Immutable NSMutableArray

Question

On my viewDidLoad I load from a webservice the first 5 items of an array and put them on a NSMutableArray of mine. Also on this View I have UITableViewController that displays the items of my NSMutableArray with the contents from the webservice. When I scroll to the last item of the UITableView I am loading the 5 next items from the webservice and add them to my NSMutableArray and reloading the table data in order to display all the 10 items.

The problem is that when I try to add to my existing NSMutableArray the 5 new objects from the webservice the program crashes with the message: Exception: -[__NSCFArray insertObject:atIndex:]: mutating method sent to immutable object

The way I am accessing the NSMutableArray in cellForRowAtIndexPath: is like this:

cell.textLabel.text = [[self.myMutableArray objectAtIndex:indexPath.row]objectForKey:@"name"];

Any idea on what it could be?

Can you paste the declaration of your myMutableArray property? — Francesco
– Francesco, Commented Jun 21, 2012 at 14:48
My guess is that you don't have an NSMutableArray, but a plain old NSArray. Just because the pointer is named "mutable" does not make the object mutable. — Hot Licks
– Hot Licks, Commented Jun 21, 2012 at 14:48

Rob Napier · Accepted Answer · 2012-06-21 14:52:48Z

3

First, look for warnings in your compiler output. You likely have are assigning an NSArray to an NSMutableArray variable. This will often generate a warning. Your build should have no warnings in it. This is critical for ObjC code (many things you would think of as "errors" are only warnings in ObjC). If you're ignoring warnings, then you're going to have a lot of problems.

You can have this situation without creating a warning if something is cast to id. For example, if you fetch your "NSMutableArray" (really an NSArray) from an array or dictionary, you won't get a warning about the type mismatch. Look for these situations. Basically, audit each point you assign myMutableArray and make sure that you're actually assigning a mutable array.

answered Jun 21, 2012 at 14:52

Rob Napier

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1 Comment

Lucas Pereira Over a year ago

I was setting the result of the webservice call directly to the nsmutablearray. Now I allocated and initialized it first and then added the objects one by one, and it worked fine. Thanks a lot! :)

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