Operator Overloading

1. Operator Overloading CS 308 – Data Structures

2. What is operator overloading? • Changing the definition of an operator so it can be applied on the objects of a class is called operator overloading. • To overload an operator, we need to write a function for the operator we are overloading.

3. Special cases • The assignment operator (=) may be used with every class without explicit overloading. • The default behavior of the assignment operator is a memberwise assignment of the data members of the class. • The address operator (&) may also be used with objects of any class without explicit overloading. • It returns the address of the object in memory.

4. Explicit overloading of the assignment operator • The default overloading is not enough for classes with pointer members. void operator=(class_name&); class string { private: char *s; int size; public: string(char *); // constructor ~string(); // destructor void operator=(string&); void print(); void copy(char *); };

5. void string::operator=(string& old_str) { char *tmp; size = old_str.size; tmp = new char[size+1]; // assign new memory strcpy(tmp, old_str.s); delete [] s; // must release previously assigned memory s = tmp; } void main() { string str1("George"); string str2("Mary"); string str3("John"); str1.print(); // what is printed ? str2.print(); str2 = str1; str3.copy("Ha ha"); str1.print(); // what is printed now ? str2.print(); }

6. Differences between copy constructor and assignment operator • The copy constructor creates a new object. • The assignment operator works on an already valid object.

7. Another example: overloading the [] operator class Array { private: int numElems; int *arr; public: Array(int); // constructor ~Array(); // destructor int& operator[](int); }; Array::Array(int n) { numElems = n; arr = new int[n]; } Array::~Array() { delete [] arr; }

8. int& Array::operator[](int index) { if ((index < 0) || (index >= numElems)) { cout << "Out of bounds error !!" << endl; exit(0); // error: invalid index !! } else return(arr[index]); } void main() { int i; Array A(10); for(i=0; i<=10; i++) // i=10: error !! A[i] = i; }

9. Comments on operator overloading • Attempting to create new operators via operator overloading is a syntax error. • Attempting to change the "arity" of an operator via operator overloading is a syntax error. • Overloading is allowed only if at least one operand is a class instance (e.g., you cannot overload an operator to take two integers as operands).