Objective-C: Assigning the value of a static variable to an instance variable

Question

I essentially want to give each instance of a class a unique id.

So, I created a static integer. I increment it each time a new object is created and then assign the value of the static variable to an ivar. But clearly I don't understand something because, let's say I create three objects, "thisPageNumber" (which is the instance variable) is always 3 no matter which object I reference.

More information:

This class creates a number of "Page" objects. I'd like each page to know it's page number so that it can display the correct page art as well as perform a number of other various actions.

.h partial code:

@interface Page : UIViewController
{
    NSNumber            *thisPageNumber;
    UIImageView         *thisPageView;
    UIImageView         *nextPageView;
    UIImageView         *prevPageView;  
    UIImageView         *pageArt;
}

.m partial code:

@implementation Page

static int pageCount = 0;

- (id)initWithNibName:(NSString *)nibNameOrNil bundle:(NSBundle *)nibBundleOrNil
{
    if ((self = [super initWithNibName:nibNameOrNil bundle:nibBundleOrNil])) {
        pageCount++;
        thisPageNumber = pageCount;
    }
    return self;
}

- (void)viewWillAppear:(BOOL)animated
{
    [super viewWillAppear:animated];

    CGRect defaultFrame = CGRectMake(0.0, 0.0, 1024.0, 768.0);

    if (thisPageView == nil) {
        thisPageView = [[UIImageView alloc] 
                        initWithImage:[UIImage 
                                       imageNamed:[NSString stringWithFormat:@"Page%i.png", [thisPageNumber intValue]]]];
        thisPageView.frame = defaultFrame;
        [self.view addSubview:thisPageView];
    }

    if (nextPageView == nil && [thisPageNumber intValue] < BOOK_PAGE_COUNT) {
        nextPageView = [[UIImageView alloc] 
                        initWithImage:[UIImage 
                                       imageNamed:[NSString stringWithFormat:@"Page%i.png", [thisPageNumber intValue]+1]]];
        nextPageView.frame = defaultFrame;
        [self.view addSubview:nextPageView];
    }

    if (prevPageView == nil && [thisPageNumber intValue] > 1) {
        prevPageView = [[UIImageView alloc] 
                        initWithImage:[UIImage 
                                       imageNamed:[NSString stringWithFormat:@"Page%i.png", [thisPageNumber intValue]-1]]];
        prevPageView.frame = defaultFrame;
        [self.view addSubview:prevPageView];
    }    
}

What you are doing looks ok, can you show the code wehre you are accessing? — Joshua Weinberg
– Joshua Weinberg, Commented Jul 12, 2010 at 23:53

kubi · Accepted Answer · 2010-07-13 20:32:42Z

2

I'm not sure why the compiler didn't complain, but part of your problem is here:

thisPageNumber = pageCount;

NSNumber is an object. To set it to the current pageCount value, use

thisPageNumber = [[NSNumber alloc] initWithInt:pageCount];

answered Jul 13, 2010 at 20:32

kubi

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1 Comment

RyJ Over a year ago

OMG, duh! Thanks, that works. "thisPageNumber" was actually an int when I first posted this topic, then by the time I added more of the code I had changed it to an NSNumber to fit in with my work around. Regardless, everything seems to be working great now. Thanks a lot!

Yuji · Accepted Answer · 2010-07-13 01:01:06Z

0

Why don't you just use self as the unique ID? It's unique.

answered Jul 13, 2010 at 1:01

Yuji

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3 Comments

Logan Capaldo Over a year ago

It's unique at a given instance of time in the program. The value of self can be reused after an object is dealloced which could be a problem for certain uniqueness requirements.

Georg Fritzsche Over a year ago

Also knowing the order of creation from the id can e.g. be helpful when evaluating logs.

Yuji Over a year ago

Ah, that makes sense. We need to know what kind of uniqueness RyJ needs.

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