Re: Bewildering of polymorhpical class

From:

"Richard Smith" <richard@ex-parrot.com>

Newsgroups:

comp.lang.c++.moderated

Date:

22 Jun 2006 06:54:29 -0400

Message-ID:

<1150905630.180804.42980@p79g2000cwp.googlegroups.com>

wij@seed.net.tw wrote:

Hi:

  The copy constructors of B and D are endless self-recursive.
  How should the copy constructor and the member clone be defined
  for such polymorphical classes containing B* member?

  class B {
      B *_pb;
    public:
      B(const B& src) : _pb(new B(src)) {};

This should probably be

B(const B& src) : _pb(src._pb ? new B(src._pb) : 0) {}

virtual ~B() { delete _pb; _pb=NULL; };

Technically there's no need to set _pb to NULL, though it does no harm.

I notice you have a user-defined copy constructor and destructor, but
no assignment operator. This is nearly always a mistake. In this
case, the assignment operator will take a copy of the raw pointer
without doing a deep copy resulting in both objects thinking they own
the object.

      virtual B* clone(void) { return new B(*this); };
  };

  class D : public B {
      B *_pb;

Why do B and D both have a member called _pb? That's seems very
counterintuitive.

    public:
      D(const D& src) : B(src), _pb(new D(src)) {};
      virtual ~D() { delete _pb; _pb=NULL; };
      virtual D* clone(void) { return new D(*this); };
  };

--
Richard Smith

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