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From: amehta@ptolemy1.rdrc.rpi.edu
Newsgroups: comp.lang.c++
Subject: Can a base class know which derived class is constructed?
Message-ID: <H1B%`K#@rpi.edu>
Date: 12 Sep 90 12:56:54 GMT
Reply-To: amehta@ptolemy1.rdrc.rpi.edu ()
Organization: Rensselaer Polytechnic Institute, Troy NY
Lines: 48

Hi,

Sorry if this has been asked before!

I have a base class, say BASE, and several derived classes.
Can the base class know which derived class is being created
at the time of construction?  I want to avoid passing parameters
if possible.

I tried a virtual identity method, but the method becomes valid
only AFTER construction.  Consider:

/*---------------------------------------------------------*/
#include <stdio.h>
class BASE { public:
   BASE () { printf ("Cons:  BASE\n"); identify(); }
   virtual void identify () { printf ("Ident: I am a BASE\n"); }
};
class DER1 : public BASE { public:
   DER1 () { printf ("Cons:  DER1\n"); identify(); }
   virtual void identify () { printf ("Ident: I am a DER1\n"); }
};
class DER2 : public DER1 { public:
   DER2 () { printf ("Cons:  DER2\n"); identify(); }
   virtual void identify () { printf ("Ident: I am a DER2\n"); }
};
main() {
   printf ("Starting\n");
   DER2 d;
   d.identify();
   printf ("Done\n");
}
/*-------- This program produces as output: -------*/
Starting
Cons:  BASE
Ident: I am a BASE
Cons:  DER1
Ident: I am a DER1
Cons:  DER2
Ident: I am a DER2
Ident: I am a DER2
Done
/*-------------------------------------------------*/

Any help would be much appreciated!

Thanks
Alok