Confusion around NSMutableArray and NSArray

I have the following code:

NSLog(@"items: %d", [items count]);
NSLog(@"allObjects: %d", [self.allObjects count]);

[self.allObjects addObjectsFromArray:items];

NSLog(@"allObjects: %d", [self.allObjects count]);

Wh开发者_如何学编程ich produces the following output:

items: 7
allObjects: 0
allObjects: 0

items is a NSArray

allObjects is a NSMutableArray.

I'm confused as to why the objects in items are not being added to allObjects. I expect it's something simple, but there is a hole in my understanding here.

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Let me guess. self.allObjects is nil. Sending messages to nil is a no-op.