Why method scoped variables not alloc+init?

http://macdevcenter.com/pub/a/mac/2001/07/27/cocoa.html?page=3

Scroll down a bit and read about Convenience constructors.

 

You are confused between what an object is and what a pointer is. A pointer is not an object, it refers to (points to) and object. [[SomeClass] alloc] init] creates an initialised object and returns a pointer to that object, which you need to store in some pointer variable, or you could never access that object. But often I don't want to create a new object, I want to refer to an existing object.

 

I think you might still be a bit confused. You need to alloc and init objects (and only objects). That process returns a pointer to the memory of that object which you need to store in a pointer variable (otherwise you've just reserved memory that you have no way of releasing later since you haven't grabbed a reference to it-- AKA a memory leak).

There are factory methods which will create a new object for you so you don't need to call alloc init (often these are autoreleased so check the documentation on any particular factory method for the specifics).

Pointers themselves are just integers. Think of them like the address on a mailbox. They're the number you go to to get to the container, but the mailbox itself (the object in this analogy) is what actually holds the data.

Are you working through a book?

 

When you do this:

Code:

NSString* myString = [[NSString alloc] init];

You are actually doing several things:

1) Creating a new pointer (myString).
2) Using NSString to allocate a new NSString object.
3) Calling init to initialise that new NSString object.
4) Pointing myString at the new object.

When you do this:

Code:

NSString* myString;

You are only doing the first step above, creating the pointer. You haven't actually created a string, just a pointer which can point to a string.

So for example:

Code:

NSString* myString;
myString = [myRandomObject path];

Here:
1) You create the myString pointer.
2) The 'path' method is creating a new NSString object via alloc & init (or calling another method which does so)
3) The myString pointer is pointed at the NSString that the 'path' method returns.

You might be confused in thinking "How can I use myString here, when I've never allocated it?". The answer is: you don't have to in the example above. If you call a method which returns an object, that method will do the "allocing" and "initing" for you.

Hope that helps.

 

You should also check out the memory management rules for cocoa (there's also a PDF version available).