Private ivar that is accessible in subclasses.

FortHill.h

#import "Monongahela.h"

@interface FortHill : Monongahela

@end

FortHill.m

#import "FortHill.h"
#import "Monongahela_Private.h"

// Only override getter; use superclass's setter
- (CGPoint)frog
{
    return CGPointMake(frog.x + 2.0, frog.y + 5.5);
}

// Override both setter and getter
- (NSData *)crow
{
    NSLog(@"This is my crow.");
    return crow;    // This is the ivar declared in the class extension
}

- (void)setCrow:(NSData *)newCrow
{
    NSLog(@"What a lovely new crow!");
    crow = [newCrow copy];
}

@end

Monongahela.h

#import <Foundation/Foundation.h>

@interface Monongahela : NSObject
{
    CGPoint frog;
}

@property (assign, nonatomic) CGPoint frog;
@property (copy, nonatomic) NSData * crow;
@property (strong, nonatomic) NSPipe * aardvark;

@end

Monongahela.m

#import "Monongahela.h"
#import "Monongahela_Private.h"

@implementation Monongahela

@synthesize frog;    // Use existing ivar from public header; create getter and setter
@synthesize crow;    // Use existing ivar from extension; create getter and setter
@synthesize aardvark; // Create ivar; use getter and setter below

- (NSPipe *)aardvark
{
    NSLog(@"Here's your aardvark!");
    return aardvark;
}

- (void)setAardvark:(NSPipe *)newAardvark
{
    NSLog(@"Saving a new aardvark.");
    aardvark = newAardvark;
}

@end

Monongahela_Private.h

#import "Monongahela.h"

@interface Monongahela ()
{
    @protected    // Ivars in class extensions are @private by default
    NSData * crow;
}

@end