Why are there private instance variables in Cocoa headers?

Question

There are multiple examples of @private instance variables all over the Foundation, UIKit and other framework headers. Here are some examples:

//CLLocationManager.h
@interface CLLocationManager : NSObject
{
@private
    id _internal;
}
<...>

//NSAutoreleasePool.h
@interface NSAutoreleasePool : NSObject {
@private
    void    *_token;
    void    *_reserved3;
    void    *_reserved2;
    void    *_reserved;
}
<...>

etc.

These instance variables are @private, so it's not possible to access them from anywhere except the class itself. What is the point of exposing them in the headers?

So why not

//CLLocationManager.m
@interface CLLocationManager<>
{
    id _internal;
}

Or even, considering modern Objective-C syntax:

//CLLocationManager.m
@interface CLLocationManager<>

@property(nonatomic, assign) id internal;

Why did apple used private instance variables in headers, exposing the internals of the class instead of hiding them in the implementation?

iain · Accepted Answer · 2014-06-25 15:07:28Z

2

Changes Apple have made over the years are to the API (Application Program Interface) but removing members from structures and classes changes the ABI (Application Binary Interface) which cannot be done without forcing all programs in existence to be recompiled or else they will crash.

The void *_reserved members in fact are very likely to be old deprecated symbols that have been removed from the API, but removing them from the class definition would have changed the ABI, and so the space they took up has been padded with void *.

answered Jun 25, 2014 at 15:07

iain

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1 Comment

Grady Player Over a year ago

your use of ABI isn't specifically correct, they are both the API in your usage... the API is more flexible now that there is a more flexible ABI...

gnasher729 · Accepted Answer · 2014-06-25 10:48:11Z

0

Many of these headers have been created in ancient times, when instance variables had to be declared in a header. And you don't want to change working code unless there is a good reason to do so - after all, it all works fine and you are just curious.

Apple cannot just update the header files; they would have to update the implementation as well, which means a huge amount of testing (or do you think you could make that change in dozens of files without making any mistakes? )

answered Jun 25, 2014 at 10:48

gnasher729

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1 Comment

Artem Abramov Over a year ago

Apple changes working code. If you're developing long enough for iOS/Mac you notice that multiple methods are getting deprecated, new properties are introduces, etc. Just for example, in the recent beta in NSArray.h - (NSUInteger)count; was replaced with @property (readonly) NSUInteger count;.

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