what is the difference between self and underscore objective c

Hi Fahad,

In Objective-C you need to initialize your class variables by hand as the compiler doesn't infer data types for the values you give them which can lead to unexpected results as a nil value could occur resulting in an app crash. To solve this we use a variable initialization which is where our friend the underscore comes into effect.

The underscore refers to a memory pointer that the Objective-C compiler assigns to the class during run-time, within our variable initialization we have access to this (magic) variable so we can assign the initial value to it otherwise return it if the variable has already being assigned a value other than nil which occurs when using self.variableName.

So as an overview this is what happens.

self.variableName -> _variableName

Hope that clears things up.

Thanks for answer , so can i say i use , _myVar if i want assign value to myVar and self.myVar if i want to only read that value ?

thanks Chris but still i will show you my example , I'm following sprite kit tutorial and i have this case

header file .h

 #import <SpriteKit/SpriteKit.h>

@interface SpeacCatNod : SKSpriteNode
+(instancetype)speaceCatAtPosition:(CGPoint)position;
-(void) preformTap;
@end    

And this is implementation file .m

@implementation SpeacCatNod
+(instancetype)speaceCatAtPosition:(CGPoint)position;
{

    SpeacCatNod*speaceCat =[self spriteNodeWithImageNamed:@"spacecat_1"];
    speaceCat.position=position;

    speaceCat.anchorPoint=CGPointMake(0.5, 0);
    speaceCat.name=@"SpeacCat";
    return speaceCat;
}

-(void) preformTap{

    [self runAction:self.tapAction];

}

- (SKAction *) tapAction {


    if ( _tapAction != nil )
    {

        return _tapAction;

    }

    NSArray *textures = @[[SKTexture textureWithImageNamed:@"spacecat_2"],
                          [SKTexture textureWithImageNamed:@"spacecat_1"]];

    _tapAction = [SKAction animateWithTextures:textures timePerFrame:0.25];

    return _tapAction;

}
@end     

look at preformTap method , you will find that teacher using self to return the value and on tapAction method he used _tapAction and if i change preformTap method to _ or i change tapAction method to self , not working , if possible explain me what happen in my example .

And thanks again Man !