Passing arguments by value or by reference in objective C

Question

Hello, I'm kind of new with objective c and I'm trying to pass an argument by reference but is behaving like it were a value. Do you know why this doesn't work?

This is the function:

- (void) checkRedColorText:(UILabel *)labelToChange {
    NSComparisonResult startLaterThanEnd = [startDate compare:endDate];
    if (startLaterThanEnd == NSOrderedDescending){
        labelToChange.textColor = [UIColor redColor];
    }
    else{
        labelToChange.textColor = [UIColor blackColor];
    }

}

And this the call:

UILabel *startHourLabel; (this is properly initialized in other part of the code)
[self checkRedColorText:startHourLabel];

Thanks for your help

Answer 1

In objective-c, there is no way to pass objects by value (unless you explicitly copy it, but that's another story). Poke around your code -- are you sure checkRedColorText: is called? What about [startDate compare:endDate], does it ever not equal NSOrderedDescending? Is labelToChange nil?

Answer 2

Did you edit out code between this line

UILabel *startHourLabel;

and this line?

[self checkRedColorText:startHourLabel];

If not, the problem is that you're re-declaring your startHourLabel variable, so you're losing any sort of initialization that was there previously. You should be getting a compiler error here.

Answer 3

Here are the possibilities for why this doesn't work:

1. the label you pass in to checkRedColorText is not the one you think it is.
2. the comparison result is always coming out the same way.
3. ... actually, there is no 3.

You claim you initialised startHourLabel elsewhere, but, if it is a label from a nib file, you should not be initialising it at all. It should be declared as an IBOutlet and connected to the label in the nib with interface builder.

If it is not a label in the nib i.e. you are deliberately creating it programmatically, you need to check the address of the label you initialise and check the address of the label passed in to checkRedColorText. Either NSLog its address at initialisation and in checkRedColorText or inspect it with the debugger.