CODEWITHSUNDEEP
objective-c
user1283776
user1283776

Reputation: 21814

Why do I need to use the @property keyword in objective-c?

Say that I have the following public methods on my class:

- (BOOL)finished
{
    return _finished;
}
- (void)setFinished:(BOOL)aValue
{
    _finished = aValue;
}

Do I also need to set finished as a property on the class:

@property SomeClass *finished;

I may have misunderstood @property, but to me it seems like I am repeating myself.

Upvotes: 5

Views: 453

Answers (2)

Amin Negm-Awad
Amin Negm-Awad

Reputation: 16660

Even there is an accepted answer, I think some information and clarification has to be added.

A. @property does not synthesize anything. It is a method declaration. Period. This is the reason why it is allowed in protocols. (Method implementations are not allowed, not even possible in protocols.)

What let people think that is the fact, that if you do not implement alls required accessors manually, the compiler implicitly assumes a @synthesize in the implementation and that synthesized the accessors.

But, yes, an implicit or explicit @synthesize requires @property. (But not the other way round!)

B. Using @property you have to declare an ivar, too, if you explicitly define the required accessors. This is, because the implicit synthesize is "turned-off" in such a case (for good reasons).

C. Beside convenience there is a real advantage of @property: It adds semantic information to the property. Beside bundling getter and setter (what is the getter for -setFinished:? -isFinished? -finished) it holds information about copying and reference strength (weak, strong, copy). Atomicity is less important, because in most cases atomicity of access does not help to solve problems of concurrent execution.

Upvotes: 2

A O
A O

Reputation: 5698

By declaring a @property, the LLVM compiler will automatically "synthesize" your accessor methods (getter and setter), as well as an instance variable (ivar), denoted by an underscore prefix (_finished, for example)

In your case there, because you aren't doing anything special with your accessors (you are just directly reading/writing to the backing ivar), simply declaring a @property will do the trick. You won't need to write the -finished and -setFinished methods

If you do not declare @property, you'll have to declare your ivar

@property also indicates you are working with a member variable of an object, and as user @Sulthan says in the comment, each class holds a list of @properties that can be searched

And another good point by the same user, @property makes your accessor methods atomic, which protects your ivar from being read while it's being written

Upvotes: 6

Related Questions