Quincey,
I just got around to reading this thread and wanted to complement you on the
clarity of your explanation.
Boyd
On Mar 22, 2013, at 3:27 PM, Quincey Morris wrote:
> (sorry, this turned into a tome)
>
> The way properties are compiled has changed over time. Assuming you're using
> the current (Xcode 4.6+) version of clang to compile your files, there is an
> explicit 'atomic' attribute for properties. This has been around for some
> time. (Keep in mind that out-of-date information never gets removed from
> StackOverflow. It's a treacherous place to use as your primary source of
> information.)
>
> The purpose of the explicit 'atomic' keyword is so that you don't let
> atomic/nonatomic default. There is a companion compiler option in the build
> settings ("Implicit Atomic Objective-C Properties") which you can set to YES
> to get a warning when you don't specify either nonatomic or atomic.
>
> The reason for this is not much about performance. ('atomic' is slower, but
> not by much.) It's more about what happens when you write custom accessors.
> For example
>
> @property id someProperty;
>
> - (void) setSomeProperty: (id) newValue {
> _someProperty = newValue;
> }
>
> This is a bug, because you've declared "someProperty" atomic (by default),
> but you didn't provide an atomic implementation. Secondarily, in the above
> example, when there's no custom getter, the compiler will object that it
> can't synthesize the getter. If you declare the property nonatomic, the
> compiler will happily synthesize the getter for you.
>
> At a different level, forcing yourself to specify nonatomic or atomic will
> help you to remember whether you need the property to be atomic or not. If
> you make it atomic, you are probably dealing with a thread synchronization
> issue. But in many cases making the property atomic does *not* produce thread
> safe code, and in many cases thread safe code doesn't need atomic properties.
> Properties that truly need to be atomic are rare in most apps.
>
> The current paradigm for declaring properties is like this:
>
> @interface MyClass : …
> @property (nonatomic) id someProperty; // or 'atomic' if that's what
> you want.
> @end
>
> @implementation MyClass;
> @end
>
> The implementation uses auto-synthesis, which does three things:
>
> 1. It synthesizes the ivar that backs the property (with the name
> _someProperty, with a leading underscore).
> 2. It synthesizes the getter.
> 3. It synthesizes the setter.
>
> Note that if you synthesize explicitly like this:
>
> @implementation MyClass;
> @synthesize someProperty;
> @end
>
> the compiler does the same three things *except* that it synthesizes the ivar
> with the name someProperty, without a leading underscore.
>
> If you want to declare a private ivar yourself, you should do it in the
> implementation and not in the interface:
>
> @implementation MyClass
> {
> NSNumber* _someProperty;
> }
> @end
>
> If you're writing your own getter and/or setter, none of the above changes.
> Except for one thing. If you provide both a getter and a setter, the compiler
> does *not* auto-synthesize the ivar. You must either declare the ivar
> explicitly, as in the immediately preceding example, or with an explicit
> synthesize:
>
> @synthesize someProperty = _someProperty;
>
> Be careful about whether the synthesize will put a leading underscore on the
> ivar name. Because the rules have been in transition, it's easy to end up
> with the wrong name, producing a hard-to-find bug.
>
>
>
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