ios - Why must I define variables twice in the Header file? -

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why must define variables twice in header file? differences there between these variables?

the first definition here:

@interface mycontroller: uiviewcontroller {     nsinteger selectedindex; }

the second definition here:

@property (nonatomic) nsinteger selectedindex;

what you're seeing required in earlier versions of objective-c, isn't more.

in first versions of objective-c used next until new runtime introduced (with objective-c 2.0 on mac os x), instance variables had declared part of class's structure in @interface. reason if subclassed class, compiler needed know instance variable layout of class see @ offset put subclass's instance variables.

when properties introduced, synthesized properties had "backed" instance variable in class's structure. therefore had declare both instance variable , property.

all of above no longer true. newer objective-c less fragile in way looks instance variable offsets, has meant few changes:

  • not instance variables need in @interface. can defined in @implementation: though not in categories due possibilities of clashing , other issues.
  • instance variables synthesized properties can inferred , created based on property definition.
  • you can programmatically add instance variables classes you're creating @ runtime (only before you've registered class available system).

so, reiterate, needed declare both instance variable , synthesized property in older versions of objective-c language. you're seeing redundant , should not considered "best practice".

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