Wiki  - Library Entry

Intent

Ensure a class has only one instance and provide a global point of access.

Solution:

1.     Declare the constructor(s) as Protected;

2.     Declare a static self pointer to hold a reference to the single instance when it is created;

3.     Declare a static instance function which creates and returns the single instance if the self pointer is NULL, otherwise it just returns the previously created instance.

4.     Declare a protected destructor that sets the self pointer to NULL

5.     Declare other methods and attributes as needed normally.

6.     Clients access the singleton by calling the static instance function to get a reference to the single instance and then using it to call other methods.

7.     Clients must not store this instance reference -- Each code segment that needs to access the singleton should call the instance function, perform the work needed and then discard the reference as quickly as possible. 

Consequences

• Insures that only one instance of the class can ever be created.
• Access to the sole instance is strictly controlled by the object itself.
• Singleton classes may be extended by subclassing. Code written to use the base singleton class will automatically use an instance of the subclass provided that the instance function of the subclass is called by the client code prior to any call to the base singleton class instance function.
• Reduces the number of global names defined in a system. Singletons avoid polluting the name space with global variables.
• More flexible than using class (static) methods for functionality, since static functions (in C++) cannot be polymorphically overridden by subclasses.