General talk about software patterns

["Eric Y. Theriault" <eric AT eyt.ca>]

Peter Horan wrote:

> Perhaps, logically, a singleton should not be destroyed, otherwise, it 
> is possible to create a different instance of the singleton.


Yes, but does a Singleton pattern define the lifetime of the class?  No, 
it only states that there will only be one instance of the class.  There 
are several different types of Singletons, and while many Singletons 
have long lives, there are some particular needs for short lifetimes.  I 
lack a clear example, but think of a Singleton that is used in a file 
parser or some genre of algorithm that is a small portion of the 
application; similar to how Design Patterns states in the Consequences 
that you can easily turn this pattern into a Multiton (multiple 
instances from a controlled entry point), the design pattern need not be 
interpreted as a single instance for the lifetime of the application 
(one instance for the entire application), but could also be interpreted 
as a single instance of an object at a given time (only one instance at 
any given time, but many instances over the application lifetime).

Furthermore, when an object's lifetime is the lifetime of the 
application's, it can still be important to release the resources that 
the Singleton holds, as previously mentioned on the forum.

Although the design pattern does not discuss the destruction of the 
object, this is as important as the pattern itself.



eyt*
--
Eric Y. Theriault
http://www.eyt.ca