Ensure a class only has one instance, and provide a global point of access to it.
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
Typical Use Case
- the logging class
- managing a connection to a database
- file manager
Real world examples
- Violates Single Responsibility Principle (SRP) by controlling their own creation and lifecycle.
- Encourages using a global shared instance which prevents an object and resources used by this object from being deallocated.
- Creates tightly coupled code that is difficult to test.
- Makes it almost impossible to subclass a Singleton.