Using the Cocoa framework, when setting a label, an explicit cast to NSString works but stringValue raises an exception. Why?

Question

I'm setting a UILabel to a value stored in as a NSNumber. If I do this

foo.label.text = [bar stringValue];

then I get a NSInvalidArgumentException

However if I cast by doing:

foo.label.text = (NSString *)bar;

then things work ok.

Can anyone explain why this could be the case?

Answer 1

If bar is an NSString instance, it is normal that you get a NSInvalidArgumentException, because NSString does not respond to stringValue selector; thus the result of the stringValue is an invalid value for the text property.

Answer 2

Using your example, if I do the following:

NSNumber *theNumber = [NSNumber numberWithInt:1];
[[self theLabel] setText:(NSString *)theNumber];

I get the NSInvalidArguementException. But if I do:

NSNumber *theNumber = [NSNumber numberWithInt:1];   
[[self theLabel] setText:[theNumber stringValue]];

My label gets set with the number 1.

Are you sure bar is NSNumber? Is foo a view with a label subview defined?

For my example, I have an IBOutlet property on an UIViewController to a UILabel. I put that label in the view using InterfaceBuilder and connected the IBOutlet.