Returning a Reference to a Non-const Object

Two common examples of returning a non-const object are overloading the assignment operator and overloading the << operator for use with cout. The first is done for reasons of efficiency, and the second for reasons of necessity.

The return value of operator=() is used for chained assignment:

String s1("Good stuff");
String s2, s3;
s3 = s2 = s1;

In this code, the return value of s2.operator=(s1) is assigned to s3. Returning either a String object or a reference to a String object would work, but, as with the Vector example, using a reference allows the function to avoid calling the String copy constructor to create a new String object. In this case, the return type is not const because the operator=() method returns a reference to s2, which it does modify.

The return value of operator<<() is used for chained output:

String s1("Good stuff");
cout << s1 << "is coming!";

Here, the return value of operator<<(cout, s1) becomes the object used to display the string "is coming!". Here, the return type has to be ostream & and not just ostream. Using an ostream return type would require calling the ostream copy constructor, and, as it turns out, the ostream class does not have a public copy constructor. Fortunately, returning a reference to cout poses no problems because cout is already in scope in the calling function.