#define iTunesFileSharingEnabled... how do I declare this - xcode

Question

I would like to create a define based on the status of iTunes File Sharing boolean status on the Info.plist file. So, I have created this:
#define itunesFileSharingEnabled [[[NSBundle mainBundle] \
objectForInfoDictionaryKey:#"UIFileSharingEnabled"] boolValue]
#if itunesFileSharingEnabled
#define something
#endif
but the if fails with the error invalid token at start of preprocessor expression.
if I substitute #define itunesFileSharingEnabled with
#define itunesFileSharingEnabled YES
than the if compiles perfectly. So the error is on the previous line. Xcode is not liking the #define line.
What I want is to define some directive to be true if itunes file sharing is YES and NO if it is not.
How do I declare that?
thanks.
edit:
the problem is that I cannot use regular if to check for iTunes file sharing at run time because I want to make the compiler fail using #error or #warning directive...
for example, later in my code I want to use this
#if iTunesFileSharingEnabled
#error TURN OFF FILE SHARING
#endif
I want the compiler to fail if file sharing is turned on.

Answer 1

Preprocessor directives are evaluated at compile time.
after the first run of the preprocessor
#define itunesFileSharingEnabled [[[NSBundle mainBundle] \
objectForInfoDictionaryKey:#"UIFileSharingEnabled"] boolValue]
#if itunesFileSharingEnabled
#define something
#endif
becomes
#if [[[NSBundle mainBundle] objectForInfoDictionaryKey:#"UIFileSharingEnabled"] boolValue]
#define something
#endif
During the second run the preprocess tries to validate [[[NSBundle mainBundle] objectForInfoDictionaryKey:#"UIFileSharingEnabled"] boolValue]. Which won't work because this can't be evaluated at compile time. This is a statement that can only be evaluated at run-time.
I don't know if the stuff you want to do is possible at all. (I don't know much about the preprocessor).
I guess you know already that you can do those checks at runtime
if ([[[NSBundle mainBundle] \
objectForInfoDictionaryKey:#"UIFileSharingEnabled"] boolValue]) {
[self foo];
}
else {
[self bar];
}
Another option would be to enforce a manually entered precompiler macro
This goes into your code, somewhere were it is only called once. Probably in the init of your library:
#if HAS_ITUNES_FILESHARING
if (![[[NSBundle mainBundle] objectForInfoDictionaryKey:#"UIFileSharingEnabled"] boolValue]) {
[NSException raise:#"MBFileSharingNotActiveButDeclaredException" format:#"File sharing is not active but \"HAS_ITUNES_FILESHARING\" is set to 1"];
}
#else
if ([[[NSBundle mainBundle] objectForInfoDictionaryKey:#"UIFileSharingEnabled"] boolValue]) {
[NSException raise:#"MBFileSharingActiveButNotDeclaredException" format:#"File sharing is active but you did not declare it as preprocessor macro. Please set \"HAS_ITUNES_FILESHARING\" to 1"];
}
#endif
This has to be entered in the Build Settings as Preprocessor Macro:
HAS_ITUNES_FILESHARING=1
If HAS_ITUNES_FILESHARING=1 is not declared in the preprocessor macro section, but File Sharing is active the code will throw an exception.
If HAS_ITUNES_FILESHARING=1 is declared, but File Sharing is not active, the code will raise an exception too.
This way you can always tell that the preprocessor macro matches the configuration. If you only call this once (in +(void)initialize there should be no performance impact at all.

Answer 2

Even though you can only check for it (in code) at runtime, iTunes File Sharing is something that an app developer sets in development. If you don't want file sharing, don't turn it on.
Presumably you're not actually having trouble with your own left and right hands working at cross purposes, though -- more likely you're in a situation where you're writing code (which wants iTunes File Sharing off) to be used by someone else (who might have it turned on)?
In that case, your best recourse is documentation (or licensing). Communicate to the user of your library that it should be used only in projects without iTunes File Sharing. To put it another way: if you're giving your source code to another programmer, you've surrendered control over what they do with it, but you can still give them recommendations as to how it's best used. (Or have your lawyer get them to agree to only use your software in ways that you specify, but that's no longer a programming question and this isn't a legal advice forum.)
(Aside: you could also consider doing something interesting with the build system: e.g. a shell script build phase that reads the project's main Info.plist. However, if your library is a dependency of the app, it'll need to be built first, which means that plist won't be there for it to read. Again, communication is your best bet.)