Question

I’ve got a doubt about assigning values to properties in Objective-C. I’ve had a lot of problems of “misteriously missed” values, for example if I have this interface:

// MyClass.h
@interface MyClass : NSObject {
    MyOtherClass *myVar;
}

@property (nonatomic, retain) MyOtherClass *myVar;

- (id)initWithValue:(NSString *)val;

And this implementation:

// MyClass.m
@implementation MyClass

@synthesize myVar;

- (id)initWithValue:(NSString *)val {
    self = [super init];

    if (self != nil) {
        /** Do some stuff with val... **/
        myVar = [method:val]; // Some method that returns a MyOtherClass value
    }

    return self;
}

At some point of the execution, the value of myVar disappears, changes or something else… And the solution is to change:

myVar = [method:val]; // Some method that returns a MyOtherClass value

for

self.myVar = [method:val]; // Some method that returns a MyOtherClass value

So… what is the diference between using self or not using it? I mean, it’s clear that I’ve to use it because if not it will cause problems but I don’t know why

Thank you in advance

Answer 1

This is a good example of why you should change your @synthesize to @synthesize myVar = _myVar; or some variation of that. Without doing that, it is possible to set your instance variable out from under you as a direct assignment of `myVar = nil” would bypass your setter.

Using the myVar = _myVar strategy will cause the compiler to complain if you attempt to do myVar = and will require you to call self.myVar which will access your setter or getter.

The only place you should then use _myVar is if you change the declaration of your setter or getter for that variable.