Pattern name: Context Intent Loosely couple behavior modification to behavior and structure Write an editing program instead of editing the program yourself. Reuse editing program later. Define collaborative behavior modification in one place instead of spreading it over the program. Context let's you write behavior modifications with minimal dependency on the class structure. Also Known As: Structure-shy behavior modification, Event-based coupling Motivation Modify the behavior of a function call without changing the object or the function directly. Applicability An object's function needs to be specialized or replaced. A function call needs to accomplish additional work. Express behavior which needs to be executed in addition to a traversal. Use the pattern when you need to change the behavior of a group of classes without changing the classes. Solution Use context objects as described in [CONTEXT]. Consequences Easy behavior adjustments Allows to use the behavior modification in other situations. Does not strongly tie behavior modification to the modified program. Implementation Use wrappers as in propagation patterns. Use context tables as proposed in [CONTEXT]. Known Uses Visitor objects in the Visitor pattern of GOF use a primitive form of the Context pattern. The Context pattern (with the wrapper implementation) is used extensively in the implementation of the Demeter Tools/C++. The work of Lang and Lockemann at the University of Karlsruhe on "Behaviorally Adaptive Objects" uses the Context pattern. They dissect an object into a crystal and temporarily associated contexts. An object is defined as a crystal together with all its current contexts, leading to dynamically changing object behavior. Sample Code See [CONTEXT]. Related patterns: Inventor's Paradox, Structure-shy Traversal. Exercise: Define a notation to express context objects. References: CONTEXT: Linda Seiter, Jens Palsberg, Karl Lieberherr, Context Objects (see URL: ==========================================================================