Weak linking on iPhone refuses to work

Question

I've got an iPhone app that's mainly targetting 3.0, but which takes advantage of newer APIs when they're available. Code goes something like this:

if (UIApplicationDidEnterBackgroundNotification != NULL)
    [nc addObserver: self selector: @selector(irrelevantCallbackName:) name: UIApplicationDidEnterBackgroundNotification object: nil];

Now, according to everything Apple's ever said, if the relevant APIs are weakly linked, that will work fine because the dynamic linker will evaluate UIApplicationDidEnterBackgroundNotification to NULL. Except that it doesn't. The application compiles, but as soon as it hits "if (UIApplicationDidEnterBackgroundNotification != NULL)" it crashes with EXC_BAD_ACCESS.

Is this simply a matter of a compiler flag I need to set? Or am I going about this the wrong way?

Answer 1

Aaand I figured it out. For symbols that are not functions (extern const int foobar, for instance), you have to compare against the address of the symbol, not the symbol itself, so:

if (&UIApplicationWillEnterForegroundNotification != NULL)
    etc;

Which in retrospect is kind of obvious, but I still fault the entire universe around me for not ever mentioning the distinction.

Answer 2

Here is what I had to do when checking for an external framework constant.

const CLLocationAccuracy * ptr = &kCLLocationAccuracyBestForNavigation;
BOOL frameworkSupports = (ptr != NULL);
if (frameworkSupports) {
    return kCLLocationAccuracyBestForNavigation;
} else {
    return kCLLocationAccuracyBest;
}

It would not work without the ptr variable.