#define or #if statement error

Question

I have read many definitions of # if and # define statement.
I tried to use the way I read, but only with the error "Invalid token at start of a preprocessor expression" on the line that defines it as a comment below:

#import <Foundation/Foundation.h>
#import <UIKit/UIKit.h>

#define is_ipad         (UI_USER_INTERFACE_IDIOM() == UIUserInterfaceIdiomPad)
#define is_ipad_hd      ( (is_ipad == 1 && [UIScreen mainScreen].bounds.size.width > 1024 ) ? YES : NO)
#define is_iphone_hd    ([UIScreen mainScreen].bounds.size.width > 480 ? YES : NO)
#define device_width    480.0f
#define device_height   320.0f

#if (is_ipad_hd == YES) // Error here
  #define device_width       = 2048.0f
  #define device_height      = 1496.0f
#endif

Why it works in simple tutorials and when we try something more complex these things happen!

Answer 1

These are preprocessor directives, so you don't have access to [UIScreen mainScreen] methods and all other objects that are defined upon compilation!

Answer 2

Macro evaluation happens at compile time.

However, (UI_USER_INTERFACE_IDIOM() == UIUserInterfaceIdiomPad) or [UIScreen mainScreen] can only be interpreted at run time.

Beyond that, you should not be relying on the screen size to do your dirty work for you. You should instead be relying on the user interface idiom and the scale of the main screen:

BOOL isiPad = UI_USER_INTERFACE_IDIOM() == UIUserInterfaceIdiomPad;
BOOL isHD = [[UIScreen mainScreen] scale] == 2.0;

if (isiPad) {
  if (isHD) {
    // retina iPad
  } else {
    // non-retina iPad
  }
} else {
  if (isHD) {
    // retina iPhone/iPod touch
  } else {
    // non-retina iPhone/iPod touch
  }
}