Objective-c Iphone how to set default value for a property

Question

Hi i've got the following code in a ViewController.h:

#import <UIKit/UIKit.h>

@interface CalcViewController : UIViewController {
    NSNumber* result;
    NSString* input;
    //NSString* input = @"";

    IBOutlet UITextField* display;
}

@property (retain) NSNumber* result;
@property (retain) NSString* input;
@property (nonatomic, retain) UITextField* display;

@end

The problem is that I want to append a string to input but this is not possible when its still null. Thats why I want to set the default value of input to be @"". But where do I put this code.

I'm aware of one possible solution where you put it in a default constructor. But I've got no idea in what file to put this. And where I should call it from.

I unfortunately only got a limited understanding of C and realise that perhaps a .h file is not the right place.

The project type is a View-Based-Application if you should need this.

Hope you can help.

Solution 1

You may find it useful to read some documentation on Objective-C or Cocoa. You probably can find some good suggestions on reading material if you perform a search here in StackOverflow or on Google.

To answer your question, you should have a @implementation of CalcViewController. One would most often place this @implementation inside of the *.m file. If your *.h file is named "ViewController.h" then the implementation would go in "ViewController.m".

You would then create a copy of the UIViewController's initialization function and place it there (I don't know what the default init function is).

For example:

@implementation CalcViewController

@synthesize result;
@synthesize input;

- (id)initWithNibName:(NSString*)aNibName bundle:(NSBundle*)aBundle
{
    self = [super initWithNibName:aNibName bundle:aBundle]; // The UIViewController's version of init
    if (self) {
        input = [[NSString alloc] initWithString:@""]; // You should create a new string as you'll want to release this in your dealloc
    }
    return self;
}

- (void)dealloc
{
    [input release];
    [super dealloc];
}
@end // end of the @implementation of CalcViewController

NOTES:

• You may want to rename the file to CalcViewController. I believe it is easier for Xcode's refactoring engine to deal with.
• You do not need to declare the @property for the display instance variable as you connect that with Interface Builder. Unless you want clients of the CalcViewController to change it often

EDIT: April 28, 2009: 10:20 AM EST: I suggest actually allocating a NSString as you should technically release it in the dealloc.

EDIT: April 28, 2009: 11:11 AM EST: I updated the @implementation to use the UIViewController's version of init.

Solution 2

Very good question. The short answer is to init the values in the init function. You need to overwrite the default init function so that the default value is ready before the object can be used. I would like to suggest people stop suggesting others to read document; please answer the question directly if you can.

Solution 3

Another option is to write your own getter for your input property, and return @"" if the instance variable is nil. This would work even if you accidentally or intentionally assigned nil to input, whereas using init to set a default value would break.

Comments

[MrHus]

Answer to note its a CalcViewController file I just met that its a ViewController. And yes the display needs to change often. Thanks

[Lyndsey Ferguson]

Just to be clear, when I wrote about the display instance variable changing often, I mean the actual instance of the variable. Not the actual data in the display object. So if you can imagine that you'll be destroying and re-creating the display instance variable, then this would make sense.

[Lyndsey Ferguson]

The actual call is "– initWithNibName:bundle:" and your code should either be creating the CalcViewController directly as in: CalcViewController* vc = [[CalcViewController alloc] initWithNibName:@"MyCalcView" bundle:nil]; Or by hooking it up directly in Interface Builder. So, as I tried to allude in the text, I didn't think "init" was the correct initializer. Again, read documentation to get a basic idea of how Cocoa works. Good luck!

[Barry Wark]

A good solution, but not common practice. Other readers of your code will likely look to the designated initializer for the default value.

[Werner]

I wouldn't say that, returning an empty object instead of nil is pretty common throughout Cocoa. That's not to say setting it in init is a bad or anything, it's more a matter of preference.

[Alex Gray]

People who say "blah blah blah maybe you should read the docs" should be spanked, ostracized, and hated upon. they are dumb.

[Dmitry Minkovsky]

Voted this down because of the "read the docs" comment. Sometimes you just want to know you're doing something correctly. I, for example, have been reading docs for a while now, and came upon this answer after Googling to confirm my understanding. Don't be a dick.

[Dmitry Minkovsky]

Up-voting alex gay all over SO because I agree with this sentiment so much.

[Lyndsey Ferguson]

It was not the intention to be offensive, rather to point out that there are some foundational concepts that should be reviewed. Otherwise other problems will pop up to confound the author of the code. How would you change my response so that I can still suggest reviewing the docs without being a dick? Thanks. I did answer the question.

[Caleb]

+1 This is all good advice, but "read the docs" is the most important part. As the saying goes: Give a man a fish and you feed him for a day. Teach a man how to fish and you feed him for a lifetime. This question goes straight to the fundamentals of Objective-C (and most OO languages) -- if the OP is unaware of the role of initializers, he/she definitely needs to review the basics. @dimadima When you want to know that you're doing something correctly, your first stop should be the documentation for whatever language or framework you're using.

[johnrubythecat]

Imagine how much space could be saved if all the recommendations that someone should read the docs could be removed.