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Re: Issues subclassing NSMutableArray

From: mickbert
Subject: Re: Issues subclassing NSMutableArray
Date: Mon, 24 Feb 2020 08:38:35 +0100
User-agent: Posteo Webmail

On 17.11.2019 16:53, Richard Frith-Macdonald wrote:
On 17 Nov 2019, at 15:08, Riccardo Mottola <address@hidden> wrote:

I wanted to subclass NSMutableArray, so that I can easily add some extra methods.
I declared my subclass like this:

@interface FileArray : NSMutableArray

However, when I run my app, I get this: Uncaught exception NSInvalidArgumentException, reason: [FileArray-addObject:] should be overridden by subclass

I did override all concrete methods in the easiest possible way, calling super. For addObject I did:

- (void)addObject:(id)anObject
  [super addObject:anObject];

why is this not enough or not working in any case?

Because NSMutable array is an abstract class.  You need to create a
concrete class with actual instance variables to store data in.

What you could have done to get the effect you seem to want is have a
concrete instance do the work for you:

@interface FileArray : NSMutableArray
  NSMutableArray        *content;
@implementation FileArray
- (void) addObject: (id)anObject
  [content addObject: anObject];
- (void) dealloc
  [content release];
  [super dealloc];
- (id) initWithCapacity: (NSUInteger)capacity
  content = [[NSMutableArray alloc] initWithCapacity: capacity];
  return self;


In this case the line
    content = [[NSMutableArray alloc] initWithCapacity: capacity];
will actually create a GSMutableArray instance (a concrete subclass)
that your class implementation can use to store the objects.

No, I do not understand. If NSMutableArray is an abstract class, how can I make the following call?

  [content addObject: anObject];

And if the call

  content = [[NSMutableArray alloc] initWithCapacity: capacity];

creates a GSMutableArray instance, why an hypothetical subclass shouldn't do the same?

  myMutableArray = [[MyMutableArray alloc] initWithCapacity: capacity];

The solution you proposed is surely an option. Bu If there is a class I can directly use (instantate an object), it should be possible to subclass it, just adding any new method, but keeping access to those of the superclass as they are, without the need of wrapping each.

Thanks for being patient with me: I am an expert C++ programmer, but with very few experience in Obj-C, even for a number of doubts like that one above.
Mick Bert

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