someClass* object = [someClass new];
someClass* object = [[someClass alloc] init];
Actually "new" is not a keyword in Objective-C, but NSObject implements a class method "new" which simply calls "alloc" and "init".
The new method actually comes from NeXT days. Back then, there was no two phase initialization, it was just a single new method. They soon realized that a two phase approach could have advantages, and introduced alloc. new was kinda deprecated, but kept in for backwards compatibility. It is exactly the same as alloc-init. Use 'new' if it suits. One shortcoming is that it only works with the basic 'init' initializer, and will not work with other initializers (eg initWithString:).
Frequently, you are going to need to pass arguments to init and so you will be using a different method, such as [[SomeObject alloc] initWithString: @"Foo"]. If you're used to writing this, you get in the habit of doing it this way and so [[SomeObject alloc] init] may come more naturally that [SomeObject new].
There was a very long thread on this same subject (alloc/init vs. new) on the cocoa-dev mailing list this week (search for "[Foo new] vs [[Foo alloc] init]"). Unfortunately the documentation is not crystal clear on this, but Bill Bumgarner (an Apple Engineer) confirmed that new is implemented as allocWithZone/alloc followed by init back at least to the beginning of OS X. So the answer is, use whichever you prefer. The current vogue in Cocoa programming is to use alloc/init because it makes the intended behavior explicit.
* new doesn't support custom initializers (like initWithString)
* alloc-init is more explicit than new
General opinion seems to be that you should use whatever you're comfortable with.
object-c学习笔记：new vs alloc init转载