|So, the customary
solution in singly dispatched object oriented languages to this problem is to
employ an idiom known as “double dispatch(ing)”.
was first described in a classic OOPSLA ’86 paper by Smalltalk master-coder
Dan Ingalls. Kurt Hebel and Ralph Johnson produced a nice, readable treatment
of Double Dispatch about 12 years ago.
|The code above
illustrate how it works.
|When an Irishman
imbibes a Dram, he can’t be sure exactly what’s in it, but he knows for sure
its been drunk by an Irishman.
|He dutifully informs
the Libation of this fact.
|By virtue of
this second method invocation, true
polymorphism is brought into play a second time.
|And, sure and
begora, the Irishman may not have been sure what he drank, but a Dram of
IrishWhisky knows just what to do when its inside an Irishman.
|Of course, using
double dispatch requires that you implement these ricochet / pinball bumper
routines. In general, the number which must be implemented is potentially,
but seldom actually, quite large. We’ll see just how large later in this