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From: bs@alice.UUCP
Newsgroups: comp.lang.c++
Subject: Re: virtual constructors
Message-ID: <7331@alice.UUCP>
Date: Wed, 30-Sep-87 21:26:35 EDT
Article-I.D.: alice.7331
Posted: Wed Sep 30 21:26:35 1987
Date-Received: Sat, 3-Oct-87 09:21:43 EDT
References: <895@saturn.ucsc.edu>
Organization: AT&T Bell Laboratories, Murray Hill NJ
Lines: 36
Keywords: constructors, derived
Summary: how to do it



# From: conrad@jupiter.ucsc.edu.UUCP (conrad @ University of California, Santa Cruz; CIS/CE)
# Virtual constructors are not allowed.  So if I need a a copy
# of a particular object which has derived classes, how do I
# do it?  For example: rectangle, circle, line, etc. are derived
# classes of the base class *shape*.  Now I want to make a copy
# of a given shape.

I think you are just thinking about it a little bit wrong. YOU cannot
make a copy of something that you don't know the exact type of, but
with a little bit forethought you can ask IT to copy itself; after
all IT (and nobody else) knows exactly what kind of shape it is
(provided it has suitable virtual functions):

class shape {
	// ...
	virtual shape* copy();
};

class circle : public shape {
	// ...
	shape* copy() { return new circle(*this); }
};

class rectangle : public shape {
	// ...
	shape* copy() { return new rectangle(*this); }
};


my_program(shape* p)
{
	shape* p2 = p->copy();	// make a copy of p
	// ...
}