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Operator Overloading

type operator operator-symbol ( parameter-list )

The operator keyword declares a function specifying what operator-symbol means when applied to instances of a class. This gives the operator more than one meaning, or "overloads" it. The compiler distinguishes between the different meanings of an operator by examining the types of its operands.

You can redefine the function of most built-in operators globally or on a class-by-class basis. Overloaded operators are implemented as functions.

The name of an overloaded operator is operatorx, where x is the operator as it appears in the following table. For example, to overload the addition operator, you define a function called operator+. Similarly, to overload the addition/assignment operator, +=, define a function called operator+=.

Redefinable Operators

Operator Name Type
, Comma Binary
! Logical NOT Unary
!= Inequality Binary
% Modulus Binary
%= Modulus assignment Binary
& Bitwise AND Binary
& Address-of Unary
&& Logical AND Binary
&= Bitwise AND assignment Binary
( ) Function call
* Multiplication Binary
* Pointer dereference Unary
*= Multiplication assignment Binary
+ Addition Binary
+ Unary Plus Unary
++ Increment1 Unary
+= Addition assignment Binary
Subtraction Binary
Unary negation Unary
–– Decrement1 Unary
–= Subtraction assignment Binary
–> Member selection Binary
–>* Pointer-to-member selection Binary
/ Division Binary
/= Division assignment Binary
< Less than Binary
<< Left shift Binary
<<= Left shift assignment Binary
<= Less than or equal to Binary
= Assignment Binary
== Equality Binary
> Greater than Binary
>= Greater than or equal to Binary
>> Right shift Binary
>>= Right shift assignment Binary
[ ] Array subscript
^ Exclusive OR Binary
^= Exclusive OR assignment Binary
| Bitwise inclusive OR Binary
|= Bitwise inclusive OR assignment Binary
|| Logical OR Binary
~ One's complement Unary
delete Delete
new New
conversion operators conversion operators Unary

1   Two versions of the unary increment and decrement operators exist: preincrement and postincrement.

See General Rules for Operator Overloading for more information. The constraints on the various categories of overloaded operators are described in the following topics:

The operators shown in the following table cannot be overloaded. The table includes the preprocessor symbols # and ##.

Nonredefinable Operators

Operator Name
. Member selection
.* Pointer-to-member selection
:: Scope resolution
? : Conditional
# Preprocessor convert to string
## Preprocessor concatenate

Although overloaded operators are usually called implicitly by the compiler when they are encountered in code, they can be invoked explicitly the same way as any member or nonmember function is called:

Point pt;
pt.operator+( 3 );  // Call addition operator to add 3 to pt.

Example

The following example overloads the + operator to add two complex numbers and returns the result.

// operator_overloading.cpp
// compile with: /EHsc
#include <iostream>

using namespace std;
class Complex
{
public:
   Complex( double r, double i ) : re(r), im(i) {}
   Complex operator+( Complex &other );
   void Display( ) {   cout << re << ", " << im << endl; }
private:
   double re, im;
};

// Operator overloaded using a member function
Complex Complex::operator+( Complex &other )
{
return Complex( re + other.re, im + other.im );
}

int main()
{
   Complex a = Complex( 1.2, 3.4 );
   Complex b = Complex( 5.6, 7.8 );
   Complex c = Complex( 0.0, 0.0 );

   c = a + b;
   c.Display();
}

See Also

C++ Operators | C++ Keywords

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