iOS: Usage of self and underscore(_) with variable
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:
when you are using the self.users
, you access the property via the setter or getter.
when you are using the _users
, you access the property directly skip the setter or getter.
here is a good demonstration of it:
- (void)setUsers:(id)users {
self.users = users; // WRONG : it causes infinite loop (and crash), because inside the setter you are trying to reach the property via setter
}
and
- (void)setUsers:(id)users {
_users = users; // GOOD : set your property correctly
}
this is the point in the case of the getter as well.
about the basic memory management (in case of MRR
or ARC
): the iOS will dealloc the object if there is no more strong pointer which keeps it alive, no matter how you release the pointer of the objects.