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Re: Wrapping C functions


From: Martin Kalbfuß
Subject: Re: Wrapping C functions
Date: Thu, 6 Aug 2009 14:13:43 -0700 (PDT)

OK. I understand that a global and load isn't needed when using a singleton.
The atexit cleanup is mainly for calling SDL_Quit;
I followed an example of apple for a singleton. Now my implementation looks
like:

static Oh *sharedOhMangager = nil;

@implementation Oh

        +(  Oh * )sharedMangager
        {
                @synchronized(self)
                {
                        if ( sharedOhInstance == nil )
                        {
                                [ [ self alloc ] init ];
                        }
                }

                return sharedOhManager;
        }

        + ( id )allocWithZone:( NSZone * )zone
        {
                @synchronized( self )
                {
                        if ( sharedOhManager == nil )
                        {
                                SDL_Init();

                                sharedOhManager = [ super allocWithZone: zone ];

                                return sharedOhManager;
                        }
                }

                return nil;
        }

        - ( id )copyWithZone:( NSZone * )zone
        {
                return self;
        }

        - ( unsigned ) retainCount
        {
                return UINT_MAX;
        }

        - ( void )release
        {
        }

        - ( id )autorelease
        {
                return self;
        }

@end

But what I don't understand is where the objects memory is going to be
freed. They don't tell. They only say it's for memory-managed code. Release
and autorelease are overwritten. Should I or the user call dealloc somwhere?

Thanks


David Chisnall wrote:
> 
> On 6 Aug 2009, at 20:56, Martin Kalbfuß wrote:
> 
>> Thank you David. That helped a lot. I decided to have a global  
>> library object
>> created with load.
> 
> You don't need a global library object; put the initialisation in the  
> +initialize method for a class that is responsible for creating the  
> library context.  Or put it in the -init method for a singleton object.
> 
>> But how to release it? Currently I set an atexit function in the  
>> load method
>> which calls the release method of the class. But is there an  
>> Objective-C
>> way?
> 
> Why do you need to release it?  If it's something that needs explicit  
> cleanup then the user should release it.  If you are using AppKit then  
> you can register an observer for the notification that is sent before  
> an app exists, but your other email indicated that you are just using  
> Foundation.  In this case, you should probably stick with atexit() if  
> you really need cleanup.
> 
>> Another point is that I want to forbid the instanciation of the  
>> class. I
>> think a singleton is the right way to go. But how can I create a  
>> singleton,
>> when I cannot hide methods?
> 
> The traditional approach is to provide a +sharedInstance method and  
> put in the documentation that people should call this instead.  If you  
> want, you can override +allocWithZone: to look a bit like this:
> 
> static id sharedInstance;
> + (void)initialize
> {
>       if (self == [YourClassName class])
>       {
>               sharedInstance = [self new];
>       }
> }
> + (id)allocWithZone: (NSZone*)aZone
> {
>       if (nil != sharedInstance)
>       {
>               return sharedInstance;
>       }
>       return [super allocWithZone: aZone];
> }
> 
> This will allocate the shared instance when class receives the first  
> message and return it whenever the class receives a +alloc message.   
> It's not a very good solution; you also need to protect -init to make  
> sure that someone who does alloc/init they are not reinitialising the  
> object and clearing all of the ivars.  Generally, just putting  
> something in the documentation saying people should use  
> +sharedInstance is enough.
> 
> David
> 
> _______________________________________________
> Discuss-gnustep mailing list
> address@hidden
> http://lists.gnu.org/mailman/listinfo/discuss-gnustep
> 
> 

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