CPS 235 Object Oriented Programming Paradigm Lecturer Aisha

CPS 235 Object Oriented Programming Paradigm Lecturer Aisha Khalid Khan Operator Overloading Computer Science Department

Operator Overloading • Operator overloading is a powerful feature of C++ • It provides programmers with a concise notation for manipulating user defined objects • It is simple to implement given a few basic rules Computer Science Department CPS 235: Operator Overloading 2

Why Operator Overloading ? int i, j, k; // integers float m, n, p; // floats // integer addition and assignment k = i + j; // floating addition and assignment p = m + n; The compiler overloads the + operator for built-in integer and float types by default, producing integer addition with i+j, and floating addition with m+n We can make object operation look like individual int variable operation, using operator functions Complex a, b, c; c = a + b; Computer Science Department CPS 235: Operator Overloading 3

Operators in C++ • C++ has a rich collection of operators most of which are common to other programming languages • Standard arithmetic and logical operators + - * / % & ! > < || && == etc • Array indexing and function evaluation operators [] () • Assignment operators = += -= *= /= etc. • Auto increment and decrement operators ++ - • Pointer de-referencing and address of operators * & • Memory management operators new delete new[] delete[] Computer Science Department CPS 235: Operator Overloading 4

Operators in C++ • We can divide up the set of operators into unary and binary operators • A unary operator has one operand • Examples if (!x) {. . } // unary operator !, operand x x++; // post-fix operator ++, operand x --x; // pre-fix operator --, operand x int y=a[5]; // operator [], operand a Computer Science Department CPS 235: Operator Overloading 5

Operators in C++ • A binary operator has two operands • Examples int z=x+y; // binary and y operator +, operands x bool z=x&&y; // binary operator && operands x and y x+=y; and y Computer Science Department // binary operator +=, operands x CPS 235: Operator Overloading 6

Operator Overload Functions • In order to overload an operator op, a function operator op must be defined – operator+ to overload + – operator+= to overload += – etc • We can either provide member functions of a class or external functions (possibly friend functions of a class) Computer Science Department CPS 235: Operator Overloading 7

Restrictions on operator overloading • Overloading an operator cannot change it’s precedence • Overloading an operator cannot change the number of operands • It is not possible to create new operators, only new versions of existing operators can be created • Operator meaning on built-in features cannot be changed – For instance you cannot change the “+” for integers • At least one argument of an operator function must be an object or reference of a user-defined type – This prevents programmers from changing how operators work on fundamental types Computer Science Department CPS 235: Operator Overloading 8

Operator Overloading Syntax • Syntax is: Examples: operator+ operator@(argument-list) operator* operator is a function operator/ @ is one of C++ operator symbols (+, -, =, etc. . ) Computer Science Department CPS 235: Operator Overloading 9

Operator Overloading Format • Format – The name of an operator is always a conjunction of the keyword operator and then the symbol itself – Examples • Operator++ • Operator- • Operator – • Operator << • Operator == • Operator = Computer Science Department CPS 235: Operator Overloading 10

Operators that can be overloaded + - * / % ^ & | ~ ! = < > += -= *= /= %= ^= &= |= << >> >>= <<= == != <= >= && || ++ -- ->* , -> [] () new delete new[] delete[] Computer Science Department CPS 235: Operator Overloading 11

Operators that cannot be overloaded. Computer Science Department . * : : CPS 235: Operator Overloading ? : 12

Forms of Overloaded Operators • Member Functions • Friend Functions • Free-Standing or Global Functions Computer Science Department CPS 235: Operator Overloading 13

Operator Functions • When to make operator functions class members, friends or global functions? • If operator overload function is a member function, then “this” is implicitly available for one of the arguments • When overloading =, ( ), [ ], ->, the operator overloading function must be declared as a class member. For other operators, the overloading functions can be non-members Computer Science Department CPS 235: Operator Overloading 14

Operator Functions • When an operator function is implemented as a member function, the left most (or only in the case of unary operators) operand must be a class object (or a reference to a class object) of operator's class • If the left operand must be an object of a different class or a built-in type, this operator must be implemented as a non-class member. eg. <<, >> operators Computer Science Department CPS 235: Operator Overloading 15

Operator Functions • An operator function implemented as a non-member must be a friend if it needs to access non-public data members of that class • The overloaded << operator must have a left operand of type ostream. Therefore, it must be a nonmember function. Also, it may require access to the private data members of the class. Thus, it needs to be a friend function for that class • Similar observation holds for >> operator which has a left operand of type istream Computer Science Department CPS 235: Operator Overloading 16

Operator Functions • Operator member functions are classed only when the left operand of a binary operator is specifically an object of that class or when the single operand of a unary operator is an object of that class • If the operator needs to be commutative (a + b = b + a), then making it a non-member function is necessary Computer Science Department CPS 235: Operator Overloading 17

Overloading Unary Operators (using member functions) class counter { private: int count; public: counter(): count(0){} counter(int c): count(c) {} int get_count() { return count; } counter operator++ () //prefix operator { ++count; counter temp; temp. count = count; return counter(count); } counter operator++(int) //postfix operator { return counter(count++); } }; Computer Science Department CPS 235: Operator Overloading 18

