In computer programming, a poltergeist (or gypsy wagon) is a short-lived, typically stateless object used to perform initialization or to invoke methods in another, more permanent class. It is considered an anti-pattern. The original definition is by Michael Akroyd at the 1996 Object World West Conference:
As a gypsy wagon or a poltergeist appears and disappears mysteriously, so does this short lived object. As a consequence the code is more difficult to maintain and there is unnecessary resource waste. The typical cause for this anti-pattern is poor object design. A poltergeist can often be identified by its name; they often include words such as "Manager", "Controller", "Supervisor", "StartProcess", etc. in the name. Sometimes, poltergeist classes are created because the programmer anticipated the need for a more complex architecture. For example, a poltergeist arises if the same method acts as both the client and invoker in a command pattern, and the programmer anticipates separating the two phases. However, this more complex architecture may actually never materialize. Poltergeists should not be confused with long-lived, state-bearing objects of a pattern such as model–view–controller, or tier-separating patterns such as business delegate pattern. To remove a poltergeist, delete the class and insert its functionality in the invoked class, possibly by inheritance or as a mixin. There have been proposed methods in detecting poltergeists in code for refactoring.
Example This Poltergeist class in this C++ example can be seen as a "poltergeist object", due to not adding additional functionality or encapsulation and only increasing complexity with unnecessary abstraction.
This could instead be more appropriately done using a smart pointer.
Another example of a poltergeist/gypsy wagon object, is the following, where UserCreator is instantiated just to perform some basic actions.
This could be more appropriately done like so, avoiding any poltergeist class entirely:
See also Anti-pattern Factory (object-oriented programming) YAGNI principle
References
Brown, William J. (1998). "Chapter 5: Software Development AntiPatterns". AntiPatterns: Refactoring Software, Architectures, and Projects in Crisis. New York, USA: John Wiley & Sons. ISBN 0-471-19713-0.
External links Development AntiPatterns