Question

Why do some objects not need to be initialized before use in objective-c?
For example why is this NSDate *today = [NSDate date]; legal?

Answer 1

They are initialized within the date method. This is a common way to create autoreleased objects in Objective-C. Allocators of that form are called convenience allocators.

To learn more about that, read the “Factory Methods” paragraph in Apple’s Cocoa Core Competencies document about Object Creation: http://developer.apple.com/library/mac/#documentation/General/Conceptual/DevPedia-CocoaCore/ObjectCreation.html

To create convenience allocator for you own classes, implement a class method, named after your class (without prefix). e.g.:

@implementation MYThing
...

+ (id)thing
{
  return [[[MYThing alloc] init] autorelease];
}

...
@end