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 Table 12.2. 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+=.
Table 12.2 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 | — |
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 Unary Operators, Binary Operators, Assignment, Function Call, Subscripting, Class-Member Access, and Increment and Decrement.
The operators shown in Table 12.3 cannot be overloaded. The table includes the preprocessor symbols # and ##.
Table 12.3 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.
#include <iostream.h>
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 );
}
void 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();
}