Re: Help with returns_ref_to_const meta-function

From:

Edd <edd@nunswithguns.net>

Newsgroups:

comp.lang.c++.moderated

Date:

Sat, 10 Nov 2007 16:41:03 CST

Message-ID:

<1194715802.501912.240770@22g2000hsm.googlegroups.com>

Hello Dave,

On 9 Nov, 21:35, David Abrahams <d...@boost-consulting.com> wrote:

on Thu Nov 08 2007, Edd <edd-AT-nunswithguns.net> wrote:

      template<class R>
      static ref_to_const
      testfunc(R&, typename enableifnonconst<R>::type = 0);

      template<class R>
      static ref_to_nonconst
      testfunc(R&, typename enableifconst<R>::type = 0);

      static val testfunc(...);

^^^
This will give you trouble when non-PODs are returned.

Why is this? I had a look in the standard for something to do with the
interaction of overload viability in relation to ellipses and non-POD
types, but couldn't find anything :(

Try the code below (untested).

   class Tester
   {
         typedef char val;
         typedef char (&ref_to_nonconst)[2];
         typedef char (&ref_to_const)[3];

         static Arg& testval;

         template<class R>
         static ref_to_const
         testfunc(R&, int, typename enableifnonconst<R>::type = 0);

         template<class R>
         static ref_to_nonconst
         testfunc(R&, int, typename enableifconst<R>::type = 0);

         template <class R>
         static val testfunc(R, ...);

   public:
         static const size_t value = sizeof(testfunc(f(testval),0));

   };

That does indeed work on the compilers I tried, but as I say, I'm
unsure why the change is needed.

I changed the f() overloads to return references to and values of a
type with a user defined constructor and both versions seemed to work
just the same. Which part of non-POD-ness will cause trouble?

Kind regards,

Edd

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