Use of __attribute__'s in ARC-managed Code

Question

When ARC came to Objective-C, I did my best to read through the Objective-C Automatic Reference Counting (ARC) guide posted on the Clang project website to get a better hang of what it was about. What I found there (and no where else) was mention of using __attribute__ declarations to signify to ARC whether certain code autoreleases its return value, for instance (__attribute__((ns_returns_autoreleased))), or whether it 'consumes' a parameter (__attribute((ns_consumed)), and so on.

However, it seems that the guide gives very little word on the actual level of necessity these declarations hold. Excluding them seems to make no difference, neither when running the static analyzer nor when running the project itself. Do these even make a difference? Is there any advantage to labeling a method with __attribute__((objc_method_family(new)))? No article I've found on ARC makes mention of these specifiers at all; perhaps an ARC guru can give word on what these are used for.

(Personally, I include all relevant specifiers just in case, but find that they make code obfuscated and messy.)

Answer 1

These attributes are expressly for abnormal cases, such as:

A function or method parameter of retainable object pointer type may be marked as consumed, signifying that the callee expects to take ownership of a +1 retain count.

A function or method which returns a retainable object pointer type may be marked as returning a retained value, signifying that the caller expects to take ownership of a +1 retain count.

You don't normally do these things, so you don't normally use these attributes. With no attributes, the normal behavior—the NARC rule, or perhaps under ARC I should say CAN—is what the compiler implements and expects.

There are two reasons to use these attributes:

• In order to violate the CAN rule; that is, to have a method not so named that returns a reference, or a method so named that doesn't. The attribute documents the violation in the method's prototype, and may even be necessary to implement it, if the implementation uses ARC.
• Working with Core Foundation types, including Core Graphics types. These aren't ARCed, so you need to use the bridging attributes to aid conversion to and from “retainable object pointer” types.

Answer 2

That's not necessary in most of the cases, since LLVM & Clang knows ObjC naming conventions. So if you follow the standard naming conventions of Cocoa, LLVM automagically assumes the corresponding family/return memory policy to follow.

Namely, if you declare a method named initWith... it will automatically consider it as the "init" family of methods, no need to specify __attribute__((objc_method_family(init))), Clang automatically detect it; same for the new family, etc.

In fact, you only need to use the __attribute__ specifiers when Clang can't guess such cases, which in practice rarely occurs (in practice I never had to use it), or only if you don't respect naming conventions:

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Quoting Clang Language Extensions Documentation:

Many methods in Objective-C have conventional meanings determined by their selectors. For the purposes of static analysis, it is sometimes useful to be able to mark a method as having a particular conventional meaning despite not having the right selector, or as not having the conventional meaning that its selector would suggest. For these use cases, we provide an attribute to specifically describe the method family that a method belongs to.

So as soon as you respect the naming conventions (which you should always do) you won't have anything do to.

Answer 3

You should definitely stick to naming conventions wherever possible.

• It's clearer to read.
• Attributes can introduce build errors if there is a conflict.
• ARC semantics combined with attributes are relatively fragile.