Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!cs.utexas.edu!samsung!think!mintaka!mit-eddie!apollo!vinoski
From: vinoski@apollo.HP.COM (Stephen Vinoski)
Newsgroups: comp.lang.c++
Subject: Re: Chameleon objects (calling virtual functions from constructors)
Keywords: virtual functions, constructor functions.
Message-ID: <4749b393.20b6d@apollo.HP.COM>
Date: 7 Dec 89 18:59:00 GMT
References: <11266@csli.Stanford.EDU>
Sender: root@apollo.HP.COM
Reply-To: vinoski@zep.UUCP (Stephen Vinoski)
Organization: Apollo Computer, Chelmsford, MA
Lines: 37

In article <11266@csli.Stanford.EDU> Neil%teleos.com@ai.sri.com writes:
>If a class is derived from such a base class, and one of the virtual
>functions is called from the base class constructor, the wrong virtual
>function is called, since during the execution of the base class constructor,
>the virtual function pointer table is set up as for the base class,
>and is not changed to show the derived class virtual functions until
>the derived class constructor is entered.

From Lippman's C++ Primer, page 352:

  "There are three cases in which an invocation of a virtual function is
resolved statically at compile time:

  1.  When a virtual function is invoked through an object of the class type.
                                   .
                                   .
                                   .

  2.  When a virtual function is explicitly invoked through a pointer or
reference using the class scope operator.
                                   .
                                   .
                                   .

  3.  When a virtual function is invoked within either the constructor or the
destructor of a base class.  In both cases, the base class instance of the
virtual function is called since the derived class object is either not yet
constructed or already destructed."

I don't see how it could be done any other way.


-steve

| Steve Vinoski       | Hewlett-Packard Apollo Div. | ARPA: vinoski@apollo.com |
| (508)256-6600 x5904 | Chelmsford, MA    01824     | UUCP: ...!apollo!vinoski |
| "My second wife isn't even born yet."                                        |