David E. Goldschmidt, Ph.D.

Example 18: Operator Overloading

Operators As Non-Member Functions:

• When it comes to sorting objects using the sort() function, we can define a function that creates a < operator for whatever type of objects we'd like to sort (e.g. Student, Date).
• Let’s start with syntax. The following expressions are actually treated as function calls, as shown in the comments:

if ( a < b )   // if ( operator< ( a, b ) )
{
  // ...

  z = x + y;   // z = operator+ ( x, y );

• When we want to write our own operators, we write them as functions using the operator keyword. For example, if we define operator< for Student objects as follows:

bool operator< ( const Student& stu1, const Student& stu2 )
{
  return stu1.last_name() < stu2.last_name() ||
         ( stu1.last_name() == stu2.last_name() && stu1.first_name() < stu2.first_name() );
}

• Then the following statement will sort Student objects into alphabetical order by last name, then first name:

sort( students.begin(), students.end() );

Exercises:

• Write an operator< function for comparing two Date objects.
• Write an operator== function for comparing two Date objects.
• Write an operator+ function for adding a given number of days (i.e. an int) to a Date object.