Structures - Objective C

Objective C array allow you to define type of variables that can hold several data item of the the same kind but structure is another user-defined data type available in Objective C programming which allows you to combine data items of different kinds.

Structures are used to represent a record , Suppose you want to keep track of your book in a library. You might want to track the following attributes about each book:

• Title
• Author
• Subject
• Book ID

Defining a Structure

To define a structure, you must use the struct statement. The struct statement defines a new data type, with more than one member for your program. The format of the struct statement is this:

struct [ structure tag]
{
         member definition;
         ..
         ..
}[one or more structure variables];

The structure tag is optional and each member definition is normal variable definition, such as int i, or float f, or any other valid variable definition. At the end of the structure's definition, before the final semicolon, you can specify one or more structure variables but it is optional. Here is the way you would declare the book structure.

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

Accessing Structure Members

To access any member of a structure, we use the member access operator(.). The member access operator is coded as a period as a period between the structure variable name and the structure member that we wish to access. You would use struct keyword to define variables of structure type. Following is the example to explain usage of structure.
 

#import <Foundation/Foundation.h>

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

int main()
{
         struct Books Book1;                  //declare Book1 of type Books
         struct Books Book2;
 
          //book 1 specification
          Book1.title = @"Objective C programming";
          Book1.author = @"Rensho";
          Book1.subject = @"Objective C programming tutorail";
          Book1.book_id = 135454;
   
          //book 2 specification
         Book2.title = @"Dot net ";
          Book1.author = @"Remo";
          Book1.subject = @"Dot net programming tutorail";
          Book1.book_id = 135456;

          //print Book1 info
         NSLog(@"Book 1 title :%@\n", Book1.title);
         NSLog(@"Book 1 author :%@\n", Book1.author);
         NSLog(@"Book 1 subject :%@\n", Book1.subject);
         NSLog(@"Book 1 book_id :%@\n", Book1.book_id);

         NSLog(@"Book 2 title :%@\n", Book2.title);
         NSLog(@"Book 2 author :%@\n", Book2.author);
         NSLog(@"Book 2 subject :%@\n", Book2.subject);
         NSLog(@"Book 2 book_id :%@\n", Book2.book_id);

         return 0;
 }

Structures as Function Arguments

You can pass a structure as a function in very similar way as you pass any other variable or pointer.You would access structure variables in the similar way as you have accessed in the above example:

#import <Foundation/Foundation.h>

struct Books
{
       NSString *title;
       NSString *author;
       NSString *subject;
       int  book_id;
};
@interface SampleClass : NSObject

                - (void)printBook : (struct Books) book ;

@end

@implementation SampleClass

     - (void) printBook : (struct Books)book
{
         NSLog(@"Book  title :%@\n", book.title);
         NSLog(@"Book  author :%@\n", book.author);
         NSLog(@"Book  subject :%@\n", book.subject);
         NSLog(@"Book  book_id :%@\n", book.book_id);
       
}
@end

int main()
{
         struct Books Book1;                  //declare Book1 of type Books
         struct Books Book2;
 
          //book 1 specification
          Book1.title = @"Objective C programming";
          Book1.author = @"Rensho";
          Book1.subject = @"Objective C programming tutorail";
          Book1.book_id = 135454;
   
          //book 2 specification
         Book2.title = @"Dot net ";
          Book1.author = @"Remo";
          Book1.subject = @"Dot net programming tutorail";
          Book1.book_id = 135456;

         SampleClass  *obj = [[SampleClass alloc]init];

         [obj printBook : Book1];

         [obj printBook : Book2]
          return 0;
 }

Pointer to Structures

You can define pointer to structures in very similar way as you define pointer to any other variable as follows:

struct Books *struct_pointer;

Now, you can store the address of a structure variable in the above-defined pointer variable. To find the address of a structure variable, place the & operator before the structure's as follows:
 

struct_pointer = &book1;

To access the member of a structure using a pointer to that structure, you must use the ->operator as follows:

struct_pointer ->title;

let us rewrite above example using structure pointer, hope this will be easy for you to understand the concept:

#import <Foundation/Foundation.h>

struct Books
{
       NSString *title;
       NSString *author;
       NSString *subject;
       int  book_id;
};
@interface SampleClass : NSObject

                - (void)printBook : (struct Books *) book ;

@end

@implementation SampleClass

     - (void) printBook : (struct Books *)book
{
         NSLog(@"Book  title :%@\n", book->title);
         NSLog(@"Book  author :%@\n", book->author);
         NSLog(@"Book  subject :%@\n", book->subject);
         NSLog(@"Book  book_id :%@\n", book->book_id);
       
}
@end

int main()
{
         struct Books Book1;                  //declare Book1 of type Books
         struct Books Book2;
 
          //book 1 specification
          Book1.title = @"Objective C programming";
          Book1.author = @"Rensho";
          Book1.subject = @"Objective C programming tutorail";
          Book1.book_id = 135454;
   
          //book 2 specification
         Book2.title = @"Dot net ";
          Book1.author = @"Remo";
          Book1.subject = @"Dot net programming tutorail";
          Book1.book_id = 135456;

         SampleClass  *obj = [[SampleClass alloc]init];

         [obj printBook : &Book1];

        [obj printBook : &Book2];

        return 0;

}

 Bit Fileds
Bit Fields allow the packing of data in a structure. This is especially useful when memory or data storage is at a premium. Typical example:

• Packing several objects into a machine e.g. 1 bit flags can be compacted.
• Reading external file formats - non standard file format could be read in E.g 9 bit integers.

Objective C allow us do this in a structure definition by putting : bit length after the variable. For example:

#import <Foundation/Foundation.h>

struct packed_struct
{
       unsigned int f1 : 1 ;
       unsigned int f2 : 1 ;
       unsigned int f3 : 1 ;
       unsigned int f4 : 1 ;
       unsigned int type : 4 ;
       unsigned int my_int : 9 ;
}pack;

Here, the packed_struct contains 6 members: Four 1 bit flags f1,f3,a 4 bit type and a 9 bit my_int.

Objective C automatically packs the above bit fields as compactly as possible, provided that the max length of the field is less than or equal to the integer word length of the computer. If this is not the case, then some compiler may allows memory overlap for the fields whilst other would store the next field in the next word.