Objective-C NAQs

Never-Asked Questions :-)

In Code Complete 2 §6.5, Steve McConnell presents a list of class-related design issues that "vary significantly depending on the language". So why don't we look at them for Objective-C? Especially as I can't find anyone else who's done so based on a couple of Google searches… N.B. as ever, I'm really considering Cocoa + Objective-C here, as the language itself doesn't provide enough policy for many of the issues to have an answer (only method resolution and the existence of isa are defined by the runtime and the compiler - ignoring details like static string instances, @protocol and so on).

Behaviour of overridden constructors and destructors in an inheritance tree. Constructors are simple. If you can initialise an object, return it. If you can't, return nil. The -[NSObject init] method is documented as simply returning self.

For destructors, we have to split the discussion into twain; half for garbage collection and half for not. In the world of the Apple GC, destruction is best done automatically, but if you need to do any explicit cleanup then your object implements -finalize. The two rules are that you don't know whether other objects have already been finalized or not, and that you must not resurrect your object. In the world of non-GC, an object is sent -dealloc when it has been released enough times not to stick around. The object should then free any memory claimed during its lifetime, and finally call [super dealloc]. Note that this means objects which use the voodoo cohesion of "OK, I'll clean up all my resources in -dealloc" probably aren't going to work too well if the garbage collection switch is flipped.

Behaviour of constructors and destructors under exception-handling conditions. Exceptions are a rarity in Cocoa code (we didn't have language-level exceptions for the first decade, and then there was a feeling that they're too expensive; C++ programmers like the Leopard 64-bit runtime because ObjC exceptions and C++ exceptions are compatible but that's not the case in the NeXT runtime) but obviously a constructor is an atomic operation. You can either initialise an object or you can't; there's no half-object. Therefore if an -init… method catches, the thing to do is unwind the initialisation and return nil. The only thing I can think to say for having exceptions around destruction time is "don't", but maybe a commenter will have better ideas.

Importance of default constructors. All classes implement -init, except those that don't ;-). Usually if there's a parameterised initialiser (-initWithFoo:) then that will be the designated initialiser, and -init will just return [self initWithFoo: someReasonableDefaultValueForFoo];. If there's no reasonable default, then there's no -init. And sometimes you just shouldn't use -init, for instance NSNull or NSWorkspace.

Time at which a destructor or finalizer is called. See How does reference counting work? for a discussion of when objects get dealloced. In the garbage-collected world, an object will be finalized at some time after it is no longer reachable in the object graph. When that is exactly really depends on the implementation of the collector and shouldn't be relied on. Temporal cohesion ("B happens after A, therefore A has been done before B") is usually bad.

Wisdom of overriding the language's built-in operators, including assignment and equality. This is such a brain-damaged idea that the language doesn't even let you do it.

How memory is handled as objects are created and destroyed or as they are declared and go out of scope. How does reference counting work? How do I start using the garbage collector?

WWDC day crosspost

WWDC summary on SophosLabs blog, written by yours truly. I wonder how many Sophos readers (I should probably avoid the term sophist, shouldn't I?) have an interest in Mac and iPhone stuff (and subequently how many comment via the e-mail address on the Labs blog).

Local KDC on Leopard

via Nigel Kersten, a great description of the operation of Leopard's built-in local KDC. I think the most exciting thing about the local KDC is the Bonjour support; could we see simple cross-system trust in the near future? Could there be someone in the world who can actually make Kerberos simple?

WWDC day 00000000000000000000000000000101

Technically day five isn't over yet, but with just one session and a taxi ride remaining I doubt I'm going to get anything prematurely wrong here. In fact, I'm no longer entirely sure that I'll remain awake through the rest of the day so even if something entirely different does happen (still haven't heard anything about IXKit this millennium) I expect I'd miss it.

Planning an east-bound flight is always difficult, because an 11-hour flight through eight time zones "takes" 19 hours. So if I sleep on the plane I'll wake up in the middle of the afternoon UK time, and going back to sleep at newly-local night-time might be complicated. Conversely if I don't sleep on the plane then I won't be going to sleep until at least 9am currently-local time, for a total of about 28 hours uptime. I was just talking to someone who recommended fasting, I'm not sure whether it's worth going through that hardship for an unverified theory (stuff all of that "in the name of science" crap). The other option would be to go into work 3pm-11pm for the beginning of the week, I'm not sure everyone else will appreciate having their meetings moved to the middle of the night and I don't think the canteen's open that late either ;-).

