Editing Double dispatch (section)

===== Visitor =====
The visitor for the classic Spaceship example also has a double-dispatch mechanism.
<syntaxhighlight lang="eiffel" line highlight="35">
make
		-- Allow SPACESHIP objects to visit and move about in a universe.
	local
		l_universe: ARRAYED_LIST [SPACE_OBJECT]
		l_enterprise,
		l_excelsior: SPACESHIP
	do
		create l_enterprise.make_with_name ("Enterprise", "A-001")
		create l_excelsior.make_with_name ("Excelsior", "A-003")
		create l_universe.make (0)
		l_universe.force (l_enterprise)
		l_universe.force (create {ASTEROID}.make_with_name ("Rogue 1", "A-002"))
		l_universe.force (create {ASTEROID}.make_with_name ("Rogue 2", "A-003"))
		l_universe.force (l_excelsior)
		l_universe.force (create {ASTEROID}.make_with_name ("Rogue 3", "A-004"))
		l_universe.force (create {SPACESTATION}.make_with_name ("Deep Space 9", "A-005"))
		visit (l_enterprise, l_universe)
		l_enterprise.set_position ("A-002")
		visit (l_enterprise, l_universe)
		l_enterprise.set_position ("A-003")
		visit (l_enterprise, l_universe)
		l_enterprise.set_position ("A-004")
		l_excelsior.set_position ("A-005")
		visit (l_enterprise, l_universe)
		visit (l_excelsior, l_universe)
		l_enterprise.set_position ("A-005")
		visit (l_enterprise, l_universe)
	end
feature {NONE} -- Implementation
visit (a_object: SPACE_OBJECT; a_universe: ARRAYED_LIST [SPACE_OBJECT])
		-- `a_object' visits `a_universe'.
	do
		across a_universe as ic_universe loop
			check attached {SPACE_OBJECT} ic_universe.item as al_universe_object then
				a_object.encounter_agent.call ([al_universe_object.sensor_data_agent])
			end
		end
	end
</syntaxhighlight>
The double-dispatch can be seen in line #35, where two indirect agents are working together to provide two co-variant calls working in perfect polymorphic concert with each other. The `a_object' of the `visit' feature has an `encounter_agent' which is called with the sensor data of the `sensor_data_agent' coming from the `al_universe_object'.
The other interesting part of this particular example is the SPACE_OBJECT class and its `encounter' feature: