Instead of using self.factbook, cant we create an instance of factbook class and use the object to access its properties

the advantage of using self.factbook is that you only have to instantiate it once and you can use it all over the controller.

It is the same with any instance right ? If i were to create an instance of a class i can use that in the whole class.. correct ?

factbook * obj = [[factbook alloc]init // in this case i can use "obj" just like self.facbook ?

It works for any object.

you can't use it like that. the factbook instance in obj will only be locally within the method (in that scope) you define it. That is why you want to have a property in your class.

to make things clear, here is an example of a viewcontroller:

#import "ViewController.h"

@interface MyViewController : ViewController

@property NSString *property;

@end

#import "MyViewController.h"

@interface MyViewController ()

@end

@implementation MyViewController
- (void)viewDidLoad {
    [super viewDidLoad];

    NSString *property = @"value1"; // only available in this scope
    self.property = @"value2"; // available within the viewcontroller

    NSLog(@"property: %@", property); // output: "property: value1"
    NSLog(@"self.property: %@", self.property); // output: "self.property: value2"
}

- (void)customMethod
{
    NSLog(@"property: %@", property); // generates undefined identifier 'property' error
    NSLog(@"self.property: %@", self.property); // output: "self.property: value2"
}

Thank you and i appreciate your effort in explaining it.....instead of creating a new object in a method if I create it outside then the scope of the object is whole class right, isn't the below code valid ?

@implementation MyViewController

NSString * obj = @"value 1";

• (void)viewDidLoad { [super viewDidLoad];

  self.property = @"value2"; // available within the viewcontroller

  NSLog(@"property: %@", obj); // output: "property: value1" NSLog(@"self.property: %@", self.property); // output: "self.property: value2" }

• (void)customMethod { NSLog(@"property: %@", obj); // displays value 1 NSLog(@"self.property: %@", self.property); // output: "self.property: value2" }

Yes your code is valid, but I would advice against this way of declaring "private" properties.

You can declare "private" properties in 2 ways, i've commented them in the example below.

// MyViewController.m

#import "MyViewController.h"

@interface MyViewController ()
// declare "private" properties here
@end

@implementation MyViewController
{
// or declare "private" properties here
NSString *_property;
}

- (void)viewDidLoad {
    [super viewDidLoad];

    _property = @"value1";

    NSLog(@"property: %@", _property); // output: "property: value1"
    NSLog(@"self.property: %@", self.property); // output: "self.property: value2"
}

- (void)customMethod
{
    NSLog(@"property: %@", _property); // output: "property: value1"
    NSLog(@"self.property: %@", self.property); // output: "self.property: value2"
}