Operator Overloading CS 308 – Data Structures

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.

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.

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 *); };

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(); }

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

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; }

int& Array::operator[](int index) { if ((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; }

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).