OK to use underscore for autosynthesized properties in block (strong reference cycle)

michaelsnowden Published at Dev

michaelsnowden

Say I have something like this:

@interface Foo : UIViewController

@property (nonatomic, strong) NSString *name;

@end

@implementation Foo

- (void)viewDidLoad
{
    [super viewDidLoad];
    _name = @"Me";
    NSArray *blackList = @[@"John", @"Malcolm", @"Jimmy"];
    [blackList enumerateObjectsUsingBlock:^(id obj, NSUInteger idx, BOOL *stop) {
        if ([obj isEqualToString:_name])  // Is _name just as bad as using self.name?
        {
            NSLog(@"You're on the blacklist!");
            *stop = YES;
        }
    }];
}

@end

The question is on this line: if ([obj isEqualToString:_name])

I know it's bad to refer to self inside a block, and you can get around it by using __weak Foo *weakSelf = self and then refer to weakSelf in the block.

__weak Foo *weakSelf = self;
[blackList enumerateObjectsUsingBlock:^(id obj, NSUInteger idx, BOOL *stop) {
    if ([obj isEqualToString:weakSelf.name])
    {
        NSLog(@"You're on the blacklist!");
        *stop = YES;
    }
}];

However, is this still bad if we don't explicitly write self.name, but write _name?

Sergey Kalinichenko

Is _name just as bad as using self.name?

Yes, it is. In fact, self reference is still there - the compiler inserts it for you.

you can get around it by using __weak Foo *weakSelf = self and then refer to weakSelf in the block.

You can use the same trick here by referring to weakSelf->_name

Note: It is not necessary to use this trick with enumerateObjectsUsingBlock:, because the enumeration finishes and the block gets released in the scope of your method. A threat of retain cycle happens only when you put the block in a variable on the self object, while also referring to self from the block.

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