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From: dsr@mir.mitre.org (Douglas S. Rand)
Newsgroups: comp.lang.c++
Subject: Re: How do you copy a Derived object?
Message-ID: <1991Mar26.175026.12955@linus.mitre.org>
Date: 26 Mar 91 17:50:26 GMT
References: <1991Mar26.152847.5853@athena.mit.edu>
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Reply-To: dsr@mir.mitre.org (Douglas S. Rand)
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In article <1991Mar26.152847.5853@athena.mit.edu>, ahodgson@athena.mit.edu (Antony Hodgson) writes:
> 
> Suppose I have a function, void f( Base& b ), and inside f() I want to
> make a local copy of whatever b is.  Since b could actually be a derived
> class, I can't simply do the following:
> 
> void f( Base& b )
>     {	Base B(b);  ...  }
> 
> nor can I do
> 
> void f( Base& b )
>     {   Base* B = new Base( b ); ...  }
> 
> What I really need to do is something like Base* B = new (typeof(b))(b).
> Is this possible?  Thanks in advance for any help.  Email replies are fine.
> 
> Tony Hodgson
> ahodgson@hstbme.mit.edu
> 
> 

You can't do it that way.  There is no direct runtime information about the
class and copying is a sticky point anyway since different people mean
different things (for example: deep copy and shallow copy).  What you can
do is define a virtual copier for each class which allocates a new instance
and copies all the interesting pieces:

virtual Foobarclass:copier(Base& ob) {
	return(new Foobarclass((Foobarclass&) ob));
}

Where you have defined Foobarclass(Foobarclass&).  

There are no reasonable automatic semantics for copy.

-- 
Douglas S. Rand 
Internet:   <dsrand@mitre.org>
Snail:	    MITRE, Burlington Road, Bedford, MA 
Disclaimer: MITRE might agree with me - then again...
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