Still, of the three WWDCs I've been to this is definitely in the top ten list of interesting and exciting WWDCs. There definitely still are fellow NeXT fans in the woodwork (actually, *tap* *tap* I think it's gypsum board) who worm their way out during these events, I guess that most have no motivation not to be working on Cocoa. Actually, I don't recall having seen Andrew Stone this week, and I know of a couple of other guys who aren't here, but have definitely managed to gain some traction for the phrase "NeXTSTEP Mobile ;-)

WWDC day four

Not so many sessions attended today - partly because I've reached the limit of what the human physiology can achieve on a diet of coffee and doughnuts. But also due to ducking out of sessions to meet with ex-NeXT guys, ex-Lighthouse guys and ex-colleagues for most of the afternoon. Of course, this was followed by the beer bash, no longer the campus bash which we all know and fondly remember (or at least we all fondly remember queueing for a couple of hours at each end for the coach) but still a good event. Speaking of ex-Lighthouse guys, Wylie and I spent a bit of time putting Sun's world to rights (essentially, if they were to stop diluting the Java brand, stop pulling themselves in every which way and try to make some money, they might do OK).

So tomorrow is the NeXT meetup at the beginning of lunch, I have a lab appointment at exactly 12 which is a little annoying but I'll try to make it over. If the G4 cube is the right shape, wrong CPU and Real Men's Objects use the NX prefix, then come over to the front of the Moscone West after the second session.

WWDC: afp548 Venn competition

I, with some help from Ken and the guys, did a better job.

WWDC day three

Today was not quite a full day, so I managed to spend about an hour or so milling around Yerba Buena park, doing a little gift shopping and generally existing outside of the Mascarpone centre. Also put in another update to the c.l.o-c FAQ (though I didn't update the date! oops…) to better discuss retain/release memory management. There are still situations (GNUstep, Cocotron, and some particular OS X environments) where garbage collection is either unavailable or unsuitable, and it turns out that a lot of questions on either the group or cocoa-dev recently have been about the memory management system.

In the evening, after dinner at Chevy's with the u.c.s.m guys (and yet more discussion of the Sophos feature set), checked in to the Apple Design Awards for the first 30 mins or so. All of the winners (and runners-up) I saw were worthy applications, although it was interesting to note that while most of the "productivity" app winners were from small shops, both of the games (runner-up was Command and Conquer 3, winner was Guitar Hero 3) were from large studios, ports of Windows/console games and in existing long-running series. They're both great games, too, of course. Ian was, of course, rooting for Delicious Library 2, and sadly disappointed ;-).

This was all followed by the AFP548 beer bash over at Thirsty Bear. Yet more "oh, you're from Sophos? Yeah, let me say this one thing…", which I really enjoy because if something's either good or bad enough to be the subject of a beer-fuelled rant in a party, I should probably hear about it. Also submitted a couple of entries to Peter's "Leopard bug Venn diagram" contest; I'm not sure I could do a better job of describing the situation than that but for those who were there, I came up with: \Omega = "Closed/Duplicate" and \Omega = "NeXTSTEP 7.0".

WWDC aside

None of Perl, Cocoa or some weird XML toolkit came up with the "simple things simple, complex things possible" quote.

WWDC day 2

Tuesday is typically associated with "starting to learn stuff" at WWDC, and today was definitely spent learning stuff. Learning, for instance, that doughnuts are considered adequate breakfast material, or that if you are prepared to pay the tiny wi-fi fee at Starbucks you can get faster INTARWEBS than all the people on the steps outside the Mascarpone. It was also a day of nice food; I've just got back from dinner with Alex at the Chevy's, having had lunch with Michael at the Ozuma sushi restaurant. More discussing Sophos-for-Mac with customers and potential customers ensued (apparently we've saved some people from the "hell that is Norton", and I point out for legal purposes that that's a direct quote and not my own opinion or words), and I got some pretty good photos of the Bay Bridge and treasure island which I'll upload as soon as I locate the USB cable.

ObjC FAQ update

Added a question about the ObjC 2.0 garbage collector. Sorry it's been so long in coming! I'll try and add a few more ObjC 2.0 questions over the coming days.

WWDC - day one

The WWDC keynote is always an odd event to attend. It's put on for the benefit of the investors and the media, with the developers being invited purely to act as braying masses expressing their adulation for His Steveness. It's rare for any technical content to make it into the session, except in unavoidable cases such as the 2005 keynote. The focus of that was the Intel transition, so by necessity there had to be some technical justification of the switch.

