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From: jbuck@galileo.berkeley.edu (Joe Buck)
Newsgroups: comp.lang.c++
Subject: Re: Virtual functions used in constructor illegal ! ??
Keywords: virtual functions, constructor
Message-ID: <39733@ucbvax.BERKELEY.EDU>
Date: 26 Nov 90 20:21:11 GMT
References: <822@orthogo.UUCP>
Sender: usenet@ucbvax.BERKELEY.EDU
Reply-To: jbuck@galileo.berkeley.edu (Joe Buck)
Lines: 33

Not illegal, they just don't do what you think.

In article <822@orthogo.UUCP>, basti@orthogo.UUCP (Sebastian Wangnick) writes:
> I just recognized that my cfront 2.0 installs the vtable
> of a derived class only *after* the constructor of the
> base class is called.

> This implies that you are not allowed to use virtual member
> functions within the constructor of an abstract base class.

This is documented in Ellis and Stroustrup's Annotated C++ Reference
Manual, which the ANSI committee is using as the base document for
the official standard.  The standard actually says that if you
do call a virtual method in the constructor, that the base class
method is always called, even if you redefine the method.  That is,
if you have

class Derived: public Base {
	...
}

any virtual methods you call in the constructor for Derived act
as though the object were of type Base.

> Now: Is this a bug or a feature ??

It's a feature (and a required one at that).  Any compiler that
does what you want violates the standard.


--
Joe Buck
jbuck@galileo.berkeley.edu	 {uunet,ucbvax}!galileo.berkeley.edu!jbuck