Preprocessor macro and BOOL weirdness

Question

Code below yields the output \"yes defined\", \"no defined\" and \"yes\". Why?

#define FOOBAR NO
- (BOOL)application:(UIApplication *)application didFinishLa         


        

Answer 1

All identifieres that the preprocessor doesn't know of are replaced with 0 for evaluation in #if directives. If you don't have defined YES and NO both are 0 (and thus equal).