How to declare instance variables and methods not visible or usable outside of the class instance?

Question

I\'ve looked through a bunch of posts on this subject. Maybe I didn\'t run across \"the one\" and someone will point me in that direction. The question is simple and proba

Answer 1

Use class extensions to add to a class in your implementation file. A class extension is basically an unnamed category with a few bonuses: properties declared in it can be synthesized and anything declared in it must be in the main implementation, so the compiler can check to make sure you didn't miss an implementation. You must put the class extension before your implementation. You can't add instance variables directly in a class extension, but you can add properties. When you synthesize accessors for properties which don't have corresponding instance variables, the new runtime (os x 10.5 and later and all versions of iOS, I believe) will create the instance variables automatically. This means you can't create your own accessors, however, unless you put the instance variable in your header. Private methods can be added to the class extension without restriction, but as Anomie noted, it is technically possible to use them if you know what they are called, and with class-dump, nothing is safe.

Example usage of a class extension:

@interface MyClass ()
@property (retain) id privateIvar;
@property (readwrite) id readonlyProperty; // bonus! class extensions can be used to make a property that is publicly readonly and privately readwrite
- (void)privateMethod;
@end
@implementation MyClass
@synthesize privateIvar; // the runtime will create the actual ivar, and we just access it through the property
- (void)privateMethod {
    ...
}
...

Another way of creating "instance variables" without putting them in the header or using a property is to use associative references, which add data to an object at runtime. They aren't technically the same as instance variables, and the syntax for them is more complex. Since they also require the new runtime, there are only two reasons you would ever really want to use them: you want to add an instance variable in a category (outside the scope of this question) or you need it to be really really private. An associative reference doesn't create any methods or add to the class's definition in the compiled code, so if you don't create wrappers for them it is impossible to find out about them without asking the object after you add the data. See the bottom of the page I linked for a complete usage example.