Not clear why init methods gets called on some class

Question

again I am reading a book and there is such implementation of class:

@implementation BNRItemStore


// Init method
- (id)init
{
    self = [super init];

    if(self)
    {
        allItems = [[NSMutableArray alloc] init];
    }

    return self;

}

#pragma mark singleton stuff

// Implementing singleton functionality
+(BNRItemStore*) sharedStore
{
    static BNRItemStore *sharedStore = nil;

    // Check if instance of this class has already been created via sharedStore
    if(!sharedStore)
    {
        // No, create one
        sharedStore = [[super allocWithZone:nil] init];
    }

    return sharedStore;

}

// This method gets called by alloc
+ (id)allocWithZone:(NSZone *)zone
{
    return [self sharedStore];
}

#pragma mark Methods

// return pointer to allItems
-(NSArray*) allItems
{
    return allItems;
}

// Create a random item and add it to the array. Also return it.
-(BNRItem*) createItem
{
    BNRItem *p = [BNRItem randomItem];
    [allItems addObject:p];
    return p;
}

@end

The thing I find strange is that Nowhere outside the class, e.g., some other class, is the init method of BNRItemStore called. However, still by some means it gets called, even when someone has typed such code outside the BNRItemStore class:

    [[BNRItemStore sharedStore] createItem]; // This still calls the init method of BNRItemStore. How?

Can someone please explain why?

score 1 · Answer 1 · answered Jul 29 '13 at 10:44

1

sharedStore = [[super allocWithZone:nil] init];

This line is responsible for the init call. First time entering sharedStore the sharedStore variable is nil, so the condition check fails and an instance of the class is initialized.

answered Jul 29 '13 at 10:44

Alexander

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Hi Alexander, that was not my question, I know the program enters the if clause because sharedStore is nil. The question is that apparently by calling `init` on `[super allocWithZone:nil]` calls `init` of the `BNRItemStore` class, eventhough `super allocWithZone` was used... do you get my point? – user2054339 Jul 29 '13 at 10:48

score 0 · Answer 2 · answered Jul 29 '13 at 10:45

-init gets called because +sharedStore calls it:

sharedStore = [[super allocWithZone:nil] init];

[super allocWithZone:nil] skips the current class' implementation of allocWithZone and calls the superclass implementation. However, init is an ordinary method call so it doesn't skip the superclass implementation.

Although the quoted line makes it look like super is an object that you're sending a message to, it really means something like "self, but skip the current class' implementation and use the superclass instead". It doesn't affect messages sent to the returned object.

Not clear why init methods gets called on some class

2 Answers2