ReadWriter

// ReadWriter.h
@interface ReadWriter
- (id)initWithFile:(NSString *)file;
@end

@interface ReadWriter (Reader)
- (NSData *)readError:(NSError * __autoreleasing *)error;
@end

@interface ReadWriter (Writer) // by using informal protocols we skip not implemented warnings from the compiler
- (BOOL)write:(NSData *)data error:(NSError * __autoreleasing *)error;
@end

// ReadWriter.m
@interface ReadWriter ()
@property (nontatomic, strong, readonly) Reader *reader;
@property (nontatomic, strong, readonly) Writer *writer;
@end

@implementation

- (id)initWithFile:(NSString *)file {
    self = [super init];
    if (!self) {
    _reader = [[Reader alloc] initWithFile:file];
    _writer = [[Writer alloc] initWithFile:file];
    }
    return self
}

- (id)forwardingTargetForSelector:(SEL)sel {
    if ([_reader respondsToSelector:sel] return _reader;
    if ([_writer respondsToSelector:sel] return _writer;
    return nil;
}

@end