Question

Possible Duplicate:
How does an underscore in front of a variable in a cocoa objective-c class work?

I have been very confused with using self or underscore with variable name after synthesizing it like below:

In .h file:
@property(nonatomic, strong) NSMutableArray *users;

In .m file:
@synthesize users = _users;

Based on my understandings when I use self.users, OS will make sure to release previously allocated memory in set method so we don’t need to take care explicitly.

_users is an instance variable for users and should be normally used while accessing the users variable. If I use _users to change its value then it won’t fire KVO delegate which will not notify a class observing users value change.

Moreover, self.users allows differentiating dummy variable in the method name like below,

- (void)assignUsers:(NSMutableArray*)users {
      self.users = users;
}

Could someone please tell me if there is anything that I understood wrong or missing while using _users or self.users?

Answer 1

I think it helps to consider how properties are (or might be) implemented by the compiler.

When you write self.users = array; the compiler translates this to [self setUsers:array];
When you write array = self.users; the compiler translates this to array = [self users];

@synthesize adds an ivar to your object (unless you added it yourself), and implements the -users and -setUsers: accessor methods for you (unless you provide your own)

If you’re using ARC, -setUsers: will look something like:

- (void)setUsers:(NSArray *)users
{
    _users = users; // ARC takes care of retaining and release the _users ivar
}

If you’re using MRC (i.e. ARC is not enabled), -setUsers: will look something like*:

- (void)setUsers:(NSArray *)users
{
    [users retain];
    [_users release];
    _users = users;
}

* – Note that this is a simplified, nonatomic implementation of -setUsers: