What is Singleton ?

Singleton class is a class whose only one object can be created at a time

Singleton provides a global, single instance by:

·  Making the class create a single instance of itself.

·  Allowing other objects to access this instance through a class method that returns a reference to the instance. A class method is globally accessible.

·   Declaring the class constructor as private so that no other object can create a new instance.

Singleton results in the following benefits and liabilities:

               Benefits

·   Instance control. Singleton prevents other objects from instantiating their own copies of the Singleton object, ensuring that all objects access the single instance.

·   Flexibility. Because the class controls the instantiation process, the class has the flexibility to change the instantiation process.

Liabilities

·   Overhead. Although the amount is minuscule, there is some overhead involved in checking whether an instance of the class already exists every time an object requests a reference. This problem can be overcome by using static initialization as described in Implementing Singleton in C#.

·   Possible development confusion. When using a singleton object (especially one defined in a class library), developers must remember that they cannot use the new keyword to instantiate the object. Because application developers may not have access to the library source code, they may be surprised to find that they cannot instantiate this class directly.

·   Object lifetime. Singleton does not address the issue of deleting the single object. In languages that provide memory management (for example, languages based on the .NET Framework), only the Singleton class could cause the instance to be deallocated because it holds a private reference to the instance. In languages, such as C++, other classes could delete the object instance, but doing so would lead to a dangling reference inside the Singleton class.