Newsgroups: comp.lang.c++
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From: jgro@lia (Jeremy Grodberg)
Subject: Re: Virtrual memory allocation
Message-ID: <1991Mar20.224829.2611@lia>
Reply-To: jgro@lia.com (Jeremy Grodberg)
References: <23659@well.sf.ca.us> <1896@news.tcs.com> <13476@helios.TAMU.EDU>
Date: Wed, 20 Mar 91 22:48:29 GMT

In article <13476@helios.TAMU.EDU> jadam@cs.tamu.edu (James P Adam) writes:
>[How do I create new objects of derived types given only their base class
> references?].
>   The problem I am having is in the constructors for class Move.  My
>basic desire is to allocate space inside these constructors to hold
>objects of class Position.  What I originally tried to do was:
>    Move::Move( Position& newFrom, Position& newTo )
>    {
>       From = new( newFrom );  // error checking removed...
>       To   = new( newTo   );      // ...for the sake of simplicity
>    }
>   The compiler doesn't like this, and I can commiserate with it.
>Obviously, I can't say something like "From = new( Position )", since
>I don't want sizeof( class Position) bytes, I want enough memory
>to hold the (unknown) class that's being passed in.
>   Three suggestions have been made to me for solving this problem.  
>  1) Overload the new operator.  I'm not really sure how this would 
>solve my problem, unless I'm supposed to say something like
>"From = newFrom.new()", and create a virtual new operator for each class.
>  2) Create a sizeOf() operator for each class, which would work as follows:
>"From = new( newFrom.sizeOf() )".
>  3)  Force the calling process to allocate the space && simply copy a 
>pointer in Move's constructor.
>
> [...]
>
>     Option #3 is weak because it requires the "user" programmer to
>properly allocate objects prior to each call to a Move constructor.
>     Is there an approved-of, or philosophically sound, way of
>accomplishing what I am trying to do?

The method which I like best, and which seems to raise the fewest objections,
is to create a virtual method called clone() in your base class (Position),
which returns a copy of the object.  Yes, you must override it everywhere,
but if you forget, you just get the base class part, instead of a core dump
(although to some people this is not an advantage).  Unfortunately, as with
asking an object for its type, there is no way in C++ to have the compiler
do all the work for you; you will have to rely on programming everything
correctly.  Clone(), however, is trivial enough that if you remember to 
do it at all, you will do it right.

-- 
Jeremy Grodberg      "Show me a new widget that's bug-free, and I'll show
jgro@lia.com         you something that's been through several releases."