Question

I have a core data stack with 2 entities: ‘Client’ and ‘Car’. It is a one to may relationship. I have two tableViewControllers to display firstly the clients, and then once a client has been chosen a list of that clients cars.

I am currently using the following code fragment in my second tableViewController viewDidLoad method…

NSSet *cars = client.cars;
carsArray = [cars allObjects];

carsArray is a NSArray and is declared and synthesized in the second tableViewCotroller. If I try and make carsArray an NSMutableArray (so I can edit it later in the event a car is deleted) I get the following compiler warning…

Incompatible Objective-C types 'struct NSArray *', expected 'struct NSMutableArray *' when passing argument 1 of 'setCarsArray:' from distinct Objective-C type

Could someone explain to me why this works with a NSArray but not an NSMutableArray please?

Many thanks

Answer 1

allObjects method returns NSArray * and if you simply try to assign it to an NSMutableArray property you are not converting the object itself.

You need to use +[NSMutableArray arrayWithArray:] or a more generic method -[NSObject mutableCopy]:

[self setCarsArray:[[cars allObjects] mutableCopy]];

This will be the case for all mutable classes in Objective-C: to create a mutable object from immutable you’ll have to explicitly call the conversion methods.