Why would you use a class method constructor versus alloc/init?

Question

There are two choices for constructors in Objective C/Cocoa:

1. Class Constructor

Product *product = [Product productWithIdentifier:@"Chocolate"];
// Use product

2. Alloc/init Constructor

Product *product = [[Product alloc] initWithIdentifier:@"Chocolate"];
// Use product
[product release];

What I do

I tend to use the class method just because it looks cleaner and you don't need a release.
I see a lot of alloc/init - what's the advantage to doing it this way?

My Question

Which one is preferable? Or is it just a matter of taste?

Code

For context, the class Product would have the following:

+(Product *)productWithIdentifier:(NSString *)identifier_ {
   return [[[[self class] alloc] initWithIdentifier:identifier] autorelease];
}

-(Product *)initWithIndentifier:(NSString *)identifier_ {
   self = [super init]
   if (self) {
       identifier = identifier_;
   }
   return self;
}

sergio sergio · Accepted Answer · 2012-02-19T14:40:38

IMO, the biggest difference between the two approaches comes from the fact that using a "class constructor" you get an autoreleased object; this is the most convenient option when you:

assign the allocated object to a retain properties;
when you create "temporary" objects (think of the various NSString methods that build a string from another string: in many case it happens you need to "chain" such calls; the constructor allows you to "forget" about memory management);
when you add the object to some object that retains it (think: addSubview)

In such cases the syntactical advantage of "class constructor" is foremost, but I think it also makes your code "safer" in terms of memory management.

On the other hand, when you create an object to assign it to an assign property (or directly to an ivar for which you have no property), then alloc/init will do perfectly and be preferable to the "constructor" (IMO).

So, in the end, it depends on the way you are going to use the objects you allocate. The class constructor is a convenience method.

Why would you use a class method constructor versus alloc/init?

2 Answers