Overloading Unary Operators (using member functions) int main() { counter c 1, c 2, c 3; ++c 1; // or c 1. operator++(); ++c 2; cout<<'n'<<c 1. get_count(); cout<<'n'<<c 2. get_count(); cout<<endl; c 3 = c 1++; //or c 3 = c 1. operator++(0); cout<<'n'<<c 3. get_count(); cout<<'n'<<c 1. get_count(); getch(); return 0; } Computer Science Department CPS 235: Operator Overloading 19

Overloading Binary Operators (using member functions) class counter { private: int count; public: counter(): count(0){} counter(int c): count(c) {} int get_count() { return count; } counter operator+(const counter & rhs) { return (counter(count+rhs. count)); } }; Computer Science Department CPS 235: Operator Overloading 20

Overloading Binary Operators (using member functions) int main() { counter c 1(10), c 2(10), c 3; c 3 = c 1 + c 2; //or c 3 = c 1. operator+(c 2); cout<<'n'<<c 3. get_count(); getch(); return 0; } Computer Science Department CPS 235: Operator Overloading 21

Implementing Operator Overloading • Two ways: – Implemented as member functions – Implemented as non-member or Friend functions • the operator function may need to be declared as a friend if it requires access to protected or private data • Expression obj 1@obj 2 translates into a function call – obj 1. operator@(obj 2), if this function is defined within the class of which obj 1 is a member – operator@(obj 1, obj 2), if this function is defined outside the class of which obj 1 is a member Computer Science Department CPS 235: Operator Overloading 22

Implementing Operator Overloading 1. Defined as a member function class Complex {. . . private: double _real, double _imag; public: . . . Complex operator +(const Complex &rhs) { double real = _real + op. _real; double imag = _imag + op. _imag; return(Complex(real, imag)); }. . . }; Computer Science Department CPS 235: Operator Overloading c = a+b; c = a. operator+(b); 23

Implementing Operator Overloading 2. Defined as a non-member function Complex operator +(Complex &op 1, Complex &op 2) { double real = op 1. real() + op 2. real(); double imag = op 1. imag() + op 2. imag(); return(Complex(real, imag)); } c = a+b; c = operator+ (a, b); Computer Science Department CPS 235: Operator Overloading 24

Implementing Operator Overloading 2. Defined as a non-member function class Complex {. . . public: . . . //need access functions double real() { return _real; } double imag() { return _imag; }. . . }; Computer Science Department CPS 235: Operator Overloading 25

Implementing Operator Overloading 3. Defined as a friend function class Complex { c = a+b; . . . public: . . . c = operator+ (a, b); friend Complex operator +( const Complex &, operator +(Complex &op 1, Complex &op 2) const Complex & { ); double real = op 1. _real +. . . op 2. _real; }; double imag = op 1. _imag + op 2. _imag; return(Complex(real, imag)); } Computer Science Department CPS 235: Operator Overloading 26

Overloading the Assignment Operator • See code example assignment. cpp void operator=(const omega& rhs) { strncpy(name, rhs. name, size); } To enable chaining i. e. , om 3 = om 2 = om 1; omega operator=(const omega& rhs) { strncpy(name, rhs. name, size); return omega(name); } Computer Science Department CPS 235: Operator Overloading 27

Overloading the Comparison Operators class Distance { int feet; float inches; public: Distance(): feet(0), inches(0. 0){} Distance(int ft, float in): feet(ft), inches(in){} void display() const { cout<<feet<<"'-"<<inches<<'"'; } bool operator<(Distance&) const; }; bool Distance: : operator<(Distance& d 1) const { float f 1 = feet + inches/12; float f 2 = d 1. feet + d 1. inches/12; return (f 1<f 2)? true : false; Computer Science Department CPS 235: Operator Overloading 28

Overloading the Comparison Operators void main() { Distance d 1(5, 11. 5); Distance d 2(6, 2. 5); if(d 1<d 2) { cout<<"the distance"; d 1. display(); cout<<"is less than"; d 2. display(); cout<<endl; } else { cout<<"the distance"; d 1. display(); cout<<"is greater than/equal to"; d 2. display(); cout<<endl; } getch(); } Computer Science Department CPS 235: Operator Overloading 29

WHY WOULD WE NEED TO MAKE OVERLOADED OPERATORS NONMEMBER FUNCTIONS? Computer Science Department CPS 235: Operator Overloading 30

class Time { private: int hours; int mins; public: Time(): hours(0), mins(0){} Time(int h, int m): hours(h), mins(m){} void display() { cout<<hours<<"-"<<mins<<endl; } Time operator*(double mult_mins) const { Time result; long totalminutes = (hours * 60) + (mins * mult_mins); result. hours = totalminutes / 60; result. mins = totalminutes % 60; return result; } }; Computer Science Department CPS 235: Operator Overloading 31

void main() { Time t 1; Time t 2(5, 40); cout<<"t 2 is: "; t 2. display(); t 1 = t 2 * 2; //t 1 = 2 * t 2; //ILLEGAL STRUCTURE OPERATION cout<<"t 1 is: "; t 1. display(); getch(); } Computer Science Department CPS 235: Operator Overloading 32

