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
?
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: