1. Operator Overloading I

2. Operator overloading
C++ operators can be overloaded to perform user-defined, object- based functions
Type of operators
Both unary and binary
Assignment
Relational
Input/output
Subscript … + - * / % ^
& | ~ ! , =
<
>
<=
>= ++ --
<<
>> == !=
&& || += -= /= %= ^=
&= |= *=
<<=
>>= [] ()
->
->*
new
new []
delete
delete[]
C++ operators that can be overloaded

3. Operator Overloading
Define a method/function using the keyword operator
Combine the keyword with the operator symbol to be overloaded
Ex:
bool operator>(int i);
This will “compare” an integer to the current object
Note: overloading can be defined for different types!
For unary operators (!, ++), the overload method should be a member of the class
For binary operators (two arguments) the overload method may be a member of the class depending on the “left-hand rule”

4. “left-hand rule” Applies to binary operators The rule:
If the object itself is on the left-hand side of the expression to be evaluated, the operator method can be made a member of the class
In this case, only the right hand side of the expression need to be passed to the method, the left hand is implied (it is the class instance itself)
If not, operator overloading can still be done, but it must be defined externally to the class
In this case, both the left and right hand sides of the expression need to be passed to the overload function (i.e. two arguments)

5. Applying the “left-hand rule”
Left hand side is the object type
Compare two Date objects
bool operator>(const Date &d) { … }
Single argument: the “other” Date object
Left hand side is another object type
Output a Date object
ostream &operator<<( ostream &output, const Date &d ) { … }
Two arguments: the stream and the date
Example: Date-OpOver1

6. Terminology For Binary Operators:
The right-hand operand is referred to as an rvalue
The left-hand operand is referred to as an lvalue

7. Friends of a class
A friend of a class is an object or function that is
Not a member of the class, but
Is allowed access to the private members of the class anyway
A friend is declared in the class definition using the keyword friend
friend ostream &operator<<( ostream &, const ClassName& ); // in class definition
Function is granted access to private data members
Usage of friends, while allowed, is discouraged
It violates the basic OOP principle of encapsulation
Example: Date-Friend