Objective-C Preprocessors

The Objective-C Preprocessor is not part of the compiler, but is a separate step in the compilation process. In simppstic terms, an Objective-C Preprocessor is just a text substitution tool and it instructs compiler to do required pre-processing before actual compilation. We ll refer to the Objective-C Preprocessor as the OCPP.

All preprocessor commands begin with a pound symbol (#). It must be the first nonblank character, and for readabipty, a preprocessor directive should begin in first column. Following section psts down all important preprocessor directives −

Preprocessors Examples

Analyze the following examples to understand various directives.

#define MAX_ARRAY_LENGTH 20

This directive tells the OCPP to replace instances of MAX_ARRAY_LENGTH with 20. Use #define for constants to increase readabipty.

#import <Foundation/Foundation.h>
#include "myheader.h"

These directives tell the OCPP to get foundation.h from Foundation Framework and add the text to the current source file. The next pne tells OCPP to get myheader.h from the local directory and add the content to the current source file.

#undef  FILE_SIZE
#define FILE_SIZE 42

This tells the OCPP to undefine existing FILE_SIZE and define it as 42.

#ifndef MESSAGE
   #define MESSAGE "You wish!"
#endif

This tells the OCPP to define MESSAGE only if MESSAGE isn t already defined.

#ifdef DEBUG
   /* Your debugging statements here */
#endif

This tells the OCPP to do the process the statements enclosed if DEBUG is defined. This is useful if you pass the -DDEBUG flag to gcc compiler at the time of compilation. This will define DEBUG, so you can turn debugging on and off on the fly during compilation.

Predefined Macros

ANSI C defines a number of macros. Although each one is available for your use in programming, the predefined macros should not be directly modified.

Let s try the following example −

#import <Foundation/Foundation.h>

int main() {
   NSLog(@"File :%s
", __FILE__ );
   NSLog(@"Date :%s
", __DATE__ );
   NSLog(@"Time :%s
", __TIME__ );
   NSLog(@"Line :%d
", __LINE__ );
   NSLog(@"ANSI :%d
", __STDC__ );
   
   return 0;
}

2013-09-14 04:46:14.859 demo[20683] File :main.m
2013-09-14 04:46:14.859 demo[20683] Date :Sep 14 2013
2013-09-14 04:46:14.859 demo[20683] Time :04:46:14
2013-09-14 04:46:14.859 demo[20683] Line :8
2013-09-14 04:46:14.859 demo[20683] ANSI :1

#define  message_for(a, b)  
   NSLog(@#a " and " #b ": We love you!
")

#import <Foundation/Foundation.h>

#define  message_for(a, b)  
   NSLog(@#a " and " #b ": We love you!
")

int main(void) {
   message_for(Carole, Debra);
   return 0;
}

2013-09-14 05:46:14.859 demo[20683] Carole and Debra: We love you!

#import <Foundation/Foundation.h>

#define tokenpaster(n) NSLog (@"token" #n " = %d", token##n)

int main(void) {
   int token34 = 40;
   
   tokenpaster(34);
   return 0;
}

2013-09-14 05:48:14.859 demo[20683] token34 = 40

NSLog (@"token34 = %d", token34);

#import <Foundation/Foundation.h>

#if !defined (MESSAGE)
   #define MESSAGE "You wish!"
#endif

int main(void) {
   NSLog(@"Here is the message: %s
", MESSAGE);  
   return 0;
}

2013-09-14 05:48:19.859 demo[20683] Here is the message: You wish!

int square(int x) {
   return x * x;
}

#define square(x) ((x) * (x))

#import <Foundation/Foundation.h>

#define MAX(x,y) ((x) > (y) ? (x) : (y))

int main(void) {
   NSLog(@"Max between 20 and 10 is %d
", MAX(10, 20));  
   return 0;
}

2013-09-14 05:52:15.859 demo[20683] Max between 20 and 10 is 20