#define directive question

Question

Hi,

I've got the following code:

#define checkLiteMessage    \
{   \
    #ifdef LITE_VERSION \
    if (alertView.tag == 100) \
    { \
     if (buttonIndex == 1) \
     [ [UIApplication sharedApplication] openURL: buyAppLink]; \
     [alertView release]; \
     return; \
    } \
    #endif \
}   \

What I want to do is to have the following code to be included every time I call checkLiteMessage:

    #ifdef LITE_VERSION
    if (alertView.tag == 100)
    {
     if (buttonIndex == 1)
     [ [UIApplication sharedApplication] openURL: buyAppLink];
     [alertView release];
     return;
    }
    #endif

What's my problem? Why doesn't this code compile?

Thanks.

Answer 1

You've specified the macro with line continuations, which is correct. However, that means the #ifdef statement is not at the start of the line, so the preprocessor isn't going to do it.

You can't have the #ifdef embedded inside the macro. You could reverse that:

#ifdef LITE_VERSION
#define checkLiteMessage  do {   \
    if (alertView.tag == 100)   \
    {   \
        if (buttonIndex == 1)   \
        [ [UIApplication sharedApplication] openURL: buyAppLink];       \
        [alertView release];    \
        return; \
    }   \ 
} while(0)
#else
#define checkLiteMessage do { ; } while (0)
#endif

I'll add that putting a 'return' statement inside a macro is pretty evil and will be confusing to everyone. Don't do it.

Answer 2

The the backslash-linefeed combination is equivalent to no letters at all so

#define x   y \
   z

is equivalent to

#define x y z

Also, according to the standard, preprocessor directives in a macro body are not expanded or interpreted and that #ifdef is passed to the compiler instead of conditionally dropping the code in between.

You may inspect this behaviour creating a file a.c with the contents

#define x y \
     z

#define v  \
       #ifdef E

x

v

and preprocessing it with the command gcc -E a.c.

In your situation, what you need to do is

 #ifdef LITE_VERSION 
 #define checkLiteMessage    \
 {   \
     if (alertView.tag == 100)   \
     {   \
         if (buttonIndex == 1)   \
         [ [UIApplication sharedApplication] openURL: buyAppLink];       \
         [alertView release];    \
         return; \
     }   \
 }   
 #else
 #define checkLiteMessage
 #endif

Answer 3

The preprocessor is one-pass only. So the sequence #ifdef is getting to your compiler, causing an error. Make it a function.

Answer 4

The problem is that you cannot include preprocessor directives in a macro. You may want to do that instead :

#ifdef LITE_VERSION
#define checkLiteMessage    \
{   \
    if (alertView.tag == 100)   \
    {   \
        if (buttonIndex == 1)   \
        [ [UIApplication sharedApplication] openURL: buyAppLink];       \
        [alertView release];    \
        return; \
    }   \
}
#else
#define checkLiteMessage
#endif

Also make sure that the "return" line does what you think it does (here, it exits the function calling checkLiteMessage, not just the macro (a macro cannot be "exited").