How to cast an object in Objective-C?

Question

Is there a way to cast objects in Objective-C similar to how objects are cast in VB.NET?

For example, I am trying to do the following:

// create the view controller for the selected item
FieldEditViewController *myEditController;
switch (selectedItemTypeID) {
    case 3:
        myEditController = [[SelectionListViewController alloc] init];
        myEditController.list = listOfItems;
        break;
    case 4:
        // set myEditController to a diff view controller
        break;
}

// load the view
[self.navigationController pushViewController:myEditController animated:YES];
[myEditController release];

However I am getting a compiler error since the list property exists in the SelectionListViewController class but not on the FieldEditViewController even though SelectionListViewController inherits from FieldEditViewController.

This makes sense, but is there a way to cast myEditController to a SelectionListViewController so I can access the list property?

For example in VB.NET I would do:

CType(myEditController, SelectionListViewController).list = listOfItems

Jim Puls · Accepted Answer · 2009-03-27 17:40:34Z

235

Remember, Objective-C is a superset of C, so typecasting works as it does in C:

myEditController = [[SelectionListViewController alloc] init];
((SelectionListViewController *)myEditController).list = listOfItems;

answered Mar 27, 2009 at 17:40

Jim Puls

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4 Comments

Dan Rosenstark Over a year ago

Or "Remember, Objective-C works like Java, just remember to add asterisks to variables that point to Obj-C objects."

Guido Anselmi Over a year ago

Great answer. You could make it a little clearer by breaking out the cast and assignment into two lines.

csmith Over a year ago

Typecasting in Objective-C is far more like old C than Java. Java hides most of this from the user, hence arguments that C should still be taught rather than Java as a first language.

Erhan Demirci Over a year ago

I added front of my object (myVC *) and worked

Community · Accepted Answer · 2017-05-23 12:02:13Z

Typecasting in Objective-C is easy as:

NSArray *threeViews = @[[UIView new], [UIView new], [UIView new]];
UIView *firstView = (UIView *)threeViews[0];

However, what happens if first object is not UIView and you try to use it:

NSArray *threeViews = @[[NSNumber new], [UIView new], [UIView new]];
UIView *firstView = (UIView *)threeViews[0];
CGRect firstViewFrame = firstView.frame; // CRASH!

It will crash. And it's easy to find such crash for this case, but what if those lines are in different classes and the third line is executed only once in 100 cases. I bet your customers find this crash, not you! A plausible solution is to crash early, like this:

UIView *firstView = (UIView *)threeViews[0];
NSAssert([firstView isKindOfClass:[UIView class]], @"firstView is not UIView");

Those assertions doesn't look very nice, so we could improve them with this handy category:

@interface NSObject (TypecastWithAssertion)
+ (instancetype)typecastWithAssertion:(id)object;
@end


@implementation NSObject (TypecastWithAssertion)

+ (instancetype)typecastWithAssertion:(id)object {
    if (object != nil)
        NSAssert([object isKindOfClass:[self class]], @"Object %@ is not kind of class %@", object, NSStringFromClass([self class]));
    return object;
}

@end

This is much better:

UIView *firstView = [UIView typecastWithAssertion:[threeViews[0]];

P.S. For collections type safety Xcode 7 have a much better than typecasting - generics

Sijmen Mulder · Accepted Answer · 2009-03-27 17:40:48Z

12

((SelectionListViewController *)myEditController).list

More examples:

int i = (int)19.5f; // (precision is lost)
id someObject = [NSMutableArray new]; // you don't need to cast id explicitly

answered Mar 27, 2009 at 17:40

Sijmen Mulder

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1 Comment

Chris Hanson Over a year ago

In general this is correct; you don't need to cast id in message expressions. But when using dot syntax to access and set properties, you must use a concrete type, not just id, so the compiler knows what method invocation to actually generate. (It can differ for properties with the same name.)

Andrew Grant · Accepted Answer · 2009-03-27 17:41:24Z

5

Sure, the syntax is exactly the same as C - NewObj* pNew = (NewObj*)oldObj;

In this situation you may wish to consider supplying this list as a parameter to the constructor, something like:

// SelectionListViewController
-(id) initWith:(SomeListClass*)anItemList
{
  self = [super init];

  if ( self ) {
    [self setList: anItemList];
  }

  return self;
}

Then use it like this:

myEditController = [[SelectionListViewController alloc] initWith: listOfItems];

answered Mar 27, 2009 at 17:41

Andrew Grant

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Comments

stephen · Accepted Answer · 2011-11-14 04:21:55Z

0

Casting for inclusion is just as important as casting for exclusion for a C++ programmer. Type casting is not the same as with RTTI in the sense that you can cast an object to any type and the resulting pointer will not be nil.

answered Nov 14, 2011 at 4:21

stephen

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