To support iOS 4.3, use "assign" instead of weak, but Interface Builder uses unsafe_unretained?

Question

To support iOS 4.3 with ARC, I think the proper way is to use assign:

@property (assign, nonatomic) UIView *view;
@property (assign, nonatomic) MYNode *node;

Is that correct? I also see the following in Apple's doc for Transitioning to ARC:

For declared properties, you should use assign instead of weak; for variables you should use __unsafe_unretained instead of __weak.

However, if I use the current Xcode (4.4.1), changing a Single View app target to 4.3, and Ctrl-drag a UIButton to the .h file to create an outlet, the generated code is:

@property (unsafe_unretained, nonatomic) IBOutlet UIButton *foo;

Why the difference and which one should be used?

Martin R · Accepted Answer · 2012-09-08T11:19:05.647

3

According to 4.1.1. Property declarations in the llvm documentation "assign" and "unsafe_unretained" are equivalent in a property declaration:

assign implies __unsafe_unretained ownership.
...
unsafe_unretained implies __unsafe_unretained ownership.

ADDED: In the clang source code http://clang.llvm.org/doxygen/SemaObjCProperty_8cpp_source.html you find

00523   // 'unsafe_unretained' is alias for 'assign'.
00524   if (Attributes & ObjCDeclSpec::DQ_PR_unsafe_unretained)
00525     PDecl->setPropertyAttributes(ObjCPropertyDecl::OBJC_PR_assign);

edited Sep 08 '12 at 11:19

answered Sep 08 '12 at 10:16

Martin R

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but could they both imply the same thing but still have differences? – Jeremy L Sep 08 '12 at 10:54
@JeremyL: I am sure that there is no difference. I assume that older Xcode versions did not understand `@property (unsafe_unretained, ...)`. I have some older versions so I can test that later if you are really interested. – Martin R Sep 08 '12 at 11:12
There is definitely no difference. – Catfish_Man Sep 08 '12 at 20:14

score 1 · Answer 2 · answered Sep 08 '12 at 10:15

1

You've quoted excerpt from answer to the question "Which classes don’t support weak references?" — actually, I guess, the excerpt is meant to be applied only to the classes listed in the answer.

From what I've read before when I was studying ARC, there is no real difference between unsafe_unretained and assign.

answered Sep 08 '12 at 10:15

ivanzoid

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an answer said they are different? http://stackoverflow.com/questions/8397511/using-arc-lifetime-qualifier-assign-and-unsafe-unretained – Jeremy L Sep 08 '12 at 10:54
That answer is referring to `__unsafe_unretained`, not `unsafe_unretained`. – omz Sep 08 '12 at 11:24

To support iOS 4.3, use "assign" instead of weak, but Interface Builder uses unsafe_unretained?

2 Answers2