When to Use Class Methods in Objective C ?

A class in Objective-C can declare two types of methods: instance methods and class methods. An instance method is a method that is triggered when a message is sent to a particular instance of the class. In other words, before you call an instance method, you must first create an instance of the class. Class methods, by comparison, do not require you to create an instance. They are triggered when a message is sent to a class. In other languages, i.e. Java, class methods are called static methods.

One common use for class methods in Objective C or static methods in Java is to provide utility methods, i.e mathematical functions. A common example for static methods in Java are the static methods in java.lang Math. In Objective C you can often rely on pure C functions for utility methods, such as mathematical functions, so that you won't find many utility class methods in the frameworks, i.e. the fondation framework.

In Objective C you typically use class methods as (static) factory methods to create new instances of the class, which is an alternative to the object allocation and initialization procedure. For example NSString contains some very useful factory methods (like + (id)StringWithFormat:(NSString *)format) that return autoreleased NSString objects.

In his Book Effective Java (2nd Edition): A Programming Language Guide Joshua Bloch explained the advantages of of using factory (class/static) methods.Although Bloch's book was initially written for Java, most of his ideas are applicable to other object oriented languages.
One advantage of static factory methods is that, unlike constructors, they have names. A static factory with a well-chosen name is easier to use and the resulting client code easier to read. 
Most of the factory methods in the apple frameworks are good example for good naming convention as you can see in NSDate. You don't need much documentation to understand which objects are created by the following methods:

+ (id)dateWithTimeInterval:(NSTimeInterval)seconds sinceDate:(NSDate *)date
+ (id)dateWithTimeIntervalSince1970:(NSTimeInterval)seconds
+ (id)dateWithTimeIntervalSinceNow:(NSTimeInterval)seconds
A second advantage of static factory methods is that, unlike constructors, they are not required to create a new object each time they’re invoked.  
You can ensure that a certain class vends a singleton instance. The factory only creates an instance if the the instance doesn't exist. This is achieved with the with the following code snippnet:

static AccountManager *DefaultManager = nil;

+ (AccountManager *)defaultManager {
    if (!DefaultManager) DefaultManager = [[self allocWithZone:NULL] init];
    return DefaultManager;


Exception Breakpoint in Xcode

In Xcode 4 when an exception is raised (wether it is ,unrecognized selector sent to instance‘ exception or an 'out of bounds' exception or another exception) the exception goes up the call stack looking for someone who can handle it. And what normally happens is that no on handles it  and the exception percolate up the call stack all the way up to main. The program just crashes and you lost the call stack information.
To avoid this, Xcode provides a very important breakpoint which gonna break every time an exception is raised and Xcode shows in the main code window the line of code on which that exception would raise.

You can add an Exception Breakpoint by opening the Breakpoint Navigator.

Breakpoint Navigator

Than you have to click the Plus Button in bottom left corner:

Add Exception Breakpoint

After clicking Add Exception Breakpoint menu item, you will see this breakpoint configuration view open up:

Click Done
After clicking the Done button and you will the new Exception Breakpoint in your list of breakpoints.

I recommend setting that exception on all the time on in Xcode because you pretty much always want to have this exception happen right a the line of code.