When you create an new instance of NSArray, inside arrayWithObjects:count: (which can be called directly and is called indirectly by other constructors and also is that method is used internally to create an array from a literal array) depending on how many items it has, under the hood NSArray may create either an instance of __NSArrayI(Runtime Header file) or an instance of __NSSingleObjectArrayI (Runtime Header file). Actually, if the array contains one item, NSArray create an instance of __NSSingleObjectArrayI, otherwise __NSArrayI gets used. 

NSArray *arr1 = @[@(1)]; // arr1 is __NSSingleObjectArrayI
NSArray *arr2 = @[@(1), @(2)]; // arr2 is __NSArrayI

By looking at __NSSingleObjectArrayI.h you will notice that this class has a protected variable, called _object, that I guess it is where the single item of the array is stored.

@interface __NSSingleObjectArrayI : NSArray {
    id  _object;
}

As soon as you learn that single object arrays are treated differently, an important question arises: What is the necessity of having a separate type for array that has just one object? My first guess was Performance, either in terms of speed or memory usage, or maybe both. I tested my hypothesizes and I like to share the results with you. If you like to learn more about benchmarking in Objective-C/Swift, read “Benchmarking” by Mattt. To test my hypothesizes I utilizeed what I had learned from this article.

Hypothesis1: It is faster

I measured how much it takes to create an instance of NSArray. The varibale of this experiment was the number of items the array contains. Also, I was curious if specifing the type of array’s items make any difference. So I repeated experiments once again but this type specified the type of arrays’ items:

Case# Array Time
Case1 NSArray *arr = @[@(1)]; 2.62712e-05 s
Case2 NSArray<NSNumber *> *arr = @[@(1)]; 2.59094e-06 s
Case3 NSArray *arr = @[@(1),@(2)]; 1.91713e-06 s
Case4 NSArray<NSNumber *> *arr = @[@(1),@(2)]; 2.41189e-06 s
Case5 NSArray *arr = @[@(1),@(2),@(3),@(4),@(5),@(6),@(7),@(8)]; 2.89804e-06 s
Case6 NSArray<NSNumber *> *arr = @[@(1),@(2),@(3),@(4),@(5),@(6),@(7),@(8)]; 3.00701e-06 s

In general, create an instance of __NSSingleObjectArrayI is slower than __NSArrayI, and specifying the type of array’s items makes the creation faster. Then I measured the amount of time it takes to access the first item of an array. The varible was the type of array, but the access method was the same:

Case# Array Time
Case1 NSArray *arr = @[@(1)]; 4.65124 s
Case2 NSArray<NSNumber *> *arr = @[@(1)]; 4.61453 s
Case3 NSArray *arr = @[@(1),@(2)]; 4.11629 s
Case4 NSArray<NSNumber *> *arr = @[@(1),@(2)]; 4.04419 s
Case5 NSArray *arr = @[@(1),@(2),@(3),@(4),@(5),@(6),@(7),@(8)]; 4.05391 s
Case6 NSArray<NSNumber *> *arr = @[@(1),@(2),@(3),@(4),@(5),@(6),@(7),@(8)]; 4.09293 s

As you can see, accessing the first item of an array of __NSSingleObjectArrayI type is slower than __NSArrayI, and specifying the type of array’s items enables us to access the first item faster.

Hypothesis2: It reduces memory usage

I ran another experiment. This time in each test case, I created 1000 instance of array with N items and used Instruments to measure how much memory was used to store these instances. This time, the variable of the experiment was N: Here is the result:

Case# Array Memory
Case1 @[@(1)] 156,25 Kb
Case2 @[@(1),@(2)] 312,50 Kb
Case3 @[@(1),@(2),@(3)] 468,75 Kb
Case4 @[@(1),@(2),@(3),@(4)] 468,75 Kb
Case5 @[@(1),@(2),@(3),@(4),@(5)] 625,00 Kb
Case6 @[@(1),@(2),@(3),@(4),@(5),@(6)] 781,25 Kb

Even when we compare memory usage, there is no significant difference between these two (size of @[@(1),@(2)] x 2 = size of @[@(1)], that makes sense).

Conclusion

I don’t still understand the reason behind having a separate type for immutable arrays that have just one item, though I know now that neither is faster nor used less memory. On the other hand, I couldn’t find a use case where someone needs to create thousands instance of single object array so might this subtle difference become important for the developer. Maybe this is why no one has ever complained about that. The only case where I can image this difference might be important is where a JSON gets decoded. Because if a JSON array has just one item, the decoded type will be __NSSingleObjectArrayI. So if you are working on a program that its performance is critical and somewhere in this program you have to decode a huge JSON which has many single item arrays, you should be more careful.