iphone - What is the difference between these two ways of allocating memory in Objective-C?

Question

I am confused about the proper means of allocating memory in Objective-C. Suppose I have an NSMutableDictionary. There are two ways I can initialize it:

   NSMutableDictionary *alpha = [[NSMutableDictionary alloc] init];

or

   NSMutableDictionary *alpha = [NSMutableDictionary dictionary];

What is the difference between them? I know the first allocates memory for alpha, but what about the second?

Which of these is recommended as the best practice for allocating memory?

Answer 1

[NSMutableDictionary dictionary];

is exactly the same thing as:

[[[NSMutableDictionary alloc] init] autorelease];

It just saves you some typing. It doesn't matter which one you use, as long as you know the difference between a retained object and an autoreleased one. If you're using ARC, then you don't even need to know that.

The convention is:

1. If you see init, new or copy: it's a retained object.
2. If the method name starts with the class name (sans the framework prefix), it's an autoreleased object.