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iOS "Unrecognized selector" error when attempting to remove an object while in a for loop

M

moonman239

Cancelled
Original poster
Mar 27, 2009
1,541
32
Here's the deal. I'm trying to have the app remove an object from a mutable array while it is looping through another array. The object to be removed and the array that the object is to be removed from are both defined. However, I just got the following error message:

Code: 
Terminating app due to uncaught exception 'NSInvalidArgumentException', reason:
'-[__NSArrayI removeObject:]: unrecognized selector sent to instance 0x95e6160
 
moonman239 said:
Here's the deal. I'm trying to have the app remove an object from a mutable array while it is looping through another array. The object to be removed and the array that the object is to be removed from are both defined. However, I just got the following error message:

Code: 
Terminating app due to uncaught exception 'NSInvalidArgumentException', reason:
'-[__NSArrayI removeObject:]: unrecognized selector sent to instance 0x95e6160
Click to expand...

Sounds like the array is not mutable. How are you creating it? Note that if you save a mutable array to a plist or use NSCoding to save it, when you read it back, it's immutable.
 
moonman239 said:
Here's the deal. I'm trying to have the app remove an object from a mutable array while it is looping through another array. The object to be removed and the array that the object is to be removed from are both defined. However, I just got the following error message:

Code: 
Terminating app due to uncaught exception 'NSInvalidArgumentException', reason:
'-[__NSArrayI removeObject:]: unrecognized selector sent to instance 0x95e6160
Click to expand...

You don't have a mutable array. Show us your code from the point where the array is created until you attempt to remove an object from it.
 
Here's what my code looks like

Code: 
NSMutableArray *array1 = (NSMutableArray *)[array2 arrayByAddingObject:object];
for (object in array3)
{
[array1 removeObject:definedObject];
}


The app crashes before it runs out of objects in array1.
 
NSArray's arrayByAddingObject: returns an NSArray. Just because you typecast it as a NSMutableArray does not make it mutable. I would look into using mutableCopy.
 
Update: After reading the Stack Overflow page entitled "Error: Mutating method sent to immutable object for NSMutableArray from JSON file," I figured out that simply casting an NSArray to an NSMutableArray does not produce an array that can be changed. I have to create a mutable copy of the immutable array and assign that array to the mutable array I mentioned in the post above.

Edit:
dejo said:
NSArray's arrayByAddingObject: returns an NSArray. Just because you typecast it as a NSMutableArray does not make it mutable. I would look into using mutableCopy.
Click to expand...

Thanks. I did not notice your post until now.
 
dejo said:
NSMutableArray doesn't have its own arrayWithArray: method, so that would still call the superclass NSArray's method (of which NSMutableArray is a subclass) and so the return value is still an (immutable) NSArray.
Click to expand...
This.

Also I suggest not using fast enumeration to add and/or remove objects.
 
dejo said:
NSMutableArray doesn't have its own arrayWithArray: method, so that would still call the superclass NSArray's method (of which NSMutableArray is a subclass) and so the return value is still an (immutable) NSArray.
Click to expand...

I have found multiple sources that state using arrayWithArray: to get a NSMutableArray from a NSArray. I feel like I have also used it before with fine results. Am I doing something wrong here? I went back and read documentation and it seems you are right that arrayWithArray is just a method from the superclass, but I still feel like it worked for me in the past.
 
waterskier2007 said:
I have found multiple sources that state using arrayWithArray: to get a NSMutableArray from a NSArray. I feel like I have also used it before with fine results. Am I doing something wrong here? I went back and read documentation and it seems you are right that arrayWithArray is just a method from the superclass, but I still feel like it worked for me in the past.
Click to expand...

Huh. Interesting. Well, I might be wrong here. A simple code test should be all it takes to verify whether that works or not. I'll let you know what I find.
 
arrayWithArray: should return an NSMutableArray, I believe, when invoked on an NSMutableArray. The method signature says that it returns an NSArray, which an NSMutableArray qualifies as.

No, I didn't actually take the 5 minutes to set up a test for this.
 
dejo said:
Huh. Interesting. Well, I might be wrong here. A simple code test should be all it takes to verify whether that works or not. I'll let you know what I find.
Click to expand...

I just tested it, and the NSMutableArray version of arrayWithArray does create a new mutable array.

The NSArray class reference defines NSArray arrayWithArray: as

Code: 
+ (instancetype)arrayWithArray:(NSArray *)anArray

That implies, in a very ambiguous fashion, that you get back an object of the type you created.
 
dejo said:
Thanks for the update, Duncan. I guess that settles it. My apologies for possibly leading the OP astray.
Click to expand...

The docs for NSMutableaArray should list a mutable array variant of the arrayWithArray method.

----------
Duncan C said:
I just tested it, and the NSMutableArray version of arrayWithArray does create a new mutable array.

The NSArray class reference defines NSArray arrayWithArray: as

Code: 
+ (instancetype)arrayWithArray:(NSArray *)anArray

That implies, in a very ambiguous fashion, that you get back an object of the type you created.
Click to expand...


What does the type "instancetype" mean, anyway? Type of the target?
 
Duncan C said:
What does the type "instancetype" mean, anyway? Type of the target?
Click to expand...

yes, as far as I recall instancetype means type of the target, and is used a lot in custom init methods instead of specifying -(id) return type

Does this then mean that if you invoke it with [NSMutableArray arrayWithArray:someArray] it returns the type of NSMutableArray

EDIT: What is interesting is that in the class reference for NSArray, under the method +(instancetype)array there is a note that states

Discussion

This method is used by mutable subclasses of NSArray.
Click to expand...

However that is not present for +(instancetype)arrayWithArray
 
waterskier2007 said:
yes, as far as I recall instancetype means type of the target, and is used a lot in custom init methods instead of specifying -(id) return type

Does this then mean that if you invoke it with [NSMutableArray arrayWithArray:someArray] it returns the type of NSMutableArray

EDIT: What is interesting is that in the class reference for NSArray, under the method +(instancetype)array there is a note that states



However that is not present for +(instancetype)arrayWithArray
Click to expand...


Yes, when used like this:

Code: 
  NSArray *anArray = @[@"one", @"two", @"three"];
  NSMutableArray *aMut = [NSMutableArray arrayWithArray: anArray];

The result is a mutable array.
 
waterskier2007 said:
yes, as far as I recall instancetype means type of the target, and is used a lot in custom init methods instead of specifying -(id) return type

Does this then mean that if you invoke it with [NSMutableArray arrayWithArray:someArray] it returns the type of NSMutableArray

EDIT: What is interesting is that in the class reference for NSArray, under the method +(instancetype)array there is a note that states



However that is not present for +(instancetype)arrayWithArray
Click to expand...
Please report the doc-error to Apple, using the on-page feedback.

Doc bugs are bugs. Report them so they can be fixed.
 
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