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From: jbuck@carson.berkeley.edu (Joe Buck)
Newsgroups: comp.lang.c++
Subject: Re: Cloning derived classes
Keywords: inheritance, cloning, derived class
Message-ID: <35956@ucbvax.BERKELEY.EDU>
Date: 2 May 90 19:42:29 GMT
References: <1019@rna.UUCP>
Sender: usenet@ucbvax.BERKELEY.EDU
Reply-To: jbuck@carson.berkeley.edu (Joe Buck)
Lines: 35

In article <1019@rna.UUCP>, kc@rna.UUCP (Kaare Christian) writes:
> I recently needed to make a copy (a clone) of a class object, given only a
> base class pointer to the object.  C++ lets you do some things with a class,
> knowing only its base type, but it doesn't provide an easy way to make
> a copy.  My solution was to make a virtual member function that
> returned the size of each of the derived classes, and then make an
> operator= function (left arg, ref pointer to base, right arg, ref to
> base) that did the work - it called the size fn of the right operand to
> determine how much space was needed, allocated it, did a bitwise copy,
> then did some housekeeping.

You're working too hard.

All you really need to do is call the copy constructor from a virtual
function.  In the base class (I'll assume it's virtual) do

class BaseType {
	virtual BaseType* copy () = 0;
}

For EACH derived class you must redefine the copy function, as follows.

BaseType* DerivedType::copy () { return new DerivedType(*this); }

If your BaseType isn't a virtual base class, then you'd define
copy in the base class to return new BaseType(*this).

This is simple; it calls the copy constructor for the appropriate type
to do the copy.  This assumes either that the default copy constructor
will do, or that you've defined copy constructors when they won't.


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