Recipe 3.7 Indirectly Overloading the , -, , and Operators

Recipe 3.7 Indirectly Overloading the +=, -=, /=, and *= Operators

Problem

You need to control the handling of the += , -= , /= , and *= operators within your data type; unfortunately , these operators cannot be directly overloaded.

Solution

Overload these operators indirectly by overloading the + , - , / , and * operators:

 public class Foo {     // Other class members...     // Overloaded binary operators     public static Foo operator +(Foo f1, Foo f2)     {         Foo result = new Foo( );         // Add f1 and f2 here...         // Place result of the addition into the result variable         return (result);     }     public static Foo operator +(int constant, Foo f1)     {         Foo result = new Foo( );         // Add the constant integer and f1 here...         // Place result of the addition into the result variable         return (result);     }     public static Foo operator +(Foo f1, int constant)     {         Foo result = new Foo( );         // Add the constant integer and f1 here...         // Place result of the addition into the result variable         return (result);     }     public static Foo operator -(Foo f1, Foo f2)     {         Foo result = new Foo( );         // Subtract f1 and f2 here...         // Place result of the subtraction into the result variable         return (result);     }    public static Foo operator -(int constant, Foo f1)     {         Foo result = new Foo( );         // Subtract the constant integer and f1 here...         // Place result of the subtraction into the result variable         return (result);     }    public static Foo operator -(Foo f1, int constant)     {         Foo result = new Foo( );         // Subtract the constant integer and f1 here...         // Place result of the subtraction into the result variable         return (result);     }     public static Foo operator *(Foo f1, Foo f2)     {         Foo result = new Foo( );         // Multiply f1 and f2 here...         // Place result of the multiplication into the result variable         return (result);     }     public static Foo operator *(int multiplier, Foo f1)     {         Foo result = new Foo( );         // Multiply multiplier and f1 here...         // Place result of the multiplication into the result variable         return (result);     }     public static Foo operator *(Foo f1, int multiplier)     {         return (multiplier * f1);     }     public static Foo operator /(Foo f1, Foo f2)     {         Foo result = new Foo( );         // Divide f1 and f2 here...         // Place result of the division into the result variable         return (result);     }     public static Foo operator /(int numerator, Foo f1)     {         Foo result = new Foo( );         // Divide numerator and f1 here...         // Place result of the division into the result variable         return (result);     }     public static Foo operator /(Foo f1, int denominator)     {         return (1 / (denominator / f1));     } } 

Discussion

While it is illegal to try and overload the += , -= , /= , and *= operators directly, you can overload them indirectly by overloading the + , - , / , and * operators. The += , -= , /= , and *= operators then use the overloaded + , - , / , and * operators for their calculations.

The four operators + , - , / , and * are overloaded by the methods in the Solution section of this recipe. You might notice that each operator is overloaded three times. This is intentional, since a user of your object may attempt to add, subtract, multiply, or divide it by an integer value. The unknown here is which position the integer constant will be in; will it be in the first parameter or the second? The following code snippet shows how this might look for multiplication:

 Foo x = new Foo( ); Foo y *= 100;    // Uses:  operator *(Foo f1, int multiplier) y = 100 * x;     // Uses:  operator *(int multiplier, Foo f1) y *= x;          // Uses:  operator *(Foo f1, Foo f2) 

The same holds true for the other overloaded operator.

If these operators were being implemented in a class, you would first check whether any were set to null . The following code for the overloaded addition operator has been modified to do this:

 public static Foo operator +(Foo f1, Foo f2) {     if (f1 == null  f2 == null)     {         throw (new ArgumentException("Neither object may be null."));     }     Foo result = new Foo( );     // Add f1 and f2 here...     // Place result of the addition into the result variable     return (result); } 

See Also

See the "Operator Overloading Usage Guideline," "Overloadable Operators," and "Operator Overloading Tutorial" topics in the MSDN documentation.