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Assignment

To assign a value to a variable, use the assignment operator ( := ).

location := expression
On the lefthand side, location is a variable name or declaration. Most expressions in the ScriptX language can be used on the righthand side. For more information on the assignment expression, see the discussion on page 241 in Appendix A, "ScriptX Reference."

local var := 15

x := "string that goes in x"

Variables can be assigned to a value without being declared first, as in the previous example. A warning is printed if you do so. Variables that are assigned a value before they are declared are automatically declared as global variables, regardless of the scope in which they are used.

Variable assignment expressions evaluate to the value assigned; this allows you to nest or cascade assignments within other expressions: 

z := 14 + (y := 3 - (x := 6))

11

In this example, the expression x := 6 evaluates to 6, and y evaluates to -3, allowing the value 11 to be assigned to z.

Variable assignment in ScriptX is reference assignment. Assigning one variable to another does not copy the object that variable contains. Both variables contain a reference to the same object. Consider the following assignment expressions:

myRect := new Rect x2:100 y2:100

[0, 0, 100, 100] as Rect

myOtherRect := myRect

[0, 0, 100, 100] as Rect

In the first assignment, a new instance of the Rect class is assigned to myRect (the new method is described in the next chapter). In the second assignment, myRect is then assigned to myOtherRect, which has the effect of setting myOtherRect to reference the same Rect object that myRect references.

Figure 2-2: Pointer assignment

If you change any properties of myRect, you also change myOtherRect, since they both refer to the same object in memory.

myRect.y2 := 60

60

myOtherRect

[0, 0, 100, 60] as Rect

Variables and the Garbage Collector

 As a ScriptX program runs, many objects are created. When there are no longer any references to those objects, the garbage collector automatically removes them from memory and reclaims the space they were using. This happens when all variables that reference an object have other values assigned to them, and there are no other references, such as by membership in an array or through another object's instance variables.

Each variable declaration uses only one word of memory, four bytes on a 32-bit processor, until an object is assigned to it. Objects are whatever size they are when instantiated. Once an object is assigned to a variable, ScriptX keeps that object in memory until there is no longer an active reference to it, including any other variables that reference that object. Of course, an object may itself contain references to other objects. Some objects, such as arrays, contain references to objects that are their members or elements.

Memory assigned to a local variable can be reclaimed at the end of the scope in which that variable is declared. Memory assigned to a global variable cannot be recovered unless the value of the variable is set to undefined, or the variable is unbound.

To allow the garbage collector to reclaim memory from unused objects, use local variables whenever possible. For global variables, set the value of a variable that references the object to undefined (or some other value) when the object is no longer needed. This allows memory to be reclaimed without undoing the declaration itself.

global garbage := "taking up space"

"taking up space"

garbage := undefined

undefined

garbage

undefined

In this example, garbage still exists as an empty slot in memory. This slot points to the undefined object, a system object.

This document is part of the ScriptX Language Guide, one of the volumes of the ScriptX Technical Reference Series. ScriptX is developed by the ScriptX Engineering Team at Apple Computer, successor to the Kaleida Engineering Team at Kaleida Labs, Inc.

Copyright 1996 Apple Computer, Inc. All Rights Reserved.