That's because you're doing it wrong

gistfile1.m

@class X

@interface Y : NSObject
@property (nonatomic, assign) int z;

- (int)times:(int)other;
- (int)timesX:(X *)xObject;
@end

@implementation Y

- (int)times:(X *)xObject
{
  return self.z * xObject.zzz
}

@end

@interface X : NSObject
@property (nonatomic, strong) NSString *name;
@property (nonatomic, assign) int zzz;
@end;

int main()
{
  std::cout << "hello" << std:endl;
  X *x = [[X alloc] init];
  x.zzz = 3; // alternatively, x->_zzz should work too I think, but that's an ivar access
  Y *y = [[Y alloc] init];

  // ... etc
  return 0;
}