Re: Virtual constructor?

From:
godfat@gmail.com
Newsgroups:
comp.lang.c++.moderated
Date:
5 Jun 2006 12:50:57 -0400
Message-ID:
<1149460779.813399.235690@u72g2000cwu.googlegroups.com>
Gene Bushuyev wrote:

The problem with using CRTP is that it introduces ambiguity due to multiple
inheritance, rather than overriding the virtual clone() function. Here is an
example,

template <class C>
class cloneable
{
public:
virtual C* clone() const { return new C(*this); }
};

class A : public cloneable<A> { public: A(const A&); A();};

class B : public A, public cloneable<B>
{ public: B(const B&); B();};

int main()
{
  B* b = new B;
  B* c = b->clone();
}

compiler cannot resolve the ambiguity between cloneable<A>::clone() and
cloneable<B>::clone().

--
Gene Bushuyev (www.gbresearch.com)
----------------------------------------------------------------
To see what is in front of one's nose needs a constant struggle ~ George Orwell


I don't know if this would work, and somehow I got different error
messages
from compiler to compiler.

template <class T>
class cloner{
protected:
     T* clone() const{ return new T(*static_cast<T const*>(this)); }
     cloner(){}
};

struct null_type{};

template <class T, class Parent = null_type>
struct cloneable: public Parent, private cloner<T>{
     virtual T* clone() const{ return cloner<T>::clone(); }
     // ^^^ note here

};

class Base: public cloneable<Base>{};
class Derived: public cloneable<Derived, Base>{};

==

Comeau C/C++ 4.3.3:

    error: return type is not identical to nor covariant
           with return type "Base *" of overridden virtual function

  If I change T* to Parent*, it compiles.

==

GCC 3.4.5:

    error: invalid covariant return type for `T* cloneable<T,
Parent>::clone() const [with T = Derived, Parent = Base]'
    error: `cloner<Base>' is an inaccessible base of `Base'

  If I change private inheritance to protected inheritance, plus
  changing T* to Parent*, it compiles.

==

VC++ 7.1:

    error C2555: 'cloneable<T,Parent>::clone': overriding virtual
function return type differs and is not covariant from
'cloneable<T>::clone'

If I change T* to Parent*, it yells another error:
    error C2244: 'cloneable<T,Parent>::clone' : unable to match function
definition to an existing declaration

  The only way to compile under VC++ 7.1 is change T* to null_type* or
  void*, etc.

==

Could anyone elaborate for me?

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