I do not understand ways of declaring instance variable in the code

Question

I do not quite understand the way of declaring instance variable and property. Can someone explain in detail the difference of the two codes below? In the second method, if I use _name for instance variable, is it the same function as the way declaring name in first code? Thanks!

First Code:

//  OrderItem.h
#import <Foundation/Foundation.h>

@interface OrderItem : NSObject

{

  @public NSString *name;

}

-(id) initWithItemName: (NSString *) itemName;

@end


//  OrderItem.m
#import "OrderItem.h"

@implementation OrderItem

-(id) initWithItemName: (NSString *) itemName {

     self = [super init];

      if (self) {

          name = itemName;

         NSLog(@"Initializing OrderItem");
   }

     return  self;

   }

@end

Second Code:

//  OrderItem.h
#import <Foundation/Foundation.h>

@interface OrderItem : NSObject

 @property (strong,nonatomic) NSString *name;

-(id) initWithItemName: (NSString *) itemName;

@end


//  OrderItem.m
#import "OrderItem.h"

@implementation OrderItem

-(id) initWithItemName: (NSString *) itemName {

     self = [super init];

      if (self) {

          _name = itemName;

         NSLog(@"Initializing OrderItem");
   }

     return  self;

   }

@end

Not an exact duplicate (though I think there is one; need to find it), but see stackoverflow.com/questions/12899305/… — Rob Napier
– Rob Napier, Commented Aug 7, 2016 at 13:26
Mike there is lot of answer for your question in Stackoverflow.Even I can give answer now but everywhere you can see the solution according to your question. — user3182143
– user3182143, Commented Aug 7, 2016 at 13:50

Sulthan · Accepted Answer · 2016-08-07 13:31:05Z

1

In the first case you have declared an instance variable (usually called an ivar in Objective-C).

In the second case you have declared a property. A property is a set of two methods, a getter and a setter, usually accessed using dot notation, e.g. self.name. However, an ivar is automatically synthesized for the property with the name _name. That instance variable is what you are accessing in your init.

You can actually change the name of the ivar using @synthesize name = _myName or not have it at all (if you declare the getter and setter manually, no ivar will be synthesized).

Objective-C properties are a rather complicated topic so don't worry if you don't understand it immediately.

answered Aug 7, 2016 at 13:31

Sulthan

130k23 gold badges223 silver badges277 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

Mike Over a year ago

Hi Sulthan, thank you very much! In practice, I prefer the second way, so if I code in the second way, there should be no problem, right?

Sulthan Over a year ago

@Mike Properties are generally preferred over instance variables.

Mike Over a year ago

@Sulthan Hi Sulthan, got it. I have accepted your answer. Have a nice Sunday!

Marcel Voss · Accepted Answer · 2016-08-07 14:18:19Z

1

Properties are public which means that other classes can read and write them (even classes that aren't subclasses of the class that declares the property). In addition to that, properties provide a getter and a setter method (mutator methods). The getter of a property gets called every time you access the property

NSString *aName = self.name;

Whereas the setter is accessed every time you write or assign to a property.

self.name = @"Some name";

Instance variables (or ivars) are, by default, only visible for the class that declares it and its subclasses (also known as being encapsulated by their class). You can change this default behavior when you add the keyword @public to your ivar declaration though.

edited Aug 7, 2016 at 14:18

answered Aug 7, 2016 at 13:34

Marcel Voss

3902 silver badges9 bronze badges

9 Comments

Marcel Voss Over a year ago

@user3182143 Thanks! I hope my answer helped you a bit.

Mike Over a year ago

Hi Marcel, thanks a lot! According to the function of the code, in practice both ways have no problem, right?

Sulthan Over a year ago

Well, ivars are public too. Or better said, there is no concept of public or private in Objective-C. If you declare your ivars in the header, they are accessible externally, too. There are only accessed using the -> notation.

Marcel Voss Over a year ago

@Mike Yes, you can use both ways. Although, I would recommend to use properties only if it's necessary since they are public to other classes as long as they are declared in your header. You shouldn't disclose too much information to other classes without having to.

Sulthan Over a year ago

@MarcelVoss There is absolutely zero difference between ivars and properties when speaking about accessibility.

|

Collectives™ on Stack Overflow

I do not understand ways of declaring instance variable in the code

2 Answers 2

3 Comments

9 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

3 Comments

9 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related