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From: adam@visix.com
Newsgroups: comp.lang.misc
Subject: Re: CHALLENGE: heterogeneous collections
Message-ID: <1991Mar27.164605.24547@visix.com>
Date: 27 Mar 91 16:46:05 GMT
References: <1991Mar22.210725.29448@neon.Stanford.EDU> <pallas.669923769@red-dwarf> <48805@nigel.ee.udel.edu> <7689:Mar2623:28:5091@kramden.acf.nyu.edu>
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In article <48805@nigel.ee.udel.edu> new@ee.udel.edu (Darren New) writes:

>-
Hmm...  How about a window with a hetrogeneous collection of buttons, sliders,
text displays, etc, all of which respond to "redraw" and "is the mouse
over you"?
>-

In article <7689:Mar2623:28:5091@kramden.acf.nyu.edu>,
brnstnd@kramden.acf.nyu.edu (Dan Bernstein) writes:

>-
struct { any object; void (*redraw)(); int (*ismouseover)(); }
*whatsinwindow();

Here ``any'' is a polymorphic type (e.g., void * in C), and *redraw
takes an ``any'' argument. Naturally, languages like C++ encapsulate the
above struct into a single ``class,'' but that's merely syntactic sugar.
>-

Let me get this straight.  To construct a heterogeneous list of M
items each guaranteeing N operations, I need to waste space for M*N
function pointers?

Yes, we need to store ONE pointer to each function, but the above
scheme wastes space if we have already stored that pointer, e.g. in a
different kind of heterogeneous list.  A large windowing application
uses hundreds of different lists.

And I need to write an explicit constructor for every kind of list?

You can do better than that, even in C.

Adam