null - In Objective-C why should I check if self = [super init] is not nil?

Question

I have a general question about writing init methods in Objective-C.

I see it everywhere (Apple's code, books, open source code, etc.) that an init method should check if self = [super init] is not nil before continuing with initialisation.

The default Apple template for an init method is:

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

    if (self != nil)
    {
        // your code here
    }

    return self;
}

Why?

I mean when is init ever going to return nil? If I called init on NSObject and got nil back, then something must be really screwed, right? And in that case, you might as well not even write a program...

Is it really that common that a class' init method may return nil? If so, in what case, and why?

Answer 1

For example:

[[NSData alloc] initWithContentsOfFile:@"this/path/doesn't/exist/"];
[[NSImage alloc] initWithContentsOfFile:@"unsupportedFormat.sjt"];
[NSImage imageNamed:@"AnImageThatIsntInTheImageCache"];

... and so on. (Note: NSData might throw an exception if the file doesn't exist). There are quite a few areas where returning nil is the expected behaviour when a problem occurs, and because of this it's standard practice to check for nil pretty much all the time, for consistency's sake.