Re: Objective-C Instance Variable Names

FROM : Jens Alfke
DATE : Thu Apr 03 21:08:26 2008

On 3 Apr '08, at 10:58 AM, Rob Napier wrote:

> Apple reserves the use of leading underscores for it's own use:
> While they append the phrase "especially in methods," they do mean 
> for everything, and you can collide with them if you name your own 
> identifiers (including instance variables) with leading underscores.


This issue came up several times while I was at Apple. The most recent 
consensus (involving members of the Cocoa and compiler groups) was 
that "_" prefixes for ivar names are mostly harmless, though they 
should be avoided in method names. The reason is that ivar names are 
less dynamic. If in the next OS release a system superclass of your 
class adds an ivar with the same name, it's not a binary compatibility 
problem (your class will continue to work, because its machine code 
references the ivar as a numeric offset into the instance data, not by 
name.) Whereas if a superclass adds a _method_ with the same name as 
yours, all hell may break loose because your method will get called 
instead.

Now, your code will break when you try to recompile it using the new 
headers, because the compiler will complain about the ivar name 
conflict. But then you just rename your ivar. It's a less serious 
problem because it doesn't break any copies of the app that are out in 
the field.

> 1. Leading underscores really will cause you to collide with Apple 
> and it really did blow up in my face.


Was it with a method or an ivar name?

> 3. XCode 3 now provides good color coding to distinguish ivars from 
> local vars.


I should try turning that specific color code on. I turned off all the 
new Xcode 3 color coding because there were too many different colors 
and it made the code look garish.

> 4. And besides, if you would use accessors properly, ivars should 
> very seldom show up in your code so you shouldn't have confusion 
> (this being the cause of #2 above).


I don't use accessors properly, then. ;) Method calls are so expensive 
compared to ivar accesses that I don't use them for accessing instance 
data, unless the accessor is nontrivial or the setter call is needed 
to trigger KV notifications. (Also, I generally don't declare 
properties for internal state that isn't part of the class's public 
API.)

—Jens