Overloadable Operators

The following table describes IDL’s operators and provides information on the object method you must implement in order to override each for a custom object class.

Operator	Non-Overloaded Operation	Operator Type	Object Method Name	Function or Procedure
++	Increment	Unary	Not overloadable	N/A
--	Decrement	Unary	Not overloadable	N/A
[ ]	Indexing, right side	Unary	_overloadBracketsRightSide (see Overloading the Array Indexing Operator for additional details)	Function
[ ]	Indexing, left side	Unary	_overloadBracketsLeftSide (see Overloading the Array Indexing Operator for additional details)	Procedure
-	Negate	Unary	_overloadMinusUnary	Function
NOT	Bitwise NOT	Unary	_overloadNOT	Function
~	Logical NOT	Unary	_overloadTilde	Function
+	Addition	Binary	_overloadPlus	Function
-	Subtraction	Binary	_overloadMinus	Function
*	Multiplication	Binary	_overloadAsterisk	Function
/	Division	Binary	_overloadSlash	Function
^	Exponentiation	Binary	_overloadCaret	Function
MOD	Modulo	Binary	_overloadMOD	Function
<	Minimum	Binary	_overloadLessThan	Function
>	Maximum	Binary	_overloadGreaterThan	Function
AND	Bitwise AND	Binary	_overloadAND	Function
OR	Bitwise OR	Binary	_overloadOR	Function
XOR	Bitwise XOR	Binary	_overloadXOR	Function
EQ	Relational Equal	Binary	_overloadEQ	Function
NE	Relational Not Equal	Binary	_overloadNE	Function
GE	Greater Than or Equal	Binary	_overloadGE	Function
GT	Greater Than	Binary	_overloadGT	Function
LE	Less Than or Equal	Binary	_overloadLE	Function
LT	Less Than	Binary	_overloadLT	Function
#	Matrix multiplication	Binary	_overloadPound	Function
##	Matrix Multiplication	Binary	_overloadPoundPound	Function
&&	Logical AND	Binary	Not overloadable, but respects truth value returned by _overloadIsTrue	N/A
||	Logical OR	Binary	Not overloadable, but respects truth value returned by _overloadIsTrue	N/A
if (obj)...	Logical: obj_valid(obj)	Unary	Not overloadable, but respects truth value returned by _overloadIsTrue	Function
(? : )	Conditional expression	Binary	Not overloadable, but respects truth value returned by _overloadIsTrue	N/A
*	Pointer dereference	Unary	Not overloadable	N/A
=	Assignment operator	Binary	Not overloadable	N/A
[ ]	Array concatenation	Unary	Not overloadable	N/A
(exp)	Grouping of expressions with parentheses	Unary	Not overloadable	N/A
.	Method invocation	Binary	Not overloadable	N/A
.	Property invocation	Binary	Not overloadable, but will call GetProperty or SetProperty	N/A

Note: The behavior of the LOGICAL_AND, LOGICAL_OR, and LOGICAL_TRUE functions is not affected by object operator overloading, but it does respect the truth values returned by the _overloadIsTrue function (see The -_overloadIsTrue Function for more information).

Overloading Compound Assignment Operators

Compound assignment operators cannot be overloaded directly. They will be overloaded if the operator that is compounded is overloaded. For example, if the addition operator “+” is overloaded, “+=” will also be overloaded using the same user method.

The _overloadIsTrue Function

By default, evaluating an object reference for TRUE or FALSE (as in an IF block) yields TRUE if the object reference is valid, and FALSE for null objects. However, you can change this default behavior by implementing the _overloadIsTrue function method. This method allows you to evaluate an object and use your own criteria to determine if its state is TRUE (1) or FALSE (0).

The signature of the _overloadIsTrue object method is:

FUNCTION className::_overloadIsTrue

   ; Determine whether the specified object meets the true

   ; criteria, and place a 1 or a 0 in truth_value

   RETURN, truth_value

END

For example, you could create an object class called highVoltage, with a VOLTAGE property. You could implement the object’s _overloadIsTrue function to check the VOLTAGE property, and return TRUE if VOLTAGE is greater than or equal to 220 volts. The method definition might look something like this:

FUNCTION highVoltage::_overloadIsTrue

   high_or_low = self->VOLTAGE GE 220 ? 1 : 0

   RETURN, high_or_low

END

Supposing you instantiated an object of the highVoltage class in ohighVoltage, the evaluation code would look like this:

IF (ohighVoltage) THEN ...

Note that the _overloadIsTrue function is a unary routine. In an expression such as (obj1 || obj2) where both objects enable overloading, the _overloadIsTrue function of obj1 is called. If the first object’s _overloadIsTrue function returns TRUE, the logical OR operator short-circuits and the expression evaluates as TRUE. If the first object’s _overloadIsTrue function returns FALSE, the logical OR does not short-circuit, and the second object’s _overloadIsTrue function is called, whose result is used as the value of the entire expression.

The _overloadIsTrue method is called whenever an overloaded object’s truth value must be evaluated. Specifically, when an overloaded object appears in any of the following constructions:

• IF...THEN...ELSE statement
• WHILE...DO statement
• REPEAT...UNTIL statement
• Logical AND (&&)
• Logical OR (||)
• Conditional expression (?:)

Overloadable Information Routines

In addition to the standard operators listed in the previous table, object classes that inherit from the IDL_Object class can customize the information returned by the following IDL routines:

Routine	Default Behavior	Object Method Name
HELP	Prints heap variable ID and object class name.	_overloadHelp
N_ELEMENTS	Always reports 1 for a scalar object.	_overloadSize
PRINT/PRINTF	Prints heap variable ID and object class name.	_overloadPrint
SIZE	Always reports information for a scalar object.	_overloadSize
Implied Print	Prints heap variable ID and object class name.	_overloadImpliedPrint

Overloadable Program Control Statements

If your object inherits from the IDL_Object class, you can iterate through the elements of your object using the FOREACH statement. You are responsible for writing your object’s ::_overloadForeach function method, which retrieves the value of the next object element in the iteration.