Double Dispatch

Andrew P. Black black at
Wed Feb 26 01:47:42 UTC 2003

This is a call for language historians!

In teaching OOP, I often ask students to read and discuss Dan 
Ingall's 1986 OOPSLA paper.  It's short, clear, and provides an 
elegant and efficient solution to a well-described problem.

The problem is how to simulate "multiple dispatch".  You have three 
kinds  of graphical objects -- rectangles, ovals and bitmaps -- and 
three kinds of display ports -- canvas, printer and remote -- and so 
you need nine different displayOn: methods.  How should one implement 
this cleanly in a "single dispatch" language like Smalltalk, i.e., in 
a language in which the method chosen depends on the message and on 
the class of the target object, but not on the class of the arguments?

Dan argued, quite correctly, that it is a BAD IDEA to do explicit 
test of class membership

	rectangle>>drawOn: aDisplayPort
		(aDisplayPort isKindOf: canvas) ifTrue:
			[ ... code to draw a rectangle on a canvas ...]
		(aDisplayPort isKindOf: printer) ifTrue:
			[ ... code to draw a rectangle on a printer ...]

because this code must be re-written each time a new kind of 
DisplayPort is added to the system.  Instead we should do a second 
dispatch, to get the effect of the case statement:

	rectangle>>drawOn: aDisplayPort
		aDisplayPort displayRectangle: self

	canvas>>displayRectangle: aRectangle
		 ... code to draw a rectangle on a canvas ...

	printer>>displayRectangle: aRectangle
		 ... code to draw a rectangle on a printer ...

This is extensible and modular, and also efficient.  It is used in 
more complex patterns like the visitor pattern, and is fairly well 
know to the readers of this list, I am sure.

As far as I am aware, this technique is called Double Dispatch. 
Several of my Smalltalk books call it Double Dispatch, for example, 
LaLonde and Pugh use this name is discussing the various 
implementations of arithmetic in the number hierarchy, which now this 
technique in preference to the "genericity numbers" used in earlier 
Smalltalks.  Skublics, Klimas and Thomas, in "Smalltalk with Style" 
also use the term Double Dispatch for this technique.

My question is: who first introduced the term "Double Dispatch" to 
refer to this technique?  I was amazed to find that this term is 
*not* used in Dan Ingall's original paper, although it is a perfectly 
natural extension of Dan's Language.  I'm asking because I recently 
came across a paper that defined "Double Dispatch" to mean the 
deprecated style of explicitly testing the class of the argument, 
which (as far as I'm concerned) is exactly what Double Dispatch is 

Does anyone have a reference to the first use of the term "Double Dispatch"?


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