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

Example

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!"
#endif

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;
}