Dynamic type cast from id to class in objective c

Question

I would like to cast dynamically in Objective C and access instance properties. Here a pseudo code:

id obj; if (condition1) obj = (Class1*)[_fetchedResults objectAtIndex:indexPath.row]; else obj = (Class2*)[_fetchedResults objectAtIndex:indexPath.row]; NSNumber *latitude = obj.latitude;

Then the compiler tells me the following: property 'latitude' not found on object of type '__strong id'

Either Class1 and Class2 are core data entities and have nearly the same kind of attributes. In condition1 _fetchedResults returns objects of type Class1 and in condition2 _fetchedResults returns objects of type Class2.

Could someone give me a hint how to solve this kind of problem?

Thanks!

I think you should make sure firstly that what [_fetchedResults objectAtIndex:indexPath.row] returns is the Class has the latitude property. Also, casting a none NSManagedObject type to a NSManagedObject type is wrong either. You need a NSManagedObjectContext whenever and however you create a NSManagedObject instance. — Owen Zhao
– Owen Zhao, Commented Aug 7, 2014 at 7:48

Monolo · Accepted Answer · 2013-04-30 12:13:15Z

7

You can access the properties through Key-Value Coding (KVC):

[obj valueForKey:@"latitude"]

answered Apr 30, 2013 at 12:13

Monolo

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5 Comments

ipmcc Over a year ago

Note that using KVC to do this robs you of any type-checking from the compiler.

Monolo Over a year ago

@ipmcc Sure, but so does using id in the first place :-)

hooleyhoop Over a year ago

@Monolo This is unnecessary and has the same effect as [obj latitude]

Monolo Over a year ago

@hooleyhoop True. I'd venture that using KVC conveys intention slightly better, but that is a matter of coding style. The error messages you'd get (at runtime, alas) are also quite different. And you might in theory have defined a getter with a different name if we want to cover that one, too.

Martin R Over a year ago

Key-Value coding is documented practice for managed objects, so the only thing I would change is to replace id by NSManagedObject *.

ipmcc · Accepted Answer · 2013-04-30 12:13:45Z

The obj variable needs to be of a type that has the property in question. If both entities have the same property, one way to achieve this would be for the property to be declared on a common base class. If it's not appropriate for these two types to share a common base class, then you could have them adopt a common protocol, like this:

@protocol LatitudeHaving @property (copy) NSNumber* latitude; @end @interface Class1 (AdoptLatitudeHaving) <LatitudeHaving> @end @interface Class2 (AdoptLatitudeHaving) <LatitudeHaving> @end

From there, you would declare obj as being an id<LatitutdeHaving>, like this:

id<LatitudeHaving> obj; if (condition1) obj = (Class1*)[_fetchedResults objectAtIndex:indexPath.row]; else obj = (Class2*)[_fetchedResults objectAtIndex:indexPath.row]; NSNumber *latitude = obj.latitude;

And that should do it. FWIW, protocols are similar to Interfaces in Java.

objectAtIndex: returns an id not an id<LatitudeHaving>. Those casts may not be strictly necessary to avoid compiler complaints, but I was trying to make the code as close to the OP's code as possible.

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