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).

Answer 2

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.