Implementing overloaded operators as non-member functions • When an overloaded operator is defined as a member function, the left operand is always an object on which the function is called • So, in the previous example we have t 1 = t 2 * 2; // t 1. operator*(2) • When we write t 1 = 2 * t 2; the left operand is no longer an object on which the function can be invoked • Solution: Implement the overloaded operator function as a non-member function – Since this function needs to access the private data members of the Time class, we declare it as a friend in the Time class declaration Computer Science Department CPS 235: Operator Overloading 33

Implementing overloaded operators as non-member functions //goes into the declaration of class Time friend Time operator* (double, const Time&); //function defined outside class declaration Time operator*(double mult_mins, const Time& t) { Time result; long totalminutes = (t. hours * 60) + (t. mins * mult_mins); result. hours = totalminutes / 60; result. mins = totalminutes % 60; return result; } Computer Science Department CPS 235: Operator Overloading 34

Overloading the stream insertion operator << (version 1) • Is it possible to do the following? Time t 1; cout<<t 1; • It can be done if we overload the << operator • Remember that cout is an object of class ostream • If you want to overload the << operator, should it be done via class member function or non-member function? • If you use a class member function to overload <<, you would have to write t 1 << cout; Computer Science Department CPS 235: Operator Overloading 35

Overloading the stream insertion operator << (version 1) • So, we define the overloaded << operator as a nonmember friend function //goes into the declaration of class Time friend void operator<<(ostream&, const Time&); //function defined outside class declaration void operator<<(ostream& os, const Time& t) { os<<t. hours<<"-"<<t. mins<<endl; } Computer Science Department CPS 235: Operator Overloading 36

Overloading the stream insertion operator << (version 1) • With the << operator overloaded as above, the following works fine Time t 1; cout<<t 1; • But if you want to write cout<<t 1<<“is the time”<<endl; • It will not work! Why? • C++ reads the output statement from left to right ((cout<<t 1)<<“is the time”)<<endl; Computer Science Department CPS 235: Operator Overloading 37

Overloading the stream insertion operator << (version 2) • The << operator as defined in iostream takes an ostream object to its left • The statement cout<<t 1 satisfies the above requirement • But the output statement also requires that the whole expression (cout<<t 1) should be a type ostream object because this expression is to the left of “is the time” • You can modify the operator<< to return an ostream object Computer Science Department CPS 235: Operator Overloading 38

Overloading the stream insertion operator << (version 2) ostream& operator<<(ostream& os, const Time& t) { os<<t. hours<<"-"<<t. mins<<endl; return os; } The statement cout<<t 1; becomes the following function call operator<<(cout, trip); And this call returns the cout object Computer Science Department CPS 235: Operator Overloading 39

Analysis cout<<t 1<<“is the time”<<endl; is actually (((cout<<t 1)<<“is the time”)<<endl); Invokes the user-defined operator<< that displays t 1 and returns the cout object, so the original statement becomes ((cout<<“is the time”)<<endl); Now, the program uses the ostream definition of << for strings to display the string and again returns the cout object. This reduces the statement to (cout<<endl); This also uses the ostream definition of << for endl Computer Science Department CPS 235: Operator Overloading 40

iostream. h • • • ostream& operator<< (bool& val ); ostream& operator<< (short& val ); ostream& operator<< (unsigned short& val ); ostream& operator<< (int& val ); ostream& operator<< (unsigned int& val ); ostream& operator<< (long& val ); ostream& operator<< (unsigned long& val ); ostream& operator<< (float& val ); ostream& operator<< (double& val ); ostream& operator<< (long double& val ); http: //www. cplus. com/reference/iostream/operator<</ Computer Science Department CPS 235: Operator Overloading 41

Returning by Reference int x; int& getnset() { return x; } void main() { getnset() = 56; cout<<"value of x is: "<<getnset(); cout<<“Address is: ”<<&getnset(); } Computer Science Department CPS 235: Operator Overloading 42

Returning by Reference • You cannot return a local variable from a function int& getnset() { int x; return x; } //x goes out of scope here • You cannot return a constant int& getnset() { return 3; } Computer Science Department CPS 235: Operator Overloading 43

Returning by Reference • You cannot return an expression int& preinc(int& x) { return x++; } Computer Science Department CPS 235: Operator Overloading 44

What would be the ouput? int& preinc(int& x) { x++; cout<<"nvalue in func: "<<x<<endl; return x; } void main() { int y = 0; cout<<"nvalue in main after incrementing once: "<<preinc(y); cout<<"n. After calling preinc again"; preinc(y) = 5; cout<<"n. Value in main is: "<<y; getch(); } Computer Science Department CPS 235: Operator Overloading 45

Compulsory Reading • Robert Lafore, Chapter 8: Operator Overloading • Deitel and Deitel (5 th edition) – Topics 11. 1 – 11. 4, 11. 6, 11. 7 • Another useful link: http: //newdata. box. sk/bx/c/htm/ch 10. htm#Heading 23 Computer Science Department CPS 235: Operator Overloading 46