Does Type assignment from incompatible pointer type should always fail in objective-c?

Question

I set an incompatible pointer type, but the code works ok. I want to know the reason. X1 is the exact copy of X2 code.

@interface ViewController : UIViewController

@property(retain,nonatomic) X1 *testProperty;

@end

// ###############################

@implementation ViewController

@synthesize testProperty;

   - (void)viewDidLoad {
      [super viewDidLoad];

      testProperty = [X2 new];  --> Warning
      [testProperty printLog];
   }
}

The warning is: Incompatible pointer types assigning to
This warning is obvious, but my problem is how this code works ok? does it cast it?

No, there is no casting, the pointer is simply assigned. As long as you don't use `testProperty` there's really nothing that can go wrong. — Gereon, Dec 22 '18 at 09:28
Please update your question to show the usage code as well, otherwise it will be impossible to answer. — Gereon, Dec 22 '18 at 09:31
if you call on testProperty method which is missing in X2 it will crash. This is the problem — Andrey Chernukha, Dec 22 '18 at 11:34
@AndreyChernukha Thanks. I know this. but I want to know the reason why in this situation it is ok. is it casting? — Fattaneh Talebi, Dec 22 '18 at 11:42
What exactly is ok? Why one pointer is assigned to another and compiler allows that? Well, a pointer is nothing but an integer. One integer can be assigned to another. This is totally valid. But compiler warns you about the pointer mismatch. There's no casting at all there — Andrey Chernukha, Dec 22 '18 at 11:45

score 2 · Accepted Answer · answered Dec 22 '18 at 17:22

2

This works because of the dynamic nature of Objective-C. The compiler correctly warns you about mismatching types, but at runtime, when it executes the message sending of printLog to its receiver, it checks if the object actually knows how to handle this message. Since both your X1 and X2 classes implement printLog, this works. Likewise, any messsage that is implemented in any commmon superclass of X1 and X2 (so at least everything from NSObject) would also work.

This is called "Duck Typing" and is really fundamental to the ObjC runtime.

answered Dec 22 '18 at 17:22

Gereon

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@Fattaneh Talebi please also bear in mind that in general this is a very dangerous way of acting, because it can take you to unpredictable behaviour. Let's say for example that in a distant future you modify X2 and remove the printLog method (assuming they are *not* sibling subclasses with that method implemented in the parent class): the compiler won't warn you about that, but the app will just crash when trying to call it – il3v Dec 24 '18 at 11:10

Does Type assignment from incompatible pointer type should always fail in objective-c?

1 Answers1