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 < 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;}

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).