How is this function return value being ignored in Actionscript? - actionscript-3

Question

I'm trying to port the Box2DFlashAS3 physics engine to another language (Xojo). I am not particularly fluent in Actionscript (but am more so than I am with C++ which Box2D was originally written in).
As I understand it, parameters passed to functions in Actionscript as done so by reference. Consider these two classes (greatly cut down for simplicity, it's the two GetInverse() functions I'm interested in):
public class b2Mat22 {
public function GetInverse(out:b2Mat22) : b2Mat22 {
var a:Number = col1.x;
var b:Number = col2.x;
var c:Number = col1.y;
var d:Number = col2.y;
var det:Number = a * d - b * c;
if (det != 0.0)
{
det = 1.0 / det;
}
out.col1.x = det * d; out.col2.x = -det * b;
out.col1.y = -det * c; out.col2.y = det * a;
return out;
}
public var col1:b2Vec2 = new b2Vec2();
public var col2:b2Vec2 = new b2Vec2();
}
and
public class b2Transform {
public function GetInverse(out:b2Transform = null) : b2Transform {
if (!out)
out = new b2Transform();
R.GetInverse(out.R);
out.position.SetV(b2Math.MulMV(out.R, position));
out.position.NegativeSelf();
return out;
}
public var position:b2Vec2 = new b2Vec2();
public var R:b2Mat22 = new b2Mat22();
}
I don't understand R.GetInverse(out.R); in the b2Transform class. Doesn't the GetInverse() function of the b2Mat22 class return a value? If so, why is it not being used?

Answer 1

That a function returns a value does not mean that this value needs to be used.
Here is an example:
Array - reverse()
Returns: Array — The new array.
Reverses the array in place.
var letters:Array = new Array("a", "b", "c");
trace(letters); // a,b,c
letters.reverse();
trace(letters); // c,b,a
You can see it modifies the array but it STILL returns the new array. This can be used for some techniques like method chaining:
myArray.reverse().concat(...) //you couldn't do this if it didn't return the new array, in that case you would have to do this:
// --->
myArray.reverse();
myArray.concat(...);
Also note that in AS (just like in JS, Java, etc.) the parameter is passed by reference if it is an object but the primitive values are not passed by reference.
Now to say the complete truth I have no idea what those methods from Box2D do, but I believe this might be the case.