Operator Overloading

1
Operator Overloading
2
Syntax

• The general syntax is
• friend returntype operatorltoperator symbolgt
• ( ltparametersgt )
• ltstatementsgt

3
Syntax

• The number of arguments in the overloaded
  operators argument list depends on two factors
• Whether its a unary operator (one argument) or a
  binary operator (two arguments).
• Whether the operator is defined as a global
  function (one argument for unary, two for binary)
  or a member function (zero arguments for unary,
  one for binary the object becomes the left-hand
  argument).

4
Operators that cant be overloaded

• All operators can be overloaded except for the
  ones below
• The member selection operator - .
• The pointer to member dereference operator - .
• Scope access/resolution operator -
• Conditional operator - ?
• Also, there are no user-defined operators and you
  cant change the precedence rules.

5
Overloaded operators as member or non-member
functions?

• The decision is based on what argument(s) is
  needed by the operator.
• In general, if it doesnt make any difference,
  they should be members, to emphasize the
  association between the operator and its class.
• When the left-hand operand is always an object of
  the current class, this works fine.
• Sometimes you want the left-hand operand to be an
  object of some other class, then the overloaded
  operator cannot be a member function.

6
Some tricky operators

• Increment and decrement operators
• a (a pre-increment) generates a call to
  operator(a)
• a (a post-increment) generates a call to
  operator(a,int)for non-member functions.
• The member function versions, would be
• Boperator( ) and
• Boperator(int)
• Note a dummy value is passed by the compiler to
  distinguish (generate different signatures) the
  two versions.

7
Some tricky operators

• The assignment operator
• Defining the assignment operator has a lot in
  common with defining the copy constructor and the
  destructor.
• List Listoperator( const List b )
• if( this ! b) //check for self assignment
• free()
• copy(b)
• return this

8
An example

• Lets look at our example implementation of the
  class Values

9
Arguments and return values

• Guidelines
• Return by value as const
• The return value optimization