Can you point me to more information on "the singleton pattern" and
singletons?
of course :
The Singleton is a Desing Pattern. Design patterns are very useful
"recipes" that are working fine. You can adapt them depending on your
needs, languages, etc.
One of the most known book about DP in computing is the one of "the
gang of four", Erich Gamma is the most known of the "gang".
From this book (c++ is used in the book but that's working the same in
any OOP language, of course in C#, just more readable in C# :-) ) :
(I give you some C# examples at the end of this post)
Intent
------
Ensure a class only has one instance, and provide a global point of
access to it.
Motivation
----------
It's important for some classes to have exactly one instance. Although
there can be many printers in a system, there should be only one
printer spooler. There should be only one file system and one window
manager. A digital filter will have one A/D converter. An accounting
system will be dedicated to serving one company.
How do we ensure that a class has only one instance and that the
instance is easily accessible? A global variable makes an object
accessible, but it doesn't keep you from instantiating multiple
objects.
A better solution is to make the class itself responsible for keeping
track of its sole instance. The class can ensure that no other instance
can be created (by intercepting requests to create new objects), and it
can provide a way to access the instance. This is the Singleton
pattern.
Applicability
-------------
Use the Singleton pattern when
* there must be exactly one instance of a class, and it must be
accessible to clients from a well-known access point.
* when the sole instance should be extensible by subclassing, and
clients should be able to use an extended instance without modifying
their code.
Participants
------------
* Singleton
-defines an Instance operation that lets clients access its unique
instance. Instance is a class operation (that is, a class method in
Smalltalk and a static member function in C++).
-may be responsible for creating its own unique instance.
Collaborations
--------------
Clients access a Singleton instance solely through Singleton's Instance
operation.
Consequences
------------
The Singleton pattern has several benefits:
*Controlled access to sole instance. Because the Singleton class
encapsulates its sole instance, it can have strict control over how and
when clients access it.
*Reduced name space. The Singleton pattern is an improvement over
global variables. It avoids polluting the name space with global
variables that store sole instances.
*Permits refinement of operations and representation. The Singleton
class may be subclassed, and it's easy to configure an application with
an instance of this extended class. You can configure the application
with an instance of the class you need at run-time.
*Permits a variable number of instances. The pattern makes it easy to
change your mind and allow more than one instance of the Singleton
class. Moreover, you can use the same approach to control the number of
instances that the application uses. Only the operation that grants
access to the Singleton instance needs to change.
*More flexible than class operations. Another way to package a
singleton's functionality is to use class operations (that is, static
member functions in C++ or class methods in Smalltalk). But both of
these language techniques make it hard to change a design to allow more
than one instance of a class. Moreover, static member functions in C++
are never virtual, so subclasses can't override them polymorphically .
Implementation
--------------
Here are implementation issues to consider when using the Singleton
pattern:
* Ensuring a unique instance. The Singleton pattern makes the sole
instance a normal instance of a class, but that class is written so
that only one instance can ever be created. A common way to do this is
to hide the operation that creates the instance behind a class
operation (that is, either a static member function or a class method)
that guarantees only one instance is created. This operation has access
to the variable that holds the unique instance, and it ensures the
variable is initialized with the unique instance before returning its
value. This approach ensures that a singleton is created and
initialized before its first use.
You can define the class operation in C++ with a static member function
Instance of the Singleton class. Singleton also defines a static member
variable _instance that contains a pointer to its unique instance.
The Singleton class is declared as
class Singleton {
public:
static Singleton* Instance();
protected:
Singleton();
private:
static Singleton* _instance;
};
-------------------
C# example :
here:
http://www.yoda.arachsys.com/csharp/singleton.html
another one here :
http://www.c-sharpcorner.com/UploadF...onPattern.aspx
--
OD___
www.e-naxos.com 
