On Feb 15, 2008, at 4:55 PM, Gregory Weston wrote:
Keith Duncan wrote:
As far as I understand, [Foo new] is exactly equivalent to [[Foo
alloc] init]
Correct.
Incorrect. Based on the documentation new (by default) does setup
work analogous to alloc and then invokes init before returning the
object. It does not actually use the alloc method, though, which
means it's entirely possible for new to not be equivalent to an
alloc/init pair.
The only ways it could be different are:
1. If +new does not in fact call +alloc, but only does something
equivalent, then if a subclass overrode the +alloc method but didn't
also override +new, then it would behave differently. Do any classes
actually override +alloc? If so then that might be a good reason not
to use +new.
2. If a class overrode +new to do something different. In that case
though the class's author probably did that on purpose, so you'd
probably still be correct to call +new.
Chaining alloc and init is just the in vogue convention as far as I
know. I've been told that -new used to be a popular way of doing it
back in the days of NeXTStep.
It's more than just the 'in vogue convention' though. The point was
to recognize that allocation and initialization are distinct actions
and there's benefit in having them be separable. With alloc/init you
can change one of the phases without concern for the other.
I never meant to imply that there isn't a benefit to having them be
separable. It's very common to want to use a different initializer
than -init, or to want to allocate an object in some other way than
what +alloc does. I'm only talking about the common case where you
ARE just chaining +alloc and -init together, and wondering why people
don't just use +new instead.
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