Path: utzoo!utgpu!news-server.csri.toronto.edu!mailrus!uunet!brunix!sdm
From: sdm@cs.brown.edu (Scott Meyers)
Newsgroups: comp.lang.c++
Subject: Getting Type Information at Runtime
Message-ID: <40876@brunix.UUCP>
Date: 25 May 90 15:58:40 GMT
Sender: news@brunix.UUCP
Reply-To: sdm@cs.brown.edu (Scott Meyers)
Organization: Brown University Department of Computer Science
Lines: 43

I'm currently investigating ways to get the type of an object at runtime.
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;
    }

This rules out simply returning a string containing the classname.

I'd also like the scheme to be fairly foolproof and robust, meaning that
multiple people working semi-independently on the same system can easily
create classes that support the protocol without stepping on each other's
toes, and adding new classes doesn't normally require modifying anything
outside that class.  This rules out using an enumerated type.  A standard
scheme seems to be to declare a static char in each class, then use the
address of that char as the identifier for the class.  This approach seems
promising, but in trying to put all the pieces together, I've found it
isn't always as smooth as I'd hoped.  For example, cfront complains if the
static char isn't initialized, so you have to put superfluous
initializations in your code...

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.
It should also be possible to test this relationship using class
identifiers, e.g., something like

    if (isa(object1, placeHolderForClassX)) ...
    if (isa(placeHolderForClassX, object1)) ...

I know that various libraries have implemented schemes that offer this kind
of functionality.  What I'd like to know is how they do it, and what the
advantages and disadvantages are.  Is there a simple, foolproof, robust,
way to get and test class relationships at runtime?

Scott Meyers
sdm@cs.brown.edu