C - typedef

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The 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 definitions, 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 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:

#include <stdio.h>
#include <string.h>
 
typedef struct Books
{
   char  title[50];
   char  author[50];
   char  subject[100];
   int   book_id;
} Book;
 
int main( )
{
   Book book;
 
   strcpy( book.title, "C Programming");
   strcpy( book.author, "Nuha Ali"); 
   strcpy( book.subject, "C Programming Tutorial");
   book.book_id = 6495407;
 
   printf( "Book title : %s\n", book.title);
   printf( "Book author : %s\n", book.author);
   printf( "Book subject : %s\n", book.subject);
   printf( "Book book_id : %d\n", book.book_id);

   return 0;
}

When the above code is compiled and executed, it produces the following result:

Book  title : C Programming
Book  author : Nuha Ali
Book  subject : C Programming Tutorial
Book  book_id : 6495407

typedef vs #define

The #define is a C-directive which is also used to define the aliases for various data types similar to typedef but with following differences:

  • The typedef is limited to giving symbolic names to types only where as #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 statements are processed by the pre-processor.

Following is a simplest usage of #define:

#include <stdio.h>
 
#define TRUE  1
#define FALSE 0
 
int main( )
{
   printf( "Value of TRUE : %d\n", TRUE);
   printf( "Value of FALSE : %d\n", FALSE);

   return 0;
}

When the above code is compiled and executed, it produces the following result:

Value of TRUE : 1
Value of FALSE : 0


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