary Operators class complex{ int re, im; public: complex(int ip1 = 0, int ip2 = 0) :re(ip1), im(ip2){} friend complex operator+(complex, complex); }; //Friend Operator + Function complex operator+(complex a, complex b) {return complex(a.re+b.re, a.im+b.im);} main(){ complex one(1,1), two(2,2), three; three = one + two; //three = operator+(one, two) } Is a friend function necessary in this case? No because LH operand is an instance of the class.

3. Operator Functions as Class Members vs. as friend Functions Non-member overloaded operator functions • Enable the operator to be commutative HugeInteger bigInteger; int integer; bigInteger = integer + bigInteger; or bigInteger = bigInteger + integer;

4. Global Operator Overloading • Similar to friend Function Overloading, Except the Keyword friend is Omitted and Global Functions CANNOT ACCESS private Members class complex{ //All Public Members! public: int re, im; complex(int ip1 = 0, int ip2 = 0) :re(ip1), im(ip2){} }; void operator!(complex a) { int temp = a.re; a.re = a.im; a.im = temp; cout << "Real: " << a.re << endl; cout << "Imaginary: " << a.im << endl; } main() { complex one(1,2); !one; }

5. Overloading of Operators Having a Variable Arity • Operators Such as - Can be Unary or Binary • Overloading of Such Operators Involves Creating a Unary Function (One Operand) and a Binary Function (Two Operands) • Only if Both the Forms are Used, They Need to be Implemented class number{ int n; public: number(int x = 0):n(x){} number operator-(){n = -n; return *this;} number operator-(number ip) {n = n – ip.n; return *this;} }; main(){ number one(1), two(2), three; one = -one; //unary operator three = one - two; //three.n = -3 }

6. Operators with Prefix and Postfix Forms • Separate Functions for Each -- Prefix and Postfix -- Forms are Needed • Prefix Form is Treated as an Unary Operator • Postfix Form is Treated as a Binary Operator

7. Prefix Overloaded Function -- Example class number{ int n; public: number(int x):n(x){}; //Constructor //prefix operator -- unary number operator++(); }; number number::operator++(){ n++; return *this;} main(){ number one(10); //one.n = 10 ++one; //one.n = 11 }

8. Postfix Overloaded Function -- Example • Postfix Operator is Implemented as a Binary Operator with an int Argument with a Default Value of 0 . When specifying an overloaded operator for the postfix form of the increment or decrement operator, the additional argument must be of type int; specifying any other type generates an error. class number{ int n; public: number(int x):n(x){}; //Constructor //postfix operator -- binary -- int argument number operator++(int); }; number number::operator++(int y) {if (y != 0) n += y; else n++; return *this;}

9. Postfix overloaded function—Example (Cont) main() { number one(10); // one.n = 10 one++; // one.n = 11 one.operator++(2); // one.n = 13 } • There is no syntax for using the increment or decrement operators to pass these values other than explicit invocation, as shown in the preceding code. A more straightforward way to implement this functionality is to overload the addition/assignment operator (+=).

10. Special Overloading Forms A Few Operators Require Special Treatments During Overloading Conversion Operator const Array Operator Function Call -- Parenthesis Operator Stream Insertion -- << Operator Stream Extraction -- >> Operator Pointer to Member -- -> Operator Assignment Operator new Operator delete Operator

11. Overloading Stream-Insertion and Stream-Extraction Operators • Overloaded << and >> operators • Must have left operand of types ostream&, istream& respectively • It must be a non-member function (left operand not an object of the class) • It must be a friendfunction if it accesses private data members

12. 1 // Fig. 18.3: fig18_03.cpp 2 // Overloading the stream-insertion and 3 // stream-extraction operators. 4 #include <iostream> 5 6 using std::cout; 7 using std::cin; 8 using std::endl; 9 using std::ostream; 10 using std::istream; 11 12 #include <iomanip> 13 14 using std::setw; 15 16 class PhoneNumber { 17 friend ostream &operator<<( ostream&, const PhoneNumber & ); 18 friend istream &operator>>( istream&, PhoneNumber & ); 19 20 private: 21 char areaCode[ 4 ]; // 3-digit area code and null 22 char exchange[ 4 ]; // 3-digit exchange and null 23 char line[ 5 ]; // 4-digit line and null 24 }; 25 26 // Overloaded stream-insertion operator (cannot be 27 // a member function if we would like to invoke it with 28 // cout << somePhoneNumber;). 29 ostream &operator<<( ostream &output, const PhoneNumber &num ) 30 {

13. 31 output << "(" << num.areaCode << ") " 32 << num.exchange << "-" << num.line; 33 return output; // enables cout << a << b << c; 34 } 35 36 istream &operator>>( istream &input, PhoneNumber &num ) 37 { 38 input.ignore(); // skip ( 39 input >> setw( 4 ) >> num.areaCode; // input area code 40 input.ignore( 2 ); // skip ) and space 41 input >> setw( 4 ) >> num.exchange; // input exchange 42 input.ignore(); // skip dash (-) 43 input >> setw( 5 ) >> num.line; // input line 44 return input; // enables cin >> a >> b >> c; 45 } 46 47 int main() 48 { 49 PhoneNumber phone; // create object phone 50 51 cout << "Enter phone number in the form (123) 456-7890:\n"; 52 53 // cin >> phone invokes operator>> function by 54 // issuing the call operator>>( cin, phone ). 55 cin >> phone; 56 57 // cout << phone invokes operator<< function by 58 // issuing the call operator<<( cout, phone ). 59 cout << "The phone number entered was: " << phone << endl; 60 return 0; 61 }

14. Enter phone number in the form (123) 456-7890: (800) 555-1212 The phone number entered was: (800) 555-1212

15. Converting between Types • Cast operator • Convert objects into built-in types or other objects • Conversion operator must be a non-static member function. • Cannot be a friend function • Do not specify return type For user-defined class A A::operator char *() const; // A to char A::operator int() const; //A to int A::operator otherClass() const; //A to otherClass • When compiler sees (char *) s it calls s.operator char*()

16. Converting between Types (cont) • The compiler can call these functions to create temporary objects. • If s is not of type char * Calls A::operator char *() const; for cout << s;

17. Special overloading forms - Example • Special Forms Example

18. Acknowledgements • These slides were originally development by Dr. Uday Murthy and Dr. Rajeev Raje. • Some contents comes from the Deitel slides that accompany your text. • Some information regarding the postfix form the increment and decrement operators comes from MSDN.