Objective-C Private Property – How to Define

You want to use a "class extension" rather than a category:

@interface MyClass ()
@property (nonatomic, retain) NSMutableArray *stuff;
@end

@implementation MyClass
@synthesize stuff; // ok

Class extensions were created in Objective-C 2.0 in part specifically for this purpose. The advantage of class extensions is that the compiler treats them as part of the original class definition and can thus warn about incomplete implementations.

Besides purely private properties, you can also create read-only public properties that are read-write internally. A property may be re-declared in a class extensions solely to change the access (readonly vs. readwrite) but must be identical in declaration otherwise. Thus you can do:

//MyClass.h
@interface MyClass : NSObject
{ }
@property (nonatomic,retain,redonly) NSArray *myProperty;
@end

//MyClass.m
@interface MyClass ()
@property (nonatomic, retain, readwrite) NSArray *myProperty;
@end

@implementation MyClass
@synthesize myProperty;
//...
@end

The best way to solve this is to create ClassA+Private.h and import it in ClassA.m and Category.m. Mind the h at the end, you only need to declare your private properties and methods there, the definition is better kept in ClassA.m.