What is the correct way to declare a readonly property for ios using ARC
automatic-ref-counting, object, objective-c, properties, readonly
Solution
`strong` is correct to use if you want to keep a strong (owning) reference to whatever it is that you are pointing to. Usually, you do want strong, but in order to prevent circular references (particularly in parent/child relationships where if the parent points to the child and the child points to the parent, they will never be released) you sometimes need to use weak references. Also, if you want to keep a pointer to an object that you don't own but want it to be valid only as long as it exists, then you want to use a weak pointer because when it gets deallocated by the owner, your pointer will automatically get set to `nil` and won't be pointing to memory that it shouldn't be.
`assign` is used with scalar values, and is the default setter. `copy` makes sense if you want to automatically make a copy of the object and set your pointer to the copy instead of pointing to the original object. It only makes sense to do this if you have a specific need (usually because you don't want the object to mutate on you).
The link that you provided which shows that __strong is the default (and therefore you don't need to specify it) refers to variables and not to declared properties. The default for declared properties is `assign` so it certainly will make a difference. If you were wanting `assign` however, it makes no difference whether you specify it or not (other than just to be clear that it is what you wanted). EDIT: However, as Jacques pointed out, this is changing with LLVM 3.1 and the default is changing from `assign` to `strong`. In this case, it makes absolutely no difference whether or not you specify `strong` and can leave it out if you want. Personally I think that it is good to spell it out (especially since there is a conflict between different versions) so that everyone looking at the code is on the same page. Others may disagree on this point though. :)
I would suggest reading the Declared Properties section of The Objective-C Programming Language here: `<document removed by Apple with no direct replacement>`.
Problem
I am new to iOS development in general and have never dealt with manual reference counting (retain, release, autorelease). As such I don't have a good understanding of what magic ARC is performing. I thought I understood until I was asked what type of ownership (`weak`, `strong`, `assign`, etc) should be given to a readonly property pointing at an object, such as: ``` @property (readonly,nonatomic) NSString* name; ``` I read here Questions about a readonly @property in ARC that leaving off the `strong`/`weak` won't actually compile unless you specify a backing variable when you `@synthesize` the property; I just so happened to be specifying a backing ivar like this: ``` @synthesize name = _name; ``` Now I understand that the default 'lifetime qualifier' of a variable is strong, from here: http://developer.apple.com/library/ios/releasenotes/ObjectiveC/RN-TransitioningToARC/Introduction/Introduction.html#//apple_ref/doc/uid/TP40011226-CH1-SW4 So to cut a long story short - I am indirectly defining my property as `(readonly,nonatomic,strong)` as the `_name` ivar is implicitly declared as `__strong`. I have a few questions: Is `strong` the correct lifetime qualifier to use? I assume that it is, otherwise the object backing my `NSString*` wouldn't be owned anywhere and would thus be freed automatically (coming from Java land this makes sense as all references are strong by default). Are there any other modifiers which make sense in this situation, such as `copy` or `assign`? Does declaring the property as `(readonly,nonatomic,strong)` and `(readonly,nonatomic)` make any difference to the code which consumes the property? eg. does declaring it without the `strong` keyword cause the object pointer to be stored as `__unsafe_unretained` where the `strong` property would be stored in a `__strong` pointer? Thanks! EDIT So as I understand now, the following applies to readonly properties: - For non-NSObject* types (int, float, void*, etc) use `(readonly, assign)`. - For object pointers, use `(readonly, strong)` or `(readonly, copy)` - these function the same for readonly properties but you may want the copy semantics if you extend/subclass and redeclare the property as `readwrite`. - For object pointers, `(readonly, weak)` only makes sense if you are going to be storing an already weak pointer in that property (that pointer must be strong elsewhere or the object will be deallocated).