Operator overloading

1
Operator overloading
2
Class time

• Class date represents the date of year
• class Date
• public
• ???
• private
• int month
• int day
• int year
• what would you write if you need to compare two
  dates for equality?
• Use accessor functions
• bool equal(const Date date1, const Date date2)
• return(
• date1.getyear() date2.getyear()
• date1.getmonth() date2.getmonth()
• date1.getday() date2.getday()
• )
• looks like accessors are not helping

3
Class time (cont.)

• Can we write something like this?
• bool equal(const Date date1,const Date date2)
• return(
• date1.year date2.year
• date1.month date2.year
• date1.day date2.day
• )
• we cannot because year, month and day are private
  variables of objects and equal is not a class
  member.
• Can we make it a class member?
• We cannot if such function is called as follows
  date1.equal(date2) then it can access the private
  vars of date1 but not date2
• what shall we do?

4
Friend functions

• To allow non-member function to access private
  class members it should be decared as friend
  function
• class Date
• public
• // compares two dates
• friend bool equal(const Date, const Date)
• private
• int month
• int day
• int year
• note that a friend-function is not a class member
  - its definition should not be preceded with
  class qualifier (Date)
• no separate prototype (outside class definition)
  is necessary for friend
• equal is invoked as non-member function - without
  dot operator
• if(equal(date1, date2)) ...
• Note, that friend-functions circumvent
  information hiding and should be used as rare as
  possible and with great care
• use member function if the task to be done
  involves only one object
• only use friend-function if the task requires
  access to private members of more than one object
  and the objects are used symmetrically
• you can almost always use accessors instead of
  friends

5
Operator overloading

• We wish we could write
• if(date1 date2)) ...
• Would this code work?
• It will not by default because equality
  comparison is defined only for basic data types
  not for objects
• one could define equality comparison for objects
  using operator overloading - a principle where an
  operator is defined to accept arguments that
  differ from ones defined previously.
• Friend functions can be used for operator
  overloading
• friend bool operator(const Date, const Date)
  
• overloaded operator can do arbitrary actions.
  Stick to intuitive meaning of operator
• most operators can be overloaded no new
  operators can be created
• cannot change the number of arguments operator
  takes
• assignment operator can be overloaded but the
  principle differs will discuss later

6
Overloading arithmetic, I/O

• Assume we need a class to store US currency
• class Money
• public
• ???
• private
• int all_cents
• Binary arithmetic operators can be overloaded
• friend Money operator -(const Money, const
  Money)
• Unary operators can also be overloaded
• friend Money operator -(const Money)
• note that overloading binary does not
  automatically overload
• I/O operators (ltlt and gtgt) can be overloaded
• to be able to stack the I/O operators they need
  to return a reference () to stream (more on
  references in later lectures)
• friend ostream operatorltlt(ostream os, const
  Money)
• os ltlt all_cents // should output dollars
• return os // note the return value