In Objective C, when is alloc used and when is it no

Question

I am having a hard time figuring out when to alloc an object. I am going through the Apress iPhone Dev for Beginners book. Sometimes it says use:

UIImage *seven = [UIImage imageNamed:@"seven.png"];

and other times,

UIImageView *sevenView = [[UIImageView alloc] initWithImage:seven];

Why don't you alloc UIImage in the first example?

Thanks - Absolute Beginner.

Answer 1

initWithImage: is an instance method - the message must be sent to a particular object. You create such an object instance with alloc.

imageNamed: is a class method. It does not need to be sent to an instance of the class, so you don't allocate an object. Such methods returning objects will often allocate and init an object "under the hood".

You can find the information which methods are class methods and which are instance methods in the class reference. Also, class method declarations start with + as in + (UIImage *)imageNamed:(NSString *)name, instance methods with - as in - (id)initWithData:(NSData *)data.

By the way, alloc is just a class method of NSObject.

Answer 2

The convention is that whenever you call [Foo alloc], you have to [release] the resulting Foo object afterwards. On the other hand, if the method is called [fooWithBar], or something similar, it returns an autoreleased object, the one that will be autoreleased when the current system-called function returns.