objective c - why does initializing subclasses require calling the super class's same init function?

Question

I have heard that when you have a subclass, you are supposed to initialize the superclass with the same init function from within the subclass's init. What I mean is that the subclass's init should call [super init] and the subclass's initWithFrame should call [super initWithFrame]. Why is this? Why does calling the super's init from a subclass's initWithFrame result in an infinite loop?

If this is required, then does this mean I can't create a new init function within a subclass such as initWithPoint and have that call super's init or initWithFrame simply because the super class doesn't have initWithPoint? I guess the heart of the question here is simply why it's improper to call a different super class, something that's confusing me possibly because of my c++ background?

Answer 1

Why is this? Why does calling the super's init from a subclass's initWithFrame result in an infinite loop?

If the -init in super is implemented as

-(id)init {
  return [self initWithFrame:CGRectZero];
}

then the call graph will loop around:

[subclass initWithFrame:]
   |     ^
   v     |
[super init]

as self always uses the current class ("subclass").

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If this is required, then does this mean I can't create a new init function within a subclass such as initWithPoint and have that call super's init or initWithFrame simply because the super class doesn't have initWithPoint?

No this is not required. What is preferred is to call super's most specialized initializer, so there's no chance super's -initXXX calls back the subclass's -initYYY.