Passing arguments by value or by reference in objective C

Question

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 is the call:

UILabel *startHourLabel; // This is properly initialized in other part of the code
[self checkRedColorText:startHourLabel];

Thanks for your help

Answer 1

Objective-C only support passing parameters by value. The problem here has probably been fixed already (Since this question is more than a year old) but I need to clarify some things regarding arguments and Objective-C.

Objective-C is a strict superset of C which means that everything C does, Obj-C does it too.

By having a quick look at Wikipedia, you can see that Function parameters are always passed by value

Objective-C is no different. What's happening here is that whenever we are passing an object to a function (In this case a UILabel *), we pass the value contained at the pointer's address.

Whatever you do, it will always be the value of what you are passing. If you want to pass the value of the reference you would have to pass it a **object (Like often seen when passing NSError).

This is the same thing with scalars, they are passed by value, hence you can modify the value of the variable you received in your method and that won't change the value of the original variable that you passed to the function.

Here's an example to ease the understanding:

- (void)parentFunction {
    int i = 0;
    [self modifyValueOfPassedArgument:i];
    //i == 0 still!
}

- (void)modifyValueOfPassedArgument:(NSInteger)j {
    //j == 0! but j is a copied variable. It is _NOT_ i
    j = 23;
    //j now == 23, but this hasn't changed the value of i.
}

If you wanted to be able to modify i, you would have to pass the value of the reference by doing the following:

- (void)parentFunction {
    int i = 0; //Stack allocated. Kept it that way for sake of simplicity
    [self modifyValueOfPassedReference:&i];
    //i == 23!
}

- (void)modifyValueOfPassedReference:(NSInteger *)j {
    //j == 0, and this points to i! We can modify i from here.
    *j = 23;
    //j now == 23, and i also == 23!
}

Answer 2

Objective-C, like Java, only has pass-by-value. Like Java, objects are always accessed through pointers. "objects" are never values directly, hence you never assign or pass an object. You are passing an object pointer by value. But that does not seem to be the issue -- you are trying to modify the object pointed to by the pointer, which is perfectly allowed and has nothing to do with pass-by-value vs. pass-by-reference. I don't see any problem with your code.

Answer 3

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 4

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 5

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.