singleton in objective c

Question

I saw a singleton example on objective-c book. However, I don\'t know if there is difference of meaning of \'singleton\' definition between objective-c and other langs. Can this

Answer 1

I don't know if there is difference of meaning of 'singleton' definition between objective-c and other langs.

It follows the common definition of languages derived from C.

Can this [[SingletonClass alloc] init] still be used to create a new object?

Yes

If yes, how to guarantee there is only one object in the memory?

Avoid enforcing the pattern (e.g. do not force it to be a singleton). Just make a normal object. Then if you really want only one instance, create an instance and save it someplace for reuse (your app delegate is one typical place for this, because it is typically created once per execution).

In practice, most (>95%) ObjC singleton implementations i've seen in the wild are used for the wrong reasons, and would have been better or as good as normal objects.

Every solution linked in the answers so far has (at minimum) subtle problems, dangers, or undesirable side-effects.