What is @[] and @{} in Objective-C?

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What are the details of “Objective-C Literals” mentioned in the Xcode 4.4 release notes?

I've got question about @[] and @{}.

Some code taken from internet:

self.searches = [@[] mutableCopy]; 
self.searchResults = [@{} mutableCopy];

score 10 · Accepted Answer · answered Nov 28 '12 at 14:12

10

No. @[] is equal to [NSArray array] or [[NSArray alloc] init].
No. @{} is equal to [NSDictionary dictionary] or [[NSDictionary alloc] init].

(depending on the context and whether you use Automatic Reference Counting (ARC) or not)

That's why you see things like [@[] mutableCopy] sometimes. This will create an empty immutable array and create a mutable copy of it.

The result is the same as using [NSMutableDictionary dictionary] or [[NSMutableDictionary alloc] init].

answered Nov 28 '12 at 14:12

DrummerB

yeah ... it's creating an autoreleased object so the answer is only correct when using arc. – Bastian Nov 28 '12 at 14:14
I was assuming anyone starting with Objective-C at this time would use ARC, but good point. – DrummerB Nov 28 '12 at 14:15

score 1 · Answer 2 · answered Nov 28 '12 at 14:12

1

@{} is equal to [NSDictionary dictionary]

@[] is equal to [NSArray array]

so [@[] mutableCopy] creates an empty immutable object and then it makes a mutablecopy of it. I don't think you can do it less efficient.

answered Nov 28 '12 at 14:12

Bastian

yeah .. i copied it from the question ;) – Bastian Nov 28 '12 at 14:15
There was a post somewhere about the performance of `mutableCopy` vs simple `alloc/init`, but I can't find it... – DrummerB Nov 28 '12 at 14:17

score 0 · Answer 3 · answered Nov 28 '12 at 14:15

First one @[] is a shorthand to create an Array, for example the following array of two elements:

NSArray *array = @[ @"First", @"Second"];

Second one @{} creates a dictionary, for example:

NSDictionary *dictionary = @{
    @"first" : someValue,
    @"second" : someValue,
};

3 Answers3