What are the negative impacts of extending classes in ActionScript 3?

Question 1

This is a pretty common problem when it comes to integrating third party libraries, especially libraries that are ports (as Box2DAS3 is), where they keep the coding and naming conventions of the parent language rather than fully integrating with the destination language (case in point: Box2DAS3 using getFoo() and setFoo() instead of a .foo getter/setter).

To answer your question quickly, no, there will be no significant performance impact with making wrapper classes; no more than you'll see in the class hierarchy in your own project. Sure, if you time a loop of 5 million iterations, you might see a millisecond or two of difference, but in normal usage, you won't notice it.

"More memory consumption to accommodate the extra level of class structure." Like any language that has class inheritence, a vtable will be used behind the scenes, so you will have a small increase in memory/perf, but it's negligible.

"Performance impact when creating new objects due to initializing an extra level of members?" No more than normal instantiation, so not something to worry about unless you're creating a huge amount of objects.

Performance wise, you should generally have no problem (favour readability and usability over performance unless you actually have a problem with it), but I'd look at it more as an architectural problem and, with that in mind, what I would consider to be a negative impact of extending/modifying classes of an external library generally fall into 3 areas, depending on what you want to do:

Modify the library
Extend the classes
Composition with your own classes

Modify the libary

As Box2DAS3 is open source, there's nothing stopping you jumping in and refactoring all the class/function names to your hearts content. I've seriously considered doing this at times.

Pros:

You can modify what you want - functions, classes, you name it
You can add any missing pieces that you need (e.g. pixel-meters conversion helpers)
You can fix any potential performance issues (I've noticed a few things that could be done better and faster if they were done in an "AS3" way)

Cons:

If you plan to keep your version up to date, you'll need to manually merge/convert any updates and changes. For popular libraries, or those that change a lot, this can be a huge pain
It's very time-consuiming - aside from modifications, you'll need a good understanding on what's going on so you can make any changes without breaking functionality
If there's multiple people working with it, they can't rely as much on external documentation/examples, as the internals might have changed

Extend the classes

Here, you simply make your own wrapper classes, which extend the base Box2D classes. You can add properties and functions as you want, including implementing your own naming scheme which translates to the base class (e.g. MyBox2DClass.foo() could simply be a wrapper for Box2DClass.bar())

Pros:

You implement just the classes you need, and make just the changes necessary
Your wrapper classes can still be used in the base Box2D engine - i.e. you can pass a MyBox2DClass object to an internal method that takes a Box2DClass and you know it'll work
It's the least amount of work, out of all three methods

Cons:

Again, if you plan to keep your version up to date, you'll need to check that any changes don't break your classes. Normally not much of a problem, though
Can introduce confusion into the class, if you create your own functions that call their Box2D equivalent (e.g. "Should I use setPos() or SetPosition()?). Even if you're working on your own, when you come back to your class in 6 months, you'll have forgotten
Your classes will lack coherence and consistency (e.g. some functions using your naming methodology (setPos()) while others use that of Box2D (SetPosition()))
You're stuck with Box2D; you can't change physics engines without a lot of dev, depending on how your classes are used throughout the project. This might not be such a big deal if you don't plan on switching

Composition with your own classes

You make your own classes, which internally hold a Box2D property (e.g. MyPhysicalClass will have a property b2Body). You're free to implement your own interface as you wish, and only what's necessary.

Pros:

Your classes are cleaner and fit in nicely with your engine. Only functions that you're interested in are exposed
You're not tied to the Box2D engine; if you want to switch to Nape, for example, you only need to modify your custom classes; the rest of your engine and games are oblivious. Other developers also don't need to learn the Box2D engine to be able to use it
While you're there, you can even implement multiple engines, and switch between them using a toggle or interfaces. Again, the rest of your engine and games are oblivious
Works nicely with component based engines - e.g. you can have a Box2DComponent that holds a b2Body property

Cons:

More work than just extending the classes, as you're essentially creating an intermediary layer between your engine and Box2D. Ideally, outside of your custom classes, there shouldn't be a reference to Box2D. The amount of work depends on what you need in your class
Extra level of indirection; normally it shouldn't be a problem, as Box2D will use your Box2D properties directly, but if your engine is calling your functions a lot, it's an extra step along the way, performance wise

Out of the three, I prefer to go with composition, as it gives the most flexibility and keeps the modular nature of your engine intact, i.e. you have your core engine classes, and you extend functionality with external libraries. The fact that you can switch out libraries with minimal effort is a huge plus as well. This is the technique that I've employed in my own engine, and I've also extended it to other types of libraries - e.g. Ads - I have my engine Ad class, that can integrate with Mochi, Kongregate, etc as needed - the rest of my game doesn't care what I'm using, which lets me keep my coding style and consistency throughout the engine, whilst still being flexible and modular.

