Difference between self.object and _object

Question

In Objective-C, what is the difference between calling an object with the underscore (calling instance variable) or calling it with the self keyword? As an example, I setup a simple .h and .m file called Methods that inherits from NSObject.h.

Contents of Methods.h

#import <Foundation/Foundation.h>

@interface Methods : NSObject

@property int firstNumber;
@property int secondNumber;

@end

Contents of Methods.m

#import "Methods.h"

@implementation Methods

- (instancetype)init
{
    self = [super init];
    if (self) {
        _firstNumber = 33;
        self.secondNumber = 67;

        NSLog(@"The value of _firstNumber is: %i", _firstNumber);
        NSLog(@"The value of self.secondNumber is: %i", self.secondNumber);
    }
    return self;
}

@end

This is the result in the command line when this code is executed:

2014-07-07 20:22:08.551 CommandLine[2771:39622] The value of _firstNumber is: 33
2014-07-07 20:22:08.552 CommandLine[2771:39622] The value of self.secondNumber is: 67

This is the expected result, as the values have the numeric value that they were set to, and I've developed using both the underscore and the self keyword to call and set objects, so what makes one better than another? Or is it a case of personal preference and custom?

Also, see that http://stackoverflow.com/questions/9086736/why-would-you-use-an-ivar — Vinzzz, Jul 08 '14 at 00:48

score 1 · Accepted Answer · answered Jul 08 '14 at 00:36

1

As you mentioned one is an instance variable and the other is a property. In the earlier days of Objective-C there were different scenarios in which one would be better. Now they both act very similar and it is mainly a preference choice. There are arguments over which one is better but it really shouldn't matter which one you're using.

answered Jul 08 '14 at 00:36

Dando18

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However Xcode suggests _ form in its tooltips. – Szymon Fortuna Jul 08 '14 at 00:39
@SzymonFortuna that's just apple not changing from their old ways. Don't worry about it – Dando18 Jul 08 '14 at 00:48
it's just that they offer me one button fix when changing from variable in a method to the property this way – Szymon Fortuna Jul 08 '14 at 01:15

Difference between self.object and _object

1 Answers1