Re: Virtual constructor?

From:

"Wu Yongwei" <wuyongwei@gmail.com>

Newsgroups:

comp.lang.c++.moderated

Date:

9 Jun 2006 19:42:10 -0400

Message-ID:

<1149864156.990341.175970@g10g2000cwb.googlegroups.com>

kanze wrote:

     template< typename Derived, typename Base >
     class Clonable : public Base
     {
     public:
         Derived* clone() const
         {
             return new Derived(
                 *dynamic_cast< Derived const* >( this ) ) ;
         }

     private:
         virtual Base* doClone() const
         {
             return clone() ;
         }

     } ;

     template< typename Base >
     class AbstractClonable
     {
     public:
         AbstractClonable* clone() const
         {
             Base* result = doClone() ;
             assert( typeid( result ) == typeid( *this ) ) ;
             return result ;
         }

     private:
         virtual Base* doClone() const = 0 ;
     } ;

Hi James,

I actually find your code quite useful. However, I do find an issue,
when the class hierarchy is as follows:

class A : public public AbstractClonable<A> {};
class B : public Clonable<B, A> {};
class C : public Clonable<C, A> {}'

B* p = new C();

Then p->clone() will really return a B object instead of C.

The solutions I find so far are:

1) Declare only leaf-node objects can be instantiated and let B inherit
from A instead of Clonable<B, A>;
2) Change Clonable::clone() to this effect:

    Derived* clone() const
    {
        if (typeid(Derived) == typeid(*this))
            return new Derived(*static_cast<const Derived*>(this));
        else
            return static_cast<Derived*>(doClone());
    }

Any comments? Any better solutions? Anyone knows about the overhead of
typeid?

Best regards,

Yongwei

      [ See http://www.gotw.ca/resources/clcm.htm for info about ]
      [ comp.lang.c++.moderated. First time posters: Do this! ]

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