ObjectiveC: where to declare private instance properties?

Question

I have the following class interface:

@interface MyClass : NSObject

@property int publicProperty;

@end

then the implementation:

@interface MyClass() // class extension

- (void)privateMethod; // private methods

@end

@implementation MyClass {
    int _privateProperty;
}

@property int privateProperty = _privateProperty;

@end

this is what the Apple guy showed in WWDC, but is there any reason for NOT putting _privateProperty in class extension like:

@interface MyClass() // class extension
{
    int _privateProperty;
}

- (void)privateMethod; // private methods

@end

Thanks!

You might find many good answers to your question here: http://stackoverflow.com/search?q=private+property — Monolo, Jul 03 '12 at 08:33
possible duplicate of [How to make a private property?](http://stackoverflow.com/questions/2768750/how-to-make-a-private-property) — Monolo, Jul 03 '12 at 08:34

score 12 · Answer 1 · answered Jul 03 '12 at 13:27

I usually "force" private with an extension in the implementation

In your header

@interface MyClass : NSObject
{
}

@property (nonatomic, assign) int publicProperty;

@end

In your implementation file:

@interface MyClass ()
@property (nonatomic, assign) int privateProperty;
@end


@implementation MyClass
@synthesize privateProperty;
@synthesize publicProperty;

@end

score 9 · Accepted Answer · edited Mar 17 '20 at 10:19

You dont have to declare your ivars in both the interface and the implementation.Because you want to make them private you can just declared them in the implementation file like so:

@implementation {

int firstVariable;
int secondVariable;
...
}
//properties and code for  your methods

If you wanted to, you can then create getter and setter methods so that you can access those variables.

The person you spoke to was right, though there is not any reason why you would NOT declare them the same way in the interface. Some books actually teach you that the @interface shows the public face of the class and what you have in the implementation will be private.

score 0 · Answer 3 · answered Jul 03 '12 at 04:12

0

Do you mean you want to declare private instance variables?

You can do this:

@interface MyClass()
{
 @private //makes the following ivar private
   int _privateProperty;
}

answered Jul 03 '12 at 04:12

Anna Fortuna

1,071
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Yes but I don't want to make it ivar, it is just a property which can be accessed by self.privateProperty using accessor, and not exposed in @interface. – hzxu Jul 03 '12 at 04:14
You can't do it by this `@property int privateProperty = _privateProperty;`, you have to do it in the implementation file. – Anna Fortuna Jul 03 '12 at 04:16

score 0 · Answer 4 · answered Jul 03 '12 at 08:22

With the "modern runtime" (64-bit MacOS post-10.5 and all versions of iOS), you don't need to declare instance variables at all.

// MyClass.h
@interface MyClass : NSObject

@property int publicProperty;

@end


// MyClass.m
@implementation MyClass

@synthesize publicProperty = _privateProperty;  // int _privateProperty is automatically synthesized for you.

@end

ObjectiveC: where to declare private instance properties?

4 Answers4