Creating vs. Initialising

[Smikey]

Ok, apologies if this is a rather daft question, but I can't quite seem to grasp the difference between creating an instance of and initialising a class.

Using the NSMutableArray as an example, there are 2 methods:

+ arrayWithCapacity:
– initWithCapacity:

I understand that the '+' denotes a Class method, where as the '-' denotes an instance method, but what exactly is the difference between these 2 methods? Presumably they both create new arrays?

Thanks

Michael

[toby]

Without getting into too much detail on the technicalities here: creating an instance gives you an instance of the class, however its not ready to be used yet, it requires some base setup work to be done. Initialising the class does the base setups that are required before you actually use your new class and in some cases may also include setting default values for the object's instance variables (or values given as an argument).

In the example below, your "arrayWithCapacity" class method does not require initialising. However if you were to have created the array using "alloc" then you would need to use the "initWithCapacity" to set it up. In allot of cases (all I think but I ain't making promises here) the instances you create using class methods similar to these (not alloc, ones which give you more than a blank instance) should not require initialisation(they should have been automatically initialised to allow their initial value to be set - in the example this would be the size of the array-), but when you create a blank instance with alloc then you need to initialise it.

[Smikey]

Right... Thanks for your reply. I'm afraid I'm still a little confused.

If you initialise a class, you create an instance of the object, with default values.

But if you initialise an instance of a class, what are you creating?

So here I am initialising an instance of a class:

NSMutableArray *myArray = [[NSMutableArray alloc] initWithCapacity: 10]

But how would I use arrayWithCapacity: instead? And what would be the advantage?

Thanks for the help

[rgronlie]

Michael,

But if you initialise an instance of a class, what are you creating?

You aren't creating anything. The alloc method created the instance for you.

Have you read about arrayWithCapacity in the Developer Documentation?

arrayWithCapacity:
Creates and returns an NSMutableArray object with enough allocated memory to initially hold a given number of objects.

So:

NSMutableArray *myArray = [NSMutableArray arrayWithCapacity: 10];

Would be the same as:

NSMutableArray *myArray = [[[NSMutableArray alloc] initWithCapacity: 10] autorelease];

Ryan

[Smikey]

Ah ok - I think I get it now. Thanks for the help!