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

You could just do it yourself in your implementation:

@interface Foo : NSObject
    @property(nonatomic, retain) NSArray *anArray;
@end

--- implementation file ---

@interface Foo()
{
    NSMutableArray *_anArray;
}
@end

@implementation Foo

- (NSArray *)anArray
{
    return _anArray;
}

- (void)setAnArray:(NSArray *)inArray
{
     if ( _anArray == inArray )
     {
         return;
     }
     [_anArray release];
     _anArray = [inArray retain];
}

- (NSMutableArray *)mutablePrivateAnArray
{
    return _anArray;
}

@end

Answer 2

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.

Answer 3

You have to declare the property in the public interface as readonly in order the compiler not to complain. Like this:

Word.h

#import <Foundation/Foundation.h>

@interface Word : NSObject
@property (nonatomic, strong, readonly) NSArray *tags;
@end

Word.m

#import "Word.h"

@interface Word()
@property (nonatomic, strong) NSMutableArray *tags;
@end

@implementation Word
@end