gjditchfield@watmsg.waterloo.edu (Glen Ditchfield) (11/12/89)
In article <654@nastar.UUCP>, joel@nastar.UUCP (Joel Rives) writes: > ... It is not at all difficult to code a QUEUE object in C++ that will > accept a generic reference upon which to operate. A new instantiation of > this QUEUE object could be passed a comparison function when created that > would be specific to the data type being queued. In article <317@shrike.AUSTIN.LOCKHEED.COM> aihaug@AUSTIN.LOCKHEED.COM (Daniel A Haug) writes: >For many of our particular applications, this is overly restrictive. >I may very well want a single queue to hold objects of vastly different >types. Perhaps I'm wrong (and often am ;-), but I don't see how to >accomplish this in a statically typed language. The "data type being queued" can be a supertype instead of a specific, "concrete" type; then the queue can contain objects of vastly different types that are subtypes of that type. Let me put it this way: no matter how vastly different the types of the elements are, you want to do _something_ to them when you take them off of the queue! Lets say you might apply the bleen() and gaggle() operations. In a dynamically typed language, you would declare a queue-of-anything, promise yourself that you will never put an object on the queue unless it has both operations, and hope that testing will uncover any mistaken enqueues before the program ships. In a statically typed language, you would declare type Moof with overridable bleen() and gaggle() operations and declare a queue-of-Moof. If you mistakenly enqueue something that isn't a subtype of Moof, you get a compile-time error. I've done this in C++; I have a macro that, given a class E, generates a class of queue that can contain objects of any type descended from E. I gain a static assurance that a certain class of bug is absent from my program, but I do lose some of the flexibility of pure dynamic typing. I find that I don't miss the lost flexibility much, and I can imagine statically checked type systems which would restore some of that flexibility. Glen Ditchfield gjditchfield@violet.uwaterloo.ca Office: DC 2517 Dept. of Computer Science, U of Waterloo, Waterloo, Ontario, Canada, N2L 3G1 Moof! is a trademark of Apple Computer Inc.