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Path: utzoo!mnetor!seismo!elsie!nih-csl!keith
From: keith@nih-csl.UUCP (keith gorlen)
Newsgroups: comp.lang.c++
Subject: Re: Questions about C++ (Rather Long)
Message-ID: <227@nih-csl.UUCP>
Date: Wed, 24-Jun-87 09:44:58 EDT
Article-I.D.: nih-csl.227
Posted: Wed Jun 24 09:44:58 1987
Date-Received: Sat, 27-Jun-87 00:52:52 EDT
References: <226@nih-csl.UUCP> <131@otc.OZ>
Distribution: comp
Organization: NIH-CSL, Bethesda, MD
Lines: 61
Summary: Suggested technique for testing the class of an object has some problems.

In article <131@otc.OZ>, mikem@otc.OZ (Mike Mowbray) writes:
> // *********************** typetst.c *************************************
> #include <stream.h>
> 
> #define  IS_TYPE(classtype,instance)  ((instance)._vptr == ::classtype\
> __vtbl)
> 	    // "classtype" must have virtual functions.
> 	    // "instance" must be an instance of a class with virtual
> 	    // functions (or a reference thereto).
> 	    // If either is not the case, there'll be a compile-time error.
> 	    //  (either "_vptr: no such member"
> 	    //   or     "xxxxx__vtbl is undefined"
> 	    //   or something like that.)
> //--------------------------------------------------------

This technique assumes that all instances of a class will have the
same vtbl address, which is not true unless you use the +e compiler
switch.  Without this switch, each .c file that #includes the
declarations of a class with virtual functions gets its own instance
of vtbl, and objects constructed by code in two different .c files
will have different vtbl addresses as a result, even though they may
belong to the same class.  Since using the +e switch is such a pain, I
wouldn't recommend this method.

> 
> Note also, that IS_TYPE doesn't work for derived cases:
> 
> Association assoc;
> 
> if (IS_TYPE(Object,assoc))	// will yield 0.
>  .....
> 
> To remedy this is harder since extra type information is needed. E.g: you
> could have a table associated with each class which keeps track of all
> its ancestors' vptrs. Each instance would need a pointer to the correct
> table. The following is an example:
> 

My OOPS class library uses instances of class Class to describe the
class hierarchy at run-time.  The programmer must declare one, static,
instance of class Class for each class he writes.  Each instance of
class Class contains a pointer to the instance of class Class that
describes the base class, the class name as a character string, the
size in bytes of instances of the class described, the address of a
function for reading instances of the class from a stream, and other
useful information.  Each class defines the virtual function const
Class* isA() to return the address of its Class descriptor instance.
Also, the (static) constructor for class Class adds all the Class
instances linked in the program into a Dictionary, keyed by the name
of the class.  This structure supports run-time type checking, object
I/O, object copying operations, object I/O (i.e., storeOn(ostream&)
and readFrom(istream&)), and allows class names to be printed in error
messages.  It has low overhead, and adds no additional member
variables to objects.  I've been using it for over a year, and am
satisfied with it, except that I wish I had named the class something
else!  (The whole idea was stolen from Smalltalk-80)
-- 
	Keith Gorlen			phone: (301) 496-5363
	Building 12A, Room 2017		uucp: seismo!elsie!nih-csl!keith
	National Institutes of Health
	Bethesda, MD 20892