What sense to copy empty array?

Question

I were reading another programmer's code, so i found this:

@property(nonatomic, strong) NSArray *assets;
----
   _assets = [@[] mutableCopy];
    __block NSMutableArray *tmpAssets = [@[] mutableCopy];

Is it some kind of trick? Why does he used mutableCopy to immutable array assets ? Why doesn't he just create it like:

self.assets = [NSArray new];
__block NSMutableArray *tmpAssets = [NSMutableArray new];

?

Just because the *property* `assets` is immutable that doesn't necessarily mean the *instance variable* that backs it, `_assets`, is immutable. It's good practice to never make any property have mutable type. It's convenient to back a mutable indexed collection property with an `NSMutableArray*` instance variable but only allow it to be mutated through the [indexed collection mutation accessors](https://developer.apple.com/library/mac/documentation/Cocoa/Conceptual/KeyValueCoding/Articles/AccessorConventions.html#//apple_ref/doc/uid/20002174-SW11). — Ken Thomases, Jun 17 '15 at 08:57

cacau · Answer 1 · 2015-06-17T08:50:54.980

1

Just a shorthand to get an empty mutable array.

Instead of [NSMutableArray alloc] init] it's slightly less code to write -

the new construct isn't really used by the majority of Objective-C programmers and was added as a convenience to newcomers from other languages.

Mind that @[] will create an (immutable) NSArray -
there is no Objective-C literal for NSMutableArray.

edited Jun 17 '15 at 08:50

answered Jun 17 '15 at 08:01

cacau

For me, `new` is much more clear than `[@[] mutableCopy];` What sense to create it and then copy? – Zaporozhchenko Oleksandr Jun 17 '15 at 08:36
Oh, I see - `@[]` creates an `NSArray`! so in order to get the mutable counterpart you send it the `mutableCopy` message. – cacau Jun 17 '15 at 08:48

1 Answers1