Typedef - Objective C

The Objective C programming language provides a keyword called typedef, which you can use to give a type a new name. Following is an example to define a term BYTE for one-byte numbers.

typedef unsigned char BYTE;

After this type definition, the identifier BYTE can be used as an abbreviation for the type unsigned char, for example:

BYTE b1, b2;

By convention, uppercase letters are used for these definitions to remind the user that the type name is really a symbolic abbreviation, but you can use lowercase, as follows:

typedef unsigned char byte;

 You can use the typedef to give a name to user-defined data type as well. For example, you can use typedef with structure to define a new data type and then use that data type to define structure variables directly as follows:

#import <Foundation/Foundation.h>

typedef struct Books
{
            NSString *title;
            NSString  *author;
            NSString  *subject;
           int book_id;
} Book;

int main( )
{
         Book book1;
          book1.title = @"Objective C programming";
          book1.author = @"Rjtenos ";
          book1.subject = @"Programming tuts";
          book1.book_id = 100;
    
         NSLog(@"Book title : %@\n", book1.title);
         NSLog(@"Book title : %@\n", book1.author);
         NSLog(@"Book title : %@\n", book1.subject);
         NSLog(@"Book title : %d\n", book1.book_id);
   
         return 0;
}

typedef vs #define
The #define is a objective C directives, which is also used the aliases for various data types similar to typedef but with following difference.

• The typedef is limited to giving symbolic names to types only whereas #define can be used to define alias for values as well, like you can define 1 as ONE etc.
• The typedef interpretation is performed by the  compiler where as #define elemenets are processed by the preprocessor.

 Following  is a simplest usage of #define.
 

#import <Foundation/Foundation.h>

#define TRUE 1
#define FALSE 0

int main( )
{
          NSLog(@"Value of TRUE : %d\n", TRUE);         NSLog(@"Value of FALSe : %d\n", FALSE);
  
         return 0;
}