iPhone static libraries: How to hide instance variable

Categories and extensions can't add instance variables to a class. I'd go for the PIMPL idiom here - use a private implementation object:

// header
@class MyObjImpl;
@interface MyObj {
    MyObjImpl* impl;
}
@end

// implementation file:
@interface MyObjImpl {
    id someIvar;
}
// ...
@end

// ... etc.

This also keeps your public interface stable in case you want to add something for internal use.

The "duplicate interface" comes from missing parentheses in the second interface declaration:

// header:
@interface MyObj 
// ...
@end

// implementation file:
@interface MyObj () // note the parentheses which make it a class extension
// ...
@end

You may also use the Objective-C 2 feature known as "Associative reference".

This is not really object-oriented API, but you can add/remove object to another object by using some simple functions of the runtime:

void objc_setAssociatedObject(id object, void * key, id value)

Sets the value or remove it when value is nil.

id objc_getAssociatedObject(id object, void * key)

Retrieve the value for specified key.

Note that this is also a mean to add "instance variable" to existing object when implementing a category.

Key is s simple pointer to private variable that you can declare as a module private by using:

static char SEARCH_INDEX_KEY = 0;