NSArray property: copy or retain?

Question

According to this: NSString property: copy or retain?

For NSString/NSMutableString, copy is recommended.

How about NSArray/NSMutableArray?

Answer 1

Since you're asking about NSArray (rather than NSMutableArray), you should use copy. NSArray is immutable, so you don't expect a property of that type to change. But NSMutableArray is a subclass of NSArray, so it's perfectly valid for someone to pass in a NSMutableArray. If you just retain that object, then it may change right under your nose. If you copy rather than retain, then the object won't change.

However, you should be aware that when you copy a container like NSArray, you're copying the container only and not its contents. If the array contains mutable objects, the contents of those objects may change even though the array itself is immutable.

Answer 2

If it is a problem when the underlying data changes, use copy. In fact, this is what you want most of the time, as changing data behind someone's back is a good source for bugs.

Note that copy will essentially just be a retain for an NSArray. Only when you throw an NSMutableArray in, there is more work involved.

Answer 3

From the link you included, it pretty much comes down to this: NSString property: copy or retain?

If you want to make sure the value of the object won't change during execution, you use the copy attribute, otherwise retain will be fine. Generally, retain will be ok for NSMutableArrays and NSArrays (as well as many other objects) as you are (usually) more interested in the object then in the value it contains. In case of an NSString you are always interested in the value, so you copy it to make sure it won't change.

@jlehr:

It depends if the developer is interested in the actual value or not. Whenever interested in the actual value, use copy (since you don't want the value to change during execution), otherwise retain is fine. From Apple's docs:

It is common practice in Objective-C code to copy value objects—objects that represent attributes. C-type variables can usually be substituted for value objects, but value objects have the advantage of encapsulating convenient utilities for common manipulations. For example, NSString objects are used instead of character pointers because they encapsulate encoding and storage.

Also from Apple's docs, on the topic of value objects:

A value object is in essence an object-oriented wrapper for a simple data element such as a string, number, or date. The common value classes in Cocoa are NSString, NSDate, and NSNumber. Value objects are often attributes of other custom objects you create.

Answer 4

choose copy, unless you have a very specific reason not to, as well as all the supporting code/interface to back that up.

i detailed the rationale and several implications here: NSMutableString as retain/copy

that example is based on NSStrings, but the same applies for NSArrays.