How do I extend an NSArray?

Question

Here's my try:

H file:

@interface Strings : NSArray
@end

M file:

@implementation Strings
- (id) init
{
    [self initWithObjects:
     @"One.",
     nil];
    return self;
}
@end

When I run I get this:

'NSInvalidArgumentException', reason: '* -[NSArray initWithObjects:count:]: method only defined for abstract class. Define -[Strings initWithObjects:count:]!'

This is what I did instead:

H file:

@interface Strings : NSObject
+ (NSArray*) getStrings;
@end

M file:

@implementation Strings
+ (NSArray*) getStrings
{
    NSArray* strings = [[NSArray alloc] initWithObjects:
     @"One.",
     nil];
    return strings;
}
@end

I thought I can extend NSArray but it crashes even it compiles. I end up writing a static function like yours inside a base class of all my models. And since it's static function, i can use `[[self alloc] init]` to create items and append to array. — John Pang, Sep 22 '18 at 04:28

score 8 · Accepted Answer · answered Jan 22 '12 at 20:54

NSArray is a class cluster (link to Apple's documentation). This means that when you try to create an NSArray, the system creates some private subclass of NSArray. The NSArray class just defines an interface; subclasses of NSArray provide implementations of the interface.

You can write your own subclass of NSArray, but you have to provide your own storage for the objects in the array. You have to initialize that storage yourself. The error message is telling you this, by saying that you need to override initWithObjects:count: in your subclass. Your override needs to put the objects into whatever storage you allocate as part of your class implementation.

The NSArray implementation of the variadic initWithObjects: method is just a wrapper around initWithObjects:count:, so you don't have to implement initWithObjects:.

Also, make sure you read and **understand** the "Subclassing Notes" section of the *[NSArray Class Reference](http://developer.apple.com/library/ios/documentation/Cocoa/Reference/Foundation/Classes/NSArray_Class/NSArray.html#//apple_ref/doc/uid/20000137-740795)* before writing your own `NSArray` subclass. — rob mayoff, Jan 22 '12 at 20:59

DarkDust · Answer 2 · 2012-01-22T21:07:59.367

Deriving from NSArray is something you should avoid. From the documentation:

Remember that NSArray is the public interface for a class cluster and what this entails for your subclass. The primitive methods of NSArray do not include any designated initializers. This means that you must provide the storage for your subclass and implement the primitive methods that directly act on that storage.

What this means is that when you initialize an array, you don't get an instance of NSArray. You'll get an instance of a totally different class that merely has the same interface. That is why subclassing doesn't work the way you think it works: you'll have to completely implement the storage yourself. This is why the documentation further states:

Any subclass of NSArray must override the primitive instance methods count and objectAtIndex:. These methods must operate on the backing store that you provide for the elements of the collection. For this backing store you can use a static array, a standard NSArray object, or some other data type or mechanism. You may also choose to override, partially or fully, any other NSArray method for which you want to provide an alternative implementation.

Last but not least you would have had the initializing wrong anyway. You would have needed to call super:

- (id)init
{
    self = [super initWithObjects:@"One", @"Two", nil];
    if (!self) return nil;

    return self;
}

But as I just said, it just doesn't work that easily. You'll get the same exception again. So you should simply avoid doing deriving from NSArray.

What you can do is add a category to add methods to all NSArray instances.

score 2 · Answer 3 · edited May 23 '17 at 12:24

2

NSArray doesn't support being subclassed in this way. You can add a category, though, although that's not universally recommended.

See Objective C - Subclassing NSArray for more thoughts.

edited May 23 '17 at 12:24

Community

1
1

answered Jan 22 '12 at 20:48

StilesCrisis

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I changed to self = [self initWithObjects: but that didn't help. – Roger C S Wernersson Jan 22 '12 at 20:50
I guess it could be that [NSArray alloc] doesn't actually allocate an NSArray so when I do, there is no implementation. Help! But why does it go on about initWithObjects:count: ? – Roger C S Wernersson Jan 22 '12 at 20:52
Why do you think you need to subclass NSArray anyway? Unless you are making a specialized backing store, it's generally not a good approach. – StilesCrisis Jan 22 '12 at 20:53

score 1 · Answer 4 · answered Jan 22 '12 at 20:49

1

perhaps

self = [super initWithObjects:...];

answered Jan 22 '12 at 20:49

Michael Krelin - hacker

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That's true, but subclassing NSArray in specific also has special rules. – StilesCrisis Jan 22 '12 at 20:54
True. Well, I'll leave the answer nonetheless. First things first. – Michael Krelin - hacker Jan 22 '12 at 20:56

score 1 · Answer 5 · answered Jan 22 '12 at 20:51

1

You need to assign self, and call your superclass' init method.

if (self = [super initWithObjects:...]) {
   ...
}

return self;

answered Jan 22 '12 at 20:51

Graham Perks

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That's true, but subclassing NSArray in specific also has special rules. – StilesCrisis Jan 22 '12 at 20:54

How do I extend an NSArray?

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