----- Update 20/9/2013 -----

Big update time! So I went back to do some testing on size and speed. The class I used is too big to paste here, so you can download it at http://divillysausages.com/files/TestExtendClass.as

In it, I test a number of classes:

An Empty instance; a Class that just extends Object and implements an empty getPostion() function. This will be our benchmark
A b2Body instance
A Box2DExtends instance; a Class that extends b2Body and implements a function getPosition() that just returns GetPosition() (the b2Body function)
A Box2DExtendsOverrides instance; a Class that extends b2Body and overrides the GetPosition() function (it simply returns super.GetPosition())
A Box2DComposition instance; a Class that has a b2Body property and a getPosition() function that returns the b2Body's GetPosition()
A Box2DExtendsProperty instance; a Class that extends b2Body and adds a new Point property
A Box2DCompositionProperty instance; a Class that has both a b2Body property and a Point property

All tests were done in the standalone player, FP v11.7.700.224, Windows 7, on a not-great laptop.

Test1: Size

AS3 is a bit annoying in that if you call getSize(), it'll give you the size of the object itself, but any internal properties that are also Objects will just result in a 4 byte increase as they're only counting the pointer. I can see why they do this, it just makes it a bit awkward to get the right size.

Thus I turned to the flash.sampler package. If we sample the creation of our objects, and add up all the sizes in the NewObjectSample objects, we'll get the full size of our object (NOTE: if you want to see what's created and the size, comment in the log calls in the test file).

Empty's size is 56 // extends Object
b2Body's size is 568
Box2DExtends's size is 568 // extends b2Body
Box2DExtendsOverrides's size is 568 // extends b2Body
Box2DComposition's size is 588 // has b2Body property
Box2DExtendsProperty's size is 604 // extends b2Body and adds Point property
Box2DCompositionProperty's size is 624 // has b2Body and Point properties

These sizes are all in bytes. Some points worth noting:

The base Object size is 40 bytes, so just the class and nothing else is 16 bytes.
Adding methods doesn't increase the size of the object (they're implemented on a class basis anyway), while properties obviously do
Just extending the class didn't add anything to it
The extra 20 bytes for Box2DComposition come from 16 for the class and 4 for the pointer to the b2Body property
For Box2DExtendsProperty etc, you have 16 for the Point class itself, 4 for the pointer to the Point property, and 8 for each of the x and y property Numbers = 36 bytes difference between that and Box2DExtends

So obviously the difference in size depends on the properties that you add, but all in all, pretty negligible.

Test 2: Creation Speed

For this, I simply used getTimer(), with a loop of 10000, itself looped 10 (so 100k) times to get the average. System.gc() was called between each set to minimise time due to garbage collection.

Empty's time for creation is 3.9ms (av.)
b2Body's time for creation is 65.5ms (av.)
Box2DExtends's time for creation is 69.9ms (av.)
Box2DExtendsOverrides's time for creation is 68.8ms (av.)
Box2DComposition's time for creation is 72.6ms (av.)
Box2DExtendsProperty's time for creation is 76.5ms (av.)
Box2DCompositionProperty's time for creation is 77.2ms (av.)

There's not a whole pile to note here. The extending/composition classes take slightly longer, but it's like 0.000007ms (this is the creation time for 100,000 objects), so it's not really worth considering.

Test 3: Call Speed

For this, I used getTimer() again, with a loop of 1000000, itself looped 10 (so 10m) times to get the average. System.gc() was called between each set to minimise time due to garbage collection. All the objects had their getPosition()/GetPosition() functions called, to see the difference between overriding and redirecting.

Empty's time for getPosition() is 83.4ms (av.) // empty
b2Body's time for GetPosition() is 88.3ms (av.) // normal
Box2DExtends's time for getPosition() is 158.7ms (av.) // getPosition() calls GetPosition()
Box2DExtendsOverrides's time for GetPosition() is 161ms (av.) // override calls super.GetPosition()
Box2DComposition's time for getPosition() is 160.3ms (av.) // calls this.body.GetPosition()
Box2DExtendsProperty's time for GetPosition() is 89ms (av.) // implicit super (i.e. not overridden)
Box2DCompositionProperty's time for getPosition() is 155.2ms (av.) // calls this.body.GetPosition()

This one surprised me a bit, with the difference between the times being ~2x (though that's still 0.000007ms per call). The delay seems entirely down to the class inheritence - e.g. Box2DExtendsOverrides simply calls super.GetPosition(), yet is twice as slow as Box2DExtendsProperty, which inherits GetPosition() from its base class.

