Passing arguments by value or by reference

In many programming languages, it’s important to understand the distinction between passing arguments by value or by reference; the distinction can affect the way code is designed.

To be passed by value means that the value of the argument is copied into a local variable for use within the function. To be passed by reference means that only a reference to the argument is passed instead of the actual value. No copy of the actual argument is made.

Instead, a reference to the variable passed as an argument is created and assigned to a local variable for use within the function. As a reference to a variable outside the function, the local variable gives you the ability to change the value of the original variable.

In ActionScript 3.0, all arguments are passed by reference, because all values are stored as objects. However, objects that belong to the primitive data types, which includes Boolean, Number, int, uint, and String, have special operators that make them behave as if they were passed by value. For example, the following code creates a function named passPrimitives() that defines two parameters named xParam and yParam, both of type int.

These parameters are similar to local variables declared inside the body of the passPrimitives() function. When the function is called with the arguments xValue and yValue, the parameters xParam and yParam are initialized with references to the int objects represented by xValue and yValue. Because the arguments are primitives, they behave as if passed by value. Although xParam and yParam initially contain only references to the xValue and yValue objects, any changes to the variables within the function body generate new copies of the values in memory.

function passPrimitives(xParam:int, yParam:int):void
{
      xParam++;
     yParam++;
     trace(xParam, yParam);
}
var xValue:int = 10;
var yValue:int = 15;
trace(xValue, yValue);                   // 10 15
passPrimitives(xValue, yValue);    // 11 16
trace(xValue, yValue);                 // 10 15

Within the passPrimitives() function, the values of xParam and yParam are incremented, but this does not affect the values of xValue and yValue, as shown in the last trace statement. This would be true even if the parameters were named identically to the variables, xValue and yValue, because the xValue and yValue inside the function would point to new locations in memory that exist separately from the variables of the same name outside the function.

All other objects—that is, objects that do not belong to the primitive data types—are always passed by reference, which gives you ability to change the value of the original variable. For example, the following code creates an object named objVar with two properties, x and y.

The object is passed as an argument to the passByRef() function. Because the object is not a primitive type, the object is not only passed by reference, but also stays a reference. This means that changes made to the parameters within the function will affect the object properties outside the function.

function passByRef(objParam:Object):void
{
     objParam.x++;
     objParam.y++;
     trace(objParam.x, objParam.y);
}
var objVar:Object = {x:10, y:15};
trace(objVar.x, objVar.y);       // 10 15
passByRef(objVar);               // 11 16
trace(objVar.x, objVar.y);      // 11 16

The objParam parameter references the same object as the global objVar variable. As you can see from the trace statements in the example, changes to the x and y properties of the objParam object are reflected in the objVar object.