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From: maverick@fir.Berkeley.EDU (Vance Maverick)
Newsgroups: comp.lang.c++
Subject: Lacking parameterized types and multi-methods....
Message-ID: <11687@pasteur.Berkeley.EDU>
Date: 5 Mar 91 17:26:34 GMT
Sender: news@pasteur.Berkeley.EDU
Reply-To: maverick@fir.Berkeley.EDU (Vance Maverick)
Lines: 32

I have a problem for which I have not yet found a clean solution in C++.
 In my application, there are various classes of which I wish to keep
lists.  So I define a single class List, which handles objects of class
Listable, and I make all the things I want lists of (musical notes,
names, and other incompatible types) subclasses of Listable.

So far so good; but I want the List code to be able to compare two
objects, so it can insert items in sorted order, where this matters.  I
define a virtual member function (actually it's an overloaded operator,
but same difference) on Listable, taking an argument of type Listable. 
On Name, I define such a function taking an argument of type Name;
similarly, a Note member function comparing two Notes.  So in my
insertion code, I have something like this:

	Listable *member, *newvalue;

	if (*newvalue < *member) ....

Unfortunately, this can't invoke my Note-to-Note or Name-to-Name
comparator, since the signature of these functions contains the derived
type, and C++ uses the declared type of the right-hand argument in
selecting the function to call.  To get this to work, I need to define
alternate comparators, comparing Note to Listable and Name to Listable. 
This is unpleasant, because it involves a distasteful cast in the
alternate comparator, and because it now allows me to explicitly compare
a Note to a Name.

Does anybody have a suggestion for how to do this "correctly", lacking
parameterized types and runtime method selection on multiple arguments? 
Thanks,

	Vance