Re: Wrapping C functions

[David Chisnall]

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