LCYThing+Transmogrify.h

#import "LCYThing.h"

@interface LCYThing (Transmogrify)

- (void) turnIntoCat;

@end

LCYThing+Transmogrify.m

#import "LCYThing+Transmogrify.h"

@implementation LCYThing (Transmogrify)

- (void) turnIntoCat;
{
    self.privateDetail = @"Put a cat mask on";
}

@end

LCYThing.h

@interface LCYThing : NSObject

- (void) doSomething;

@end

LCYThing.m

#import "LCYThing.h"

@interface LCYThing()
@property (nonatomic, retain) NSString *privateDetail;
@end

@implementation LCYThing

@synthesize privateDetail = _privateDetail;

- (void) dealloc;
{
    [_privateDetail release]; _privateDetail = nil;    
    [super dealloc];
}

- (id) init;
{
    self = [super init];
    if (self)
    {
        self.privateDetail = @"A private detail";
    }
    return self;
}

- (void) doSomething;
{
    NSLog(@"%@ did something using: %@", self, self.privateDetail);
}

@end