#ifdef doesn't work. But why? - iphone

Question

#ifdef doesn't work. But why?
CGFloat maxScale;
if ( [[UIScreen mainScreen] respondsToSelector: #selector (scale)] == YES )
{
NSLog (#"case1");
#define GLOBAL1
}
else
{
NSLog (#"case2");
#undef GLOBAL1
}
#ifdef GLOBAL1
NSLog (#"first");
maxScale = 1.0 / [[UIScreen mainScreen] scale];
#else
NSLog (#"second");
maxScale = 1.0;
#endif
#undef GLOBAL1
my log:case1, second. But it must be case1, first.

Answer 1

#define, #ifdef are pre-processor macros/conditionals. That means that the logic contained in them is compiled before your code is compiled. It is not actually part of your code.
See this guide for learning what pre-processor macros/conditionals are and do.
[EDIT]
This is what your pre-processor sees when it reads your code.
#define GLOBAL1
#undef GLOBAL1
#ifdef GLOBAL1
//...
#else
//...
#endif
#undef GLOBAL1
it IGNORES all other code and logic.
This is the actual code output the compiler makes:
if ( [[UIScreen mainScreen] respondsToSelector: #selector (scale)] == YES )
{
NSLog (#"case1");
}
else
{
NSLog (#"case2");
}
// because the pre-processor #undef GLOBAL1
NSLog (#"second");
maxScale = 1.0;
The pre-processor code is "executed" telling the compiler how to compile, and will not be used or run during run-time.
Hope that helps!

Answer 2

The preprocessor does not care that the #define is inside a coded if statement - it is processed before the code and only cares about other preprocessor definitions. You can't use #defines and other preprocessor commands (such as #undef) as code- they will not be hit each time the code enters the conditional branches.