|Type :||Cr (Creational)|
|Intent :||Ensure a class only has one instance, and provide a global point of access to it.|
|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.