Question

As an Objective-C beginner, I’m very confused about the init function, and how and when to override it. So here are few questions :

• Apparently, it works fine when the init function is not overridden, so is it just a good practice to do it? If it is, then is it a very bad practice not to do it?
• Let’s assume I’m overriding the function because I have to assign a default value to a variable. Do I have to allocate and initialize all the other ivars, including IBOutlets?

Please note I’m aware of the syntax :

if ((self = [super init]))
{
    _foo = [[Bar alloc] init];
}
return self;

Answer 1

Per “Initialization”:

A class generally implements an initializer for its objects, but is not required to. If a class does not implement an initializer, Cocoa calls the initializer of the nearest ancestor of the class. However, subclasses often define their own initializer or override an initializer of their superclass to add class-specific initializations. If a class does implement an initializer, it should invoke an initializer of its superclass as the first step. This requirement ensures a series of initializations for an object down the inheritance chain, starting with the root object. The NSObject class declares the init method as the default object initializer, so it is always invoked last but returns first.

As this says, you override your superclass’s designated initializer when you need to do some initialization after it’s done with its initialization. Don’t need to do that? Then you don’t need to override.

When your object is instantiated from a NIB, -init is not called. Instead, your newly allocated object will receive an -initWithCoder: or -initWithFrame: message depending on the object’s type. The NIB loading process sends your object -awakeFromNib after it and all the other NIB-created objects it references have been established. This lets you avoid overriding -initWithCoder:/-initWithFrame: when you wish to do some configuration after the NIB has been loaded. If you can do what you want to do by overriding -awakeFromNib rather than an initializer, you should do that.

See also “Multiple Initializers”, which explains the “designated initializer” concept and how different classes can have different designated initializers, and “Allocating and Initializing Objects” for a less friendly but more in-depth description of the allocation and initialization conventions adopted by Objective-C.