Previouslywe learn how to use directives in ObjC: Objective-C : Property and Synthesize directive 

Objective-C arrays allow you to define type of variables thatcan hold several data items of the same kind but structure is anotheruser-defined data type available in Objective-C programming which allows you tocombine data items of different kinds.

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

• Title
• Author
• Subject
• Book ID

Defininga Structure

To define astructure, you must use the struct statement. The struct statementdefines a new data type, with more than one member for your program. The formatof the struct statement is this:

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

The structuretag is optional and each member definition is a normal variable definition,such as int i; or float f; or any other valid variable definition. At the endof the structure's definition, before the final semicolon, you can specify oneor more structure variables but it is optional. Here is the way you woulddeclare the Book structure:

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

Structure (structs)

Structs area C construct that allows for grouping of items into one variable. For example,if you wanted to store a date, you could use three separate variables for theday, month, and year. This approach would work if you had to store one or twodates. But if you’re writing a calendar application, for example, having tokeep track of which month belonged with which year would be a nightmare.Fortunately, C structs allow you to group these values into one type. Thesyntax for a date struct might look like this:

struct date {int month; int day; int year;};

You can thendefine a variable of this type as struct date today, yesterday;. However, wecan use our friend the typedef statement to make this code a bit neater:

typedef struct {int month; int day; int year;} Date;
Date today, yesterday; 

To accessvalues within a struct, you use the dot operator. To set the month, you mightwrite today.month = 5;. To get a value, the same procedure applies: int year =today.year;.

In action,it might look like this:

#import <Foundation/Foundation.h>
int main (int argc, char *argv[]){
    NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
    
    typedef struct {int month; int day; int year;} Date;
    Date today;
    today.month = 5;
    today.day = 22;
    today.year = 2011;
    NSLog(@"Today's date is %i/%i/%i", today.month, today.day, today.year);
    [pool drain];
    return 0;
}

After execution the result will look like this:

2015-07-13 04:36:58.342 demo[11911] Today's date is 5/22/2011

InitializingStructures

The fastestway to initialize a structure is to enclose the values in a pair of braces:

Date today = {5,22, 2011};You can list fewer values, and they will be filled inuntil the end of the values you list. The remaining values will be undefined.

Date today ={5};This sets the month to 5, but leaves the remaining fields withgarbage values from RAM.

You can alsoinitialize the values out of order:

Date today ={.day = 22, .month = 5; .year = 2011};

StructsWithin Structs

C allows youto put structs with structs. For example, Apple provides the CGRect data type,which defines a data type by being a struct containing a CGPoint and CGSize:

struct CGRect {
    CGPoint origin;
    CGSize size;
};
typedef struct CGRect CGRect;

You can thenaccess the values by chaining the dot operator. To get the width, you wouldwrite float width =rect.size.width; 

Next, we will learn about: Objective-C : Unions