Re: Automatically create a clone() function for derived classes

From:

"Thomas J. Gritzan" <phygon_antispam@gmx.de>

Newsgroups:

comp.lang.c++

Date:

Fri, 20 Feb 2009 18:03:36 +0100

Message-ID:

<gnmnpk$13d$1@newsreader2.netcologne.de>

Juha Nieminen schrieb:

I'm sure this is a frequently asked question, but...

It is (but it's not in the FAQ)!

  Suppose that I have a hierarchy of objects which are typically
allocated dynamically and handled with some smart pointer or whatever.
However, I want to be able to create copies of those objects, while
having only a pointer-to-base-class. The solution is to write a clone()
member function like this:

class Base
{
public:
    virtual Base* clone() const { return new Base(*this); }

    // other stuff here
};

  The problem is that this clone() function must be replicated in all
the derived classes as well, and their implementation is always the
same, except for the type which is being allocated. For example:

[...]

1) You can use CRTP to avoid writing all the code again and again.
2) You can try some programming by contract to check if someone forgot
to implement its clone().

Both is described in this posting (the complete thread might be of
interest, too):
http://groups.google.de/group/comp.lang.c++.moderated/msg/993079dca61f3a20?hl=de

--
Thomas