Question

Code below yields the output “yes defined”, “no defined” and “yes”. Why?

#define FOOBAR NO
- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
#ifdef YES
    NSLog(@"yes defined");
#endif

#ifdef NO
    NSLog(@"no defined");
#endif

#if FOOBAR == YES
    NSLog(@"yes");
#else
    NSLog(@"no");
#endif
    // ...
}

YES and NO are not undefined, objc.h defines them as:

typedef signed char     BOOL;
#define YES             (BOOL)1
#define NO              (BOOL)0

Answer 1

What is the value of NO? If it’s undefined (like YES), they will both evaluate to 0.

This means your expression is essentially

#if 0 == 0

which is of course true, and thus causes the first call to be compiled.

UPDATE: Not sure how BOOL is defined, but casting to what might be a typedef:ed type is not a very good idea when dealing with the preprocessor. Remember that the the #if is evaluated by the preprocessor, not by the compiler. Read something like this for more information about expressions in the preprocessor. Especially:

The preprocessor does not know anything about types in the language.