Objective C _underscore vs self.variable

Question

I know this question has been asked before but possibly not in the same context. My question is that I have a singleton class that is only dipatched_once in the lifetime of the app. In the class I have some methods that are accessing the instance variables and acting upon if they have been set on the instance. This is example of how I am attempting to access them:

// .m file:

Interface:

@property (nonatomic, assign, readwrite) BOOL userLoggedIn;

Implementation: // method:

-(void)someMethod{ if(!_userLoggedIn){ } else { } }

I know I can also use self to evaluate the value like this:

-(void)someMethod{ if(self.userLoggedIn){ } else { } }

Wondering which is correct way of accessing the value? I am not synthesizing the properties since they are all declared in the interface only in the .m file. Thanks for the help!

It depends, but if you have -(BOOL)userLoggedIn{} implemented, that could change the behavior. — Larme
– Larme, Commented Jan 24, 2017 at 19:06
Possible duplicate of iOS: Usage of self and underscore(_) with variable — holex
– holex, Commented Jan 24, 2017 at 19:12
If userLoggedIn is only defined for use in the implementation and you are using direct access to the variable, is there a reason you are declaring a property at all? Why not just use an instance variable (as in @implementation { BOOL userLoggedIn; } ...? — CRD
– CRD, Commented Jan 24, 2017 at 22:22

bbum · Accepted Answer · 2017-01-24 18:57:51Z

It depends.

Do you want the accessor invoked or not? _variable is direct access. self.variable invokes -variable, which is automatically synthesized by the compiler.

The former does not trigger KVO when the value changes. The latter does. That may be a feature or anti-feature.

But, whichever you choose, especially for write operations, make it consistent or else you'll be tracking down bugs in the future.

A general rule:

access directly in -init/-dealloc
access through setter/getter (dot syntax) everywhere else

Note also that direct access will not respect atomic.

@randomorb2110 _variable = 10; directly sets the instance variable. self.variable = 10; translates to [self setVariable:10];. One is a direct write, one calls the setter method.

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