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From: jss@hector.UUCP (Jerry Schwarz)
Newsgroups: comp.lang.c++
Subject: Re: cant use virtuals in base constructors
Message-ID: <11492@ulysses.homer.nj.att.com>
Date: 5 May 89 13:56:06 GMT
References: <DOVE.89May4223113@rocket.stars.flab.Fujitsu.JUNET>
Sender: netnews@ulysses.homer.nj.att.com
Reply-To: jss@hector.UUCP (Jerry Schwarz)
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In article <DOVE.89May4223113@rocket.stars.flab.Fujitsu.JUNET> dove@rocket.stars.flab.Fujitsu.JUNET (Webster Dove) writes:
>

The reasons for calls to a virtual member function  in a base
constructor to be made to the base version of the virtual have been
discussed in this group several times recently.  In summary, its
because the members added in the derived class have not yet been
initialized.

>
>Consider the case of someone implementing a base class for others to
>use (e.g. an error handler).  I wanted to use this technique to tell
>at construction time that a derived class had failed to implement its
>own method for something (e.g. type_string()).  I would do this by
>invoking the virtual in the base constructor, and having a dummy
>method in the base class that I could detect.
>

In the pre-release of 2.0 there is a way to detect this at compile
time.

class B {
public: virtual void f() = 0 ; // f must be supplanted in all
			       // derived classes.  Allocation of
			       // a B (directly or via new) is not
			       // allowed.  
} ;