With this in mind, it's not hard to see that the keynote can be a somewhat dull affair. Obviously as both an Apple customer and member of the "economic ecosystem" of the Mac, it's always good to be as informed as possible of the company's position and direction. That said, yesterday's keynote (no wi-fi in this hotel, so a late post) contained less of interest to me than usual.

As I mentioned I'm financially dependent on Apple (in an indirect sense of course; I'm paid to write Mac software for Sophos, therefore no Mac = no job at Sophos), though as I'm not an indie dev I have a bit more of a comfort buffer than many people. The enterprise iPhone video Steve showed was basically a backslap in front of the shareholders; look, there are people who really do use this stuff! Then the laundry list of every developer who's downloaded the SDK and managed to get something to compile; interesting to see the wealth of different domains into which the iPhone is entering, but seriously. Two demos, three tops. Not all four thousand of the known apps. Good to see TEH CHEAP being applied to the 3G iPhone, though; I may have to
have a discussion with Orange about a PAC when that's available.

Which left Mobile Me. This is actually a pretty cool reboot of iTools^W.Mac, OK it looks like there might be no more iCards but on the other hand the Mobile Me syncing is really beneficial. I can see that becoming more of a cash cow for Apple, though mainly because they opened it up to the PC; people who have an iPod and Windoze could buy MM to synchronise their contacts, mail and so on, as well as getting webmail access (and webmail access which doesn't suck balls as much as
Exchange's OWA, may I add). That then might make them more amenable to the Halo Effect and the purchase of a Mac down the line.

The rest of the day was interesting but obviously undisclosable, except for the evening I spent in a couple of bars down the financial district (the Golden%Braeburn event at 111 Minna, where I went with Steffi from BNR and a couple of Cocotron committers; then Dave's bar where I met Nigel and most of Apple UK). Conversation ranged from Sophos feature requests to the drinkability of American IPAs; all good stuff!

WWDC part 0

well, here it is, the pre-WWDC "I'm jetlagged so you have to put up with my wittering" post. I'm just waiting for a softwareupdate to finish so that I can go out with my camera, taking some early-morning pictures before heading off to stand in line for the Stevenote. I was out for beers with Ian and Neil last night, we'd all heard rumours of a 5 a.m. start to the queue. On the two previous occasions that I've been, 9 a.m. has been sufficient; but with the sellout nature of the event it's likely that the room will fill up rather quickly so we've compromised on a 7 a.m. start. Actually, forget the 5 a.m. nonsense, there's a line of overnight campers - I can't decide whether they're deliberately trying to re-enact a Joy of Tech cartoon, or actually have nothing to do with their lives.

This means business

This is the design of the business card I'll be taking to WWDC. Let's look at some notable features.

• Photo in top left. I don't know about you, but I find it much easier to remember what someone looks like than who they are - this card is available so that people can combine the two.


• Plenty of blank space. The back is also entirely empty and can be written on. When people exchange business cards there'll usually be some context, it can help you remember what that is if you write it down. For instance, I've got cards from previous WWDCs with handwritten notes like "webobjects employer", "bindings", "parallels openstep" and "huge hat" (bonus points for identifying all four).


• Questionable source code snippit. This is a deliberate ploy to annoy fellow-developers with our compulsive attention to detail, thus helping to cement the meeting in their mind as well as providing an inoffensive conversation starter.


• Minimal contextual detail. I'm pretty sure I won't get through all 250 cards in one sitting, so they ought to remain relevant for as long as possible.

Little hack to help with testing

Want the ability to switch in different test drivers, mock objects, or other test-specific behaviour? Here's a pattern I came up with (about a year ago) to do that in a GNUstep test tool, which can readily be used in Cocoa:

NSString *driverClassName = [[NSUserDefaults standardUserDefaults] stringForKey: @"Class"];
Class driverClass = NSClassFromString(driverClassName);
id myDriver = [[driverClass alloc] init];

With a healthy dose of no, seriously, don't do this in production code, you now have the ability to specify your test driver on the command-line like this:

$ ./myTestingTool -Class GLTestDriver

This uses the oft-neglected behaviour of NSUserDefaults, in which it parses the executable's command-line arguments to create a defaults domain, higher in priority than even the user's preferences file. You can use that behaviour in a graphical app too, where it comes in handy when working in Xcode. It then uses a combination of the runtime's duck typing and introspection capabilities to create an instance of the appropriate class.