Which conditional compile to use to switch between Mac and iPhone specific code?

Question

I am working on a project that includes a Mac application and an iPad application that share code. How can I use conditional compile switches to exclude Mac-specific code from the iPhone project and vice-versa? I've noticed that TARGET_OS_IPHONE and TARGET_OS_MAC are both 1, and so they are both always true. Is there another switch I can use that will only return true when compiling for a specific target?

For the most part, I've gotten the files to cooperate by moving #include <UIKit/UIKit.h> and #include <Cocoa/Cocoa.h> into the precompile headers for the two projects. I'm sharing models and some utility code that fetches data from RSS feeds and Evernote.

In particular, the [NSData dataWithContentsOfURL:options:error:] function takes a different constant for the options parameter iOS 3.2 and earlier and Mac OS 10.5 and earlier than it does for iOS 4 and Mac OS 10.6. The conditional I'm using is:

#if (TARGET_OS_IPHONE && (__IPHONE_OS_VERSION_MAX_ALLOWED > __IPHONE_3_2)) || (TARGET_OS_MAC && (MAC_OS_X_VERSION_MIN_REQUIRED > MAC_OS_X_VERSION_10_5))

This seems to work, but I want to make sure this is bulletproof. My understanding is that if the Mac version is set to 10.6, but the iOS version is set to 3.2, it will still use the new constants even if it's compiling for iOS 3.2, which seems incorrect.

Thanks in advance for any help!

Answer 1

"The correct thing to do is just use the newer constants, because if you look at the header you will see they are declared equivalent to the old ones in the enum, which means the new constants will work even on the old releases (both constants compile to the same thing, and since enums are compiled into the app they can't change without breaking binary compatibility). The only reason not to do that is if you need to continue building agains the older SDKs (that is a different thing than supporting older releases, which you can do while compiling against the newer SDKs).

If you actually wanted to use different flags based on the OS version (because the new version actually added new functionality, as opposed to just renaming a constant) then there are two sensible things you can do, neither of which your above macro accomplishes:

1. To always use the old flags unless the min version allowed is greater than version they were introduced in (something like this):

   #if (__IPHONE_OS_VERSION_MIN_REQUIRED >= 40000 || __MAC_OS_X_VERSION_MIN_REQUIRED >= 1060)
     NSDataReadingOptions  options = NSDataReadingMapped;
   #else
     NSDataReadingOptions  options = NSMappedRead;
   #end
   

2. Conditionally use only the new values in builds that can on only the new versions, and compile in code to determine the flags at runtime for builds that support both versions:

   #if (__IPHONE_OS_VERSION_MIN_REQUIRED >= 40000 || __MAC_OS_X_VERSION_MIN_REQUIRED >= 1060)
     NSDataReadingOptions  options = NSDataReadingMapped;
   #else
     NSDataReadingOptions  options;
     if ([[UIDevice currentDevice] systemVersion] compare:@"4.0"] != NSOrderedAscending) {
        options = NSDataReadingMapped;
     } else {
       options = NSMappedRead;
     }
   #end
   

Note that if you actually were doing this comparison a lot you would want to stash the result of the [[UIDevice currentDevice] systemVersion] compare:@"4.0"] somewhere. You also generally want to explicitly test for features using things like weak linking instead of doing version compares, but that is not an option for enums.

Answer 2

You've made a mistake in your observations. :)

TARGET_OS_MAC will be 1 when building a Mac or iPhone application. You're right, it's quite useless for this sort of thing.

However, TARGET_OS_IPHONE is 0 when building a Mac application. I use TARGET_OS_IPHONE in my headers all the time for this purpose.

Here's a great chart on this: http://sealiesoftware.com/blog/archive/2010/8/16/TargetConditionalsh.html