Re: Ill-formed C++0x code or compiler bug (GCC) ?

From:
Marc <marc.glisse@gmail.com>
Newsgroups:
comp.lang.c++
Date:
Fri, 28 Jan 2011 14:39:51 +0000 (UTC)
Message-ID:
<ihukfn$v4t$1@news-rocq.inria.fr>
SG wrote:

In the following C++0x code I tried to clone an object by using a
clone member function (if it exists) and falling back on a copy
constructor:

   #include <type_traits>


What for?

   struct use_copy_ctor {};
   struct prefer_clone_func : use_copy_ctor {};

   template<class T>
   auto clone(T const* ptr, prefer_clone_func)
   -> decltype(ptr->clone())
   { return ptr->clone(); }

   template<class T>
   auto clone(T const* ptr, use_copy_ctor)
   -> decltype(new T(*ptr))
   { return new T(*ptr); }

   struct abc {
     virtual ~abc() {}
     virtual abc* clone() const =0;
   };

   struct derived : abc
   {
     derived* clone() const { return new derived(*this); }
   };

   int main()
   {
     derived d;
     abc* p = &d;
     abc* q = clone(p,prefer_clone_func());
     delete q;
   }

The idea is to use auto...->decltype(expr) to weed out ill-formed
expressions as part of the template argument deduction (SFINAE) and to
resolve a possible ambiguity between both clone function templates via
partial ordering w.r.t. the second function parameter.

Unfortunately, GCC 4.5.1 doesn't accept this program:


clang++ accepts it. It gained a lot of C++0x features last week, now
it is just missing a library so we can start playing with it... (it
can't handle the C++0x parts of libstdc++ and libc++ only works on
Mac)

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