Objective C Tutorial - Objective C Preprocessors

Preprocessor is taking effect before the real compiling.

A preprocessor is a text substitution step happended before actual compilation.

All preprocessor commands begin with a pound symbol #.

The following table lists the often used preprocessor directives.

Directive Description
#define define a preprocessor macro
#include Inserts a particular header from another file
#undef Undefines a preprocessor macro
#ifdef Returns true if this macro is defined
#ifndef Returns true if this macro is not defined
#if Tests if a condition is true
#else The alternative for #if
#elif #else #if in one statement
#endif Ends preprocessor conditional
#error Prints error message on stderr
#pragma Issues special commands to the compiler


The following line defines MAX_LENGTH as 20.

#define MAX_LENGTH 20

After this definition we can have code like

int i = 20*MAX_LENGTH;

When the compiler sees the MAX_LENGTH it will replace it with 20, which is defined in the #define.

The following directives load foundation.h from Foundation Framework and add the text to the current source file and then load myheader.h from the local directory and add the content to the current source file.

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

The following two lines first undefine a value and then define that value.

#undef  FILE_SIZE
#define FILE_SIZE 42

The following line defines MESSAGE only if MESSAGE isn't already defined.

#ifndef MESSAGE
   #define MESSAGE "You wish!"

Predefined Macros

The following table lists existing macros.

Macro Description
__DATE__ The current date as a character literal in "MMM DD YYYY" format
__TIME__ The current time as a character literal in "HH:MM:SS" format
__FILE__ This contains the current filename as a string literal.
__LINE__ This contains the current line number as a decimal constant.
__STDC__ Defined as 1 when the compiler complies with the ANSI standard.
#import <Foundation/Foundation.h>

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

Example 2

The following code uses preprocessor to create a function like macro.

#import <Foundation/Foundation.h>

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

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

The following code uses preprocessor to find out the bigger value.

#import <Foundation/Foundation.h>

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

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