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 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 book; book.title = @"Objective-C Programming"; book.author = @"TutorialsPoint"; book.subject = @"Programming tutorial"; book.book_id = 100; NSLog( @"Book title : %@n", book.title); NSLog( @"Book author : %@n", book.author); NSLog( @"Book subject : %@n", book.subject); NSLog( @"Book Id : %dn", book.book_id); return 0; }
When the above code is compiled and executed, it produces the following result −
2013-09-12 12:21:53.745 demo[31183] Book title : Objective-C Programming 2013-09-12 12:21:53.745 demo[31183] Book author : TutorialsPoint 2013-09-12 12:21:53.745 demo[31183] Book subject : Programming tutorial 2013-09-12 12:21:53.745 demo[31183] Book Id : 100
typedef vs #define
The #define is a Objective-C directive, which is also used to define the aliases for various data types similar to typedef but with following differences −
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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.
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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 −
#import <Foundation/Foundation.h> #define TRUE 1 #define FALSE 0 int main( ) { NSLog( @"Value of TRUE : %dn", TRUE); NSLog( @"Value of FALSE : %dn", FALSE); return 0; }
When the above code is compiled and executed, it produces the following result −
2013-09-12 12:23:37.993 demo[5160] Value of TRUE : 1 2013-09-12 12:23:37.994 demo[5160] Value of FALSE : 0