Re: Clone an object with an abstract base class

From:

Paavo Helde <myfirstname@osa.pri.ee>

Newsgroups:

comp.lang.c++

Date:

Sun, 12 Oct 2014 02:57:48 -0500

Message-ID:

<XnsA3C46F8259AAFmyfirstnameosapriee@216.196.109.131>

Urs Thuermann <urs@isnogud.escape.de> wrote in
news:ygfppdy2sa9.fsf@tehran.isnogud.escape.de:

I want to clone an object through a pointer to its abstract base
class.
With GCC and -std=c++11 I can write

    void f2(B *b) {
            B *b1 = new auto(*b);
    }

Is this the correct and preferred way in C++11?

To work as expected, one needs to have dynamic type lookup at runtime
here, but 'auto' is a compile-time concept, so I don't think it helps
here.

Moreover, it should know all possible run-time types of B-derived classes
at compile time, but this is in principle impossible (as one can add new
dynamic libraries with new classes at runtime).

In C++98 the only way I found is to add a pure virtual function

    B *B::clone() const

and

    B *D1::close() const { return new D1(*this); }
    B *D2::close() const { return new D2(*this); }

and then call b->clone() instead of the new operator.

Is there a way to do this in C++98 without the need to add a new
function like clone() in each derived class?

Yes, that's the way to do it. 1 extra line per class is not so much.

Cheers
Paavo