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From: wevg0324@uxa.cso.uiuc.edu
Newsgroups: comp.lang.c++
Subject: Re: Getting Type Information at Runtime
Message-ID: <47700002@uxa.cso.uiuc.edu>
Date: 26 May 90 14:01:00 GMT
References: <40876@brunix.UUCP>
Lines: 68
Nf-ID: #R:brunix.UUCP:40876:uxa.cso.uiuc.edu:47700002:000:3089
Nf-From: uxa.cso.uiuc.edu!wevg0324    May 26 09:01:00 1990


	First let me say that I DO recognize that sometimes it is very
useful to determine an object's class at runtime, expecially in debug printf's.
HOWEVER:
> What I want to come up with is a simple way to do things like
>    switch (object->type()) {
>      case class1:  dosomething1(); break;
>      case class2:  dosomething2(); break;
>      case class3:  dosomething3(); break;
>    }
	You almost certainly do NOT want to use a switch statement here.
Instead use virtual methods (they are both faster, and "more object oriented").
Replace that switch statement with:
	virtual class1::switch_dosomething() { dosomething1(); };
	virtual class2::switch_dosomething() { dosomething2(); };
	virtual class3::switch_dosomething() { dosomething3(); };
and the call:
	object->switch_dosomething();
===============================================================================
> Finally, I'd like the approach to be extendable so that I can test "isa"
> relationships.  That is, I can say things like
>    if (isa(object1, object2)) ...
> and the condition will evaluate to true iff object1 and object2 are of the
> same class or there is a public inheritance path from object2 to object1.

This is a seperate problem.  First create a root super class for all
the objects you wish to compare with isa relations.  Then for each
subclass $name, add the following method to the "root" class, expanding
"$name" as appropriate:
	virtual kindOf$name() { return(false); };
Then in each class "$ThisClass" including "root" add the methods:
	virtual kindOf$ThisClass() { return(true); };
	virtual isa(root *object2) { return(object2->kindOf$ThisClass()); };

NOTE: The call object->kindOf$name() answers the question,
NOTE: is it legal to assign the "object" pointer to a
NOTE: variable of type "$name *ptr" without a cast.

Now rewrite your "if (isa(object1, object2)) ..."
statement as     "if (object1->isa(object2)) ..."

> It should also be possible to test this relationship using class
> identifiers, e.g., something like
>     if (isa(object1, placeHolderForClassX)) ...
>     if (isa(placeHolderForClassX, object1)) ...

Just use the "kindOf$name()" methods directly.
===============================================================================
A closely related trick you may find useful is the method "asA$name()."
The purpose of this method is to allow you to do the following in a
typesafe manner (no casts):
	root_class *root;
	child_class *child1, *child2;
	...
	root = child1;   // Always legal in C++
	...
	child2 = root;   // Normally a cast is needed on this line
			 // but a cast tells the compiler "trust me."

In the root class, for each subclass $name add the method:
	virtual *$name asA$name() { return(0); };  // Or perhaps assert(0).
Then in each class add the method:
	virtual *$name asA$name() { return(this); };
===============================================================================
billvoss@uiuc.edu  or  voss@cs.uiuc.edu  or voss@a.cs.uiuc.edu

Bill Voss  --  Graduate Student, Department of Computer Science
	       University of Illinois in Urbana/Champaign