Demonstrate call-by-ref. See: http://stackoverflow.com/questions/21226504/using-delegation-vs-property-reverse-scope?noredirect=1#comment31997063_21226504

CallByRef.m

#import <Foundation/Foundation.h>

@interface ClassA : NSObject

@property NSMutableString *string;

- (void)changeString;
- (void)printString;

@end

@implementation ClassA

- (void)changeString {
    [self.string appendString:@"ClassA Did This"];
}

- (void)printString {
    NSLog(@"ClassA string='%@'", self.string);
}

@end

@interface ClassB : NSObject

@property NSMutableString *string;

- (void)changeString;
- (void)printString;

@end

@implementation ClassB

- (void)changeString {
    [self.string appendString:@" ClassB Did This"];
}

- (void)printString {
    NSLog(@"ClassB string='%@'", self.string);
}

@end

int main(int argc, const char * argv[])
{
    @autoreleasepool {
        ClassA *a = [[ClassA alloc] init];
        ClassB *b = [[ClassB alloc] init];
        NSMutableString *string = [[NSMutableString alloc] init];
        a.string = string;
        b.string = string;

        [a changeString];
        [a printString];
        [b printString];

        [b changeString];
        [a printString];
        [b printString];

    }
    return 0;
}

Output

2014-01-21 06:33:00.798 CallByRef[37837:303] ClassA string='ClassA Did This'
2014-01-21 06:33:00.801 CallByRef[37837:303] ClassB string='ClassA Did This'
2014-01-21 06:33:00.802 CallByRef[37837:303] ClassA string='ClassA Did This ClassB Did This'
2014-01-21 06:33:00.802 CallByRef[37837:303] ClassB string='ClassA Did This ClassB Did This'
Program ended with exit code: 0