I guess it has to do with the overhead of function lookups and calling, though I took a look at the generated bytecode using swfdump in the FlexSDK, and they're identical, so either it's lying to me (or doesn't include it), or there's something I'm missing :) While the steps might be the same, the time between them probably isn't (e.g. in memory, it's jumping to your class vtable, then jumping to the base class vtable, etc)

The bytecode for var v:b2Vec2 = b2Body.GetPosition() is simply:

getlocal        4
callproperty    :GetPosition (0)
coerce          Box2D.Common.Math:b2Vec2
setlocal3

whilst var v:b2Vec2 = Box2DExtends.getPosition() (getPosition() returns GetPosition()) is:

getlocal        5
callproperty    :getPosition (0)
coerce          Box2D.Common.Math:b2Vec2
setlocal3

For the second example, it doesn't show the call to GetPosition(), so I'm not sure how they're resolving that. The test file is available for download if someone wants to take a crack at explaining it.

Some points to keep in mind:

GetPosition() doesn't really do anything; it's essentially a getter disguised as a function, which is one reason why the "extra class step penalty" appears so big
This was on a loop of 10m, which you're unlikely to doing in your game. The per-call penalty isn't really worth worrying about
Even if you do worry about the penalty, remember that this is the interface between your code and Box2D; the Box2D internals will be unaffected by this, only the calls to your interface

All-in-all, I'd expect the same results from extending one of my own classes, so I wouldn't really worry about it. Implement the architecture that works the best for your solution.

Question 2

I know this answer will not qualify for the bounty as I am way to lazy to write benchmarks. But having worked on the Flash code base I can maybe give some hints:

The avm2 is a dynamic language, so the compiler will not optimize anything in this case. Wrapping a call as a sub class call will have a cost. However that cost will be constant time and small.

Object creation cost will also at most be affected by a constant amount of time and memory. Also the time and amount will probably be insignificant compared to the base cost.

But, as with many things the devil is in the details. I never used box2d, but if it does any kind of object pooling things might not work well anymore. In general games should try to run without object allocations at play time. So be very careful not to add functions that allocate objects just to be prettier.

function addvectors(a:vec,b:vec,dest:vec):void

Might be ugly but is much faster than

function addvectors(a:vec,b:vec):vec

(I hope I got my AS3 syntax right...). Even more useful and more ugly might be

function addvectors(a:Vector.<vec>, b:Vector.<vec>, dest:Vector.<vec>, offset:int, count:int):void

So my answer is, if you are only wrapping for readability, go for it. It's a small, but constant cost. But be very, very careful to change how functions work.

Question 3

I don't know if there is a big impact for instanciation time, but I will answer your question differently: what are your other options? Do they seem they'll do better?

There is a beautiful benchmark made by Jackson Dunstan about function performance: http://jacksondunstan.com/articles/1820

To sum it up:

closures are expensive
static is slow: http://jacksondunstan.com/articles/1713
overriding, calling a function inside a subClass does not seem to have a big impact

So, if you want to not use inheritance, maybe you'll have to replace it with static calls, and it is bad for performance. Personally, I'll extend those classes and add an eager instanciation of all objects I'll need at runtime: if it is big, make a beautiful loading screen...

Also, take a look at post bytecode optimizations such as apparat: http://code.google.com/p/apparat/

Question 4

I don't think that extending will impact the performance a lot. Yes, there is some cost, but it's not so high as long as you don't use composition. I.e. instead of extending Box2d classes directly, you create an instance of that classes and work with it inside your class. For example this

public class Child extends b2Body {
    public function Child() {
        // do some stuff here
    }
}

instead of this

public class Child  {
    private var _body:b2Body;
    public function Child() {
        ...
        _body = _world.CreateBody(...);
        ...
    }
}

I guess you know that as less objects as you create the better. As long as you keep the number of created instances you will have same performance.

From another point of view: a) adding one more layer of abstractions may change the Box2d a lot. If you work in a team this may be an issue, because the other developers should learn your naming b) be careful about Middle Man code smell. Usually when you start wrapping already existing functionality you end up with classes which are just delegators.

Question 5

Some great answers here but I'm going to throw my two cents in.

There are two different concepts you have to recognize: when you extend a class and when you implement a class.

Here is an example of extending MovieClip

public class TrickedOutClip extends MovieClip {
   private var rims = 'extra large'
   public function TrickedOutClip() { 
     super();
   }
}

Here is an example of implementing MovieClip

public class pimpMyClip {
   private var rims = 'extra large';
   private var pimpedMovieClip:MovieClip;
   public function pimpMyClip() { 
     pimpedMovieClip = new MovieClip();
     pimpedMovieClip.bling = rims;
   }

 public function getPimpedClip() { 
     return pimpedMovieClip;
   }
}

I think you probably do not want to extend these box2D classes but implement them. Here's a rough outline:

 public class myBox2DHelper {
   private var box2d = new Box2D(...);

  public function MyBox2DHelper(stage) {

  }

   public function makeBox2DDoSomeTrickyThing(varA:String, varB:Number) { 
       // write your custom code here
     }

   public function makeBox2DDoSomethingElse(varC:MovieClip) { 
       // write your custom code here
     }
  }

Good luck.