Clone an object with an abstract base class

From:
Urs Thuermann <urs@isnogud.escape.de>
Newsgroups:
comp.lang.c++
Date:
11 Oct 2014 23:46:54 +0200
Message-ID:
<ygfppdy2sa9.fsf@tehran.isnogud.escape.de>
I want to clone an object through a pointer to its abstract base
class. Say I have the following classes

    class B {
            virtual void foo() = 0; // abstract base class
    };
    class D1 : public B {
            virtual void foo();
    };
    class D2 : public B {
            virtual void foo();
    };

Then I can clone objects when I know the subclass of B

    void f1(D1 *d1, D2 *d2) {
            B *b1 = new D1(*d1);
            B *b2 = new D2(*d2);
    }

but not if I only have a pointer to the abstract base class

    void f2(B *b) {
            B *b1 = new B(*b); // error: cannot create a B
    }

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?

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?

urs

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