How to declare an immutable property backed by a mutable type?

Question

I’d like to declare a public immutable property:

@interface Foo
@property(strong, readonly) NSSet *items;
@end

…backed with a mutable type

Answer 1

The easiest solution is

@interface Foo {
@private
    NSMutableSet* _items;
}

@property (readonly) NSSet* items;

and then just

@synthesize items = _items;

inside your implementation file. Then you can access the mutable set through _items but the public interface will be an immutable set.