A thread safe set written in Objective-C

SafeSet.m

#import <Foundation/Foundation.h>

@interface SafeSet : NSObject {
    NSMutableSet *set;
    dispatch_queue_t queue;
}
@end

@implementation SafeSet

- (id)init {
    self = [super init];
    set = [NSMutableSet new];
    queue = dispatch_queue_create("Safe Set", NULL);
    return self;
}

- (void)dealloc {
    dispatch_release(queue);
}

- (void)add:(id)val {
    dispatch_async(queue, ^{
        [set addObject:val];
    });
}

- (void)remove:(id)val {
    dispatch_async(queue, ^{
        [set removeObject:val];
    });
}

- (BOOL)contains:(id)val {
    __block BOOL result;
    dispatch_sync(queue, ^{
        result = [set containsObject:val];
    });
    return result;
}
@end


int main(void) {
    @autoreleasepool {
        SafeSet *set = [SafeSet new];
        [set add:@"First"];
        [set add:@"Second"];
        [set add:@"Third"];
        BOOL result = [set contains:@"Third"];
        printf("%s\n", result ? "True" : "False");
    }
}