life time of objects in NSMutableArray

Question

There are two classes, let's call them Class A and Class B.

Also, in class B, there's a NSMutableArray called arrayB.

@interface A {
   // for reference functions in B
   B *obj;
}
@property (assign) B *obj;
- (void) addObject: (id) objectB;

And I access B by calling:

obj = [[B alloc] init];

Here's definition of B:

@interface B {
  NSMutableArray *arrayB;
}
@property (assign) NSMutableArray *arrayB;
- (void) printArray;

Now, I tried to add objects to arrayB by functions of A:

[self addObject: objectB]

- (void) addObject: (id) objectB {
   [obj.arrayB addObject:objectB];
   NSLog(@"%@", obj.arrayB);
   // here prints arrayB with objectB
}

However when I tried to access the objects again in B:

[self.obj printArray];

- (void) printArray {
   NSLog(@"%@", arrayB);
   // now it prints an empty array
}

I have no clue what happened here...

Objects will be auto-retained by NSMutableArray's addObject, don't they?

Why it becomes empty every time? >_<

Please help me, thanks!

Answer 1

It appears that you never initialize the NSMutableArray in your B object, so your -addObject calls are actually:

[nil addObject:myObject];

which in Objective-C is pretty much the same thing as doing nothing at all.

Answer 2

It's hard to tell where the bug is with only tiny, context-free snippets of code. But most likely these you're creating two different B objects.

Answer 3

NSMutableArray will retain any object you add with addObject, your problem is somewhere else.

NSObject *someObject = [[NSObject alloc] init]; [someMutableNSArray addObject:someObject]; [someObject release];

the only reference to someObject is now in the array.