gistfile1.m

// https://twitter.com/rob_rix/status/155082958443786241
// https://twitter.com/rob_rix/status/155083105739341824

@interface ClassA : NSObject {
    @private
    int topSecretIvar;
}
- (void)publicMethod;
@end

@implementation ClassA

static void topSecretMethod(ClassA *self) {
    self->topSecretIvar++;
}

- (void)publicMethod {
    topSecretMethod(self);
}

@end