8

I can't seem to find a way to set the static int I have created to assign unique ids to every object I save to persistent memory. The following gives me a 'no setter method 'setIdGen' for assignment to property.

-(void)viewDidLoad
{
    PlayerMenuController.idGen = [[NSUserDefaults standardUserDefaults] floatForKey:@"idGen"];
}

As well as the above I've tried creating a static setIdGen method that would return bad access errors, and making NSIntegers with their own set methods. My static NSMutableArray gave the same errors when I tried to assign it using = but worked fine when using setArray.

idGen method:

+ (int) idGen
{
    /*static int idGen;
    if(!idGen)
    {
        idGen = 0;
        NSLog(@"idGen reset");
    }*/
    return self.idGen;
}
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2
  • 1
    Why do you expect a property to be there when it isn't there? return idGen; and uncomment that block. Commented Apr 12, 2013 at 20:07
  • 1
    God damn it why do my down-voted questions always get the most views :p Commented Apr 15, 2013 at 23:40

3 Answers 3

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43

Update 2017

Xcode 8 introduced class properties to Objective-C, from the release notes:

Objective-C now supports class properties, which interoperate with Swift type properties. They are declared as @property (class) NSString *someStringProperty;, and are never synthesised.

This means our sample interface below can become:

@interface PlayerMenuController : NSObject

@property (class) int idGen;

@end

However you must still implement the methods yourself, as shown below, as class properties are never synthesised. Note that this also means if you specify property attributes, such as copy, that your methods must implement the semantics.

Original Answer

It looks like you are trying to implement a class property, but there is not such thing in Objective-C - a property is a pair of instance methods.

However, you can fake it...

While the @property declaration is not available to you, if you declare class methods which follow the right naming convention then your compiler may (tested on Xcode 4.6.1, "may" as I cannot offhand point to this being supported, but it's simple to test and will compile time error if not) allow you to use dot notation, i.e. it looks like a class property even if it lacks an @property.

A sample interface:

@interface PlayerMenuController : NSObject

// a class "property"
+ (int) idGen;
+ (void) setIdGen:(int)value;

@end

The implementation:

@implementation PlayerMenuController

static int idGen = 0;

+ (int) idGen { return idGen; }
+ (void) setIdGen:(int)value { idGen = value; }

@end

And test it:

NSLog(@"initial value: %d", PlayerMenuController.idGen);
PlayerMenuController.idGen = 42;
NSLog(@"updated value: %d", PlayerMenuController.idGen);

producing:

initial value: 0
updated value: 42

So we have a "class property" - it looks, walks and quacks like a property ;-)

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7 Comments

Actually, you can declare a property, then implement the setter/getter methods to operate on the static field. (Not that this is recommended.)
If I'm reading the original question correctly, the OP is trying to use a global variable to provide a unique id for each instance, which is of course impossible.
@HotLicks - The OP has written "class.property", hence my code. But you could be right, they might be after an instance property accessing a "class variable" (static)...
@jleher - I beg to differ, you can use a static variable to provide a unique id to every instance. It is by no means impossible, not even hard (as I've done it myself ;-)). But you don't need a "class property" to do it.
@Deco - a global property would be one which stands alone and is visible from everywhere, a class property would belong to a class and require reference via the class. Two different classes could have a class property with the same name, but you couldn't have two globals properties with the same name.
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3

If you use a static variable and you ever want to subclass this, the static variable will be the same for both parent and child, so any change when addressing the child class, will also change the same 'property' on it's parent. (and vice versa)

The safe way to do it is using objc/runtime.h associated objects

+(int) idGen
{
    NSNumber* idGen = objc_getAssociatedObject(self, @selector(idGen));

    if (idGen == nil)
    {
        idGen = @0;
    }
    else
    {
        idGen = @([idGen intValue] + 1);
    }

    self.idGen = [idGen intValue];

    return [idGen intValue];
}

+(void)setIdGen:(int)idGen
{
    objc_setAssociatedObject(self, @selector(idGen), @(idGen), OBJC_ASSOCIATION_RETAIN);
}
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Comments

1

You shouldn't be returning self.idGen because an int is not a property. Your code should work if you do this:

 + (int) idGen {
     static int idGen;
     return idGen;
   }
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13 Comments

if (!idGen) idGen = 0; is a no-op. It's one of a superfluous no-op even more since static variables are initialized to 0.
Yeah, you can just initialize scalar types immediately. Only objects need the id obj = nil; if (!obj) obj = [[Obj alloc] init]; trick.
@mipadi or anything you want to lazy-initialize to something other than zero.
Good point. I was thinking of a usage where you instantiate if it is nil
@H2CO3: When would you want to lazily initialize a scalar?
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