and maneuverable. Other objects, like Asteroids are none of these things. A SPACESTATION, on the other hand, is both dock-able and manned, but is not maneuverable. Thus, when one object has an encounter with another, it first checks to see if the positions put them in the vicinity of each other and if they are, then the objects interact based upon their basic properties. Note that objects with the same type and name are considered to the same object, so an interaction is logically disallowed. Eiffel example conclusion With regards to double-dispatch, Eiffel allows the designer and programmer to further remove a level of direct object-to-object knowledge by decoupling class routines from their classes by way of making them agents and then passing those agents instead of making direct object feature calls. The agents also have specific signatures and possible results (in the case of queries), making them ideal static type checking vehicles without giving up specific object details. The agents are fully polymorphic so that the resulting code has only the specific knowledge required to get its local job done. Otherwise, there is no maintenance burden added by having specific internal class feature knowledge spread around many co-variant objects. The use and mechanics of agents ensure this. One possible downside of the use of agents is that an agent is computationally more expensive than its direct call counterpart. With this in mind, one ought never to presume the use of agents in the double-dispatch and their application in visitor patterns. If one can clearly see a design limit as to the domain of class types that will be involved in the co-variant interactions, then a direct call is the more efficient solution in terms of computational expense. However, if the class domain of participating types is expected to grow or differ substantially, then agents present an excellent solution to lessening the maintenance burden in the double-dispatch pattern. See also Visitor pattern Multiple dispatch Hash function Virtual table References Software design patterns Method (computer programming) Articles with example C++ code