Why is my Objective-C class instance variable not being set?

Question

this is a noob question but I cannot for the life of me figure out why my MSMutableArray class variable is not being set. Below is my code:

@interface MyClass : NSObject {
   NSMutableArray *imageArr;
}

@implementation MyClass
-(id)init
{
   self = [super init];
   if (self) {
      imageArr = [[NSMutableArray alloc] init]; 
   }
   return self;
}

- (void) checkImageArr {
   NSLog(@"imageArr size::%@",[imageArr count]);
}

When this code runs, the following is set to the log:

imageArr size::(null)

Why is that variable not being set? I looked at the following question and don't see anything different between my code and the accepted answer.

Thanks.

JeremyP · Accepted Answer · 2012-04-04 14:55:09Z

The %@ specifier expects the argument to be an id (or pointer to an object). -count returns an NSUInteger which is not an object. Since your array is empty, the count is zero and so the argument is being interpreted as a nil object, which comes out as (null) when used with the %@ specifier.

If the argument was not nil, -description would be sent to it to get a string to insert in the log message. So, if you add an object to your array, NSLog will try to interpret 1 as an object pointer and send -description to it. This will cause an EXC_BAD_ACCESS exception (try it!).

You need a format specifier that interprets the argument as a number. NSUinteger has the following definition

#if __LP64__ || TARGET_OS_EMBEDDED || TARGET_OS_IPHONE || TARGET_OS_WIN32 || NS_BUILD_32_LIKE_64
typedef unsigned long NSUInteger;
#else
typedef unsigned int NSUInteger;
#endif

so it's probably an unsigned long. For safety, I always cast it to make sure, so you need this:

NSLog(@"imageArr size::%lu", (unsigned long)[imageArr count]);

Extra Savoir-Faire · Accepted Answer · 2012-04-04 14:41:11Z

1

Change %@ to %lu and you'll see.

answered Apr 4, 2012 at 14:41

Extra Savoir-Faire

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Comments

Felix Lamouroux · Accepted Answer · 2012-04-04 14:41:28Z

1

The count method returns an NSUInteger.

Try this log format instead:

NSLog(@"imageArr size::%u",[imageArr count]);

answered Apr 4, 2012 at 14:41

Felix Lamouroux

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1 Comment

JeremyP Over a year ago

It's a good idea to put a cast in because NSUInteger is not guaranteed to be an unsigned int

Nikita Ilyasov · Accepted Answer · 2012-04-04 14:44:00Z

0

NSLog(@"imageArr size::%@",[imageArr count]);

You should use %@ when you want to "write an object" AFAIK, it is like write [object description]. [imageArr count] is a int, right? It is not a pointer to an object.

You should use %i instead of %@. And size will be 0.

Try NSLog(@"imageArr size::%@",imageArr); if you want to write a address

answered Apr 4, 2012 at 14:44

Nikita Ilyasov

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1 Comment

JeremyP Over a year ago

NSUInteger is unsigned long on the iPhone so %i is wrong.

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