Question

Why must I define variables twice in the header file? What differences are there between these variables?

The first definition is here:

@interface MyController: UIViewController
{
    NSInteger selectedIndex;
}

The second definition is here:

@property (nonatomic) NSInteger selectedIndex;

Answer 1

What you’re seeing was required in earlier versions of Objective-C,
but isn’t any more.

In the first versions of Objective-C used by NeXT up until the new
runtime was introduced (with Objective-C 2.0 on Mac OS X), all
instance variables had to be declared as part of the class’s structure
in its @interface. The reason was that if you subclassed a class,
the compiler needed to know the instance variable layout of the class
so it could see at what offset to put the subclass’s instance
variables.

When properties were introduced, synthesized properties had to be
“backed” by an instance variable in the class’s structure. Therefore
you had to declare both an instance variable and the property.

All of the above is no longer true. Newer Objective-C is less fragile
in the way it looks up instance variable offsets, which has meant a
few changes:

• not all instance variables need to be in the @interface. They can now be defined in the @implementation: though not in categories due
  to the possibilities of clashing and other issues.
• instance variables for synthesized properties can be inferred and created based on the property definition.
• you can programmatically add instance variables to classes you’re creating at runtime (only before you’ve registered the class as
  available to the system).

So, to reiterate, you only needed to declare both the instance
variable and a synthesized property in older versions of the
Objective-C language. What you’re seeing is redundant and should not
be considered a “best practice”.

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