Re: [Boost-users] SFINAE broken in Visual C++ Express?

From: Ovanes Markarian (om_boost_at_[hidden])
Date: 2007-10-31 11:57:27

Ok! Thanks! I got it... The problem is, that I thought that SFINAE would
work from the template class in the template function which does not depend
on the argument.

is_const is a member of the traits_class which this type is derived from...
Pretty much like:

template<class T>
struct traits_class
{
 // some type calcs here
  typedef ...     result_type;
  enum { is_const = false };
};

template<class T>
struct traits_class<const T>
{
  // some type calcs here
  typedef ...     result_type;
  enum { is_const = true };
};

template<class T>
struct some_class : traits_class<T>
{

  template<class LookUpType_>
       typename boost::disable_if_c<is_const, result_type>::type
           at(LookUpType_ const& key)
       {
           return has_key(view_, key); //has key is some external function,
which receives the result_type from the view_
       }

};

My idea was to enable only
result_type at(LookUpType_ const& key) const;
if T is const

OR
enable non-const version.
result_type at(LookUpType_ const& key);
if T is not const

On the other hand Compiler does this for me automatically if T is const (but
if T is non-const I would like to disable the const access).

Anyway in that case I thought SFINAE should work, but it did not. Do I
really need a type to be a template param of the member function, even if
the class is a template itself?

Thanks for your help!

Ovanes

On 10/31/07, David Abrahams <dave_at_[hidden]> wrote:
>
>
> on Tue Oct 30 2007, "Ovanes Markarian" <om_boost-AT-keywallet.com> wrote:
>
> > Hello!
> >
> > Does anybody else experience a problem using enable_if templates in
> Visual C++ Express 2005 SP1?
> > I get compiler errors in this code:
> >
> >         template<class LookUpType_>
> >         typename boost::enable_if_c<is_const, result_type>::type
> >             at(LookUpType_ const& key)const
> >         {
> >             return has_key(view_, key);
> >         }
> >
> >         template<class LookUpType_>
> >         typename boost::disable_if_c<is_const, result_type>::type
> >             at(LookUpType_ const& key)
> >         {
> >             return has_key(view_, key);
> >         }
> >
> > It blames that type is not a member of enable_if_c or disable_if_c?
> > This is really strance, since this should not be considered by
> > compiler as a possible template instantiation. is_const is an enum
> > value containing either true or false.
>
> In order for SFINAE to work, at least one argument to [dis|en]able_if
> has to depend on a template argument (in this case, LookUpType_ is the
> only one).  As written, none of them do.
>
> I don't know what you're doing with is_const, but at least in the
> Boost world that is a template that requires a template argument in
> that context.  For example,
>
>         template<class LookUpType_>
>         typename boost::disable_if_c<boost::is_const<LookUpType_>,
> result_type>::type
>             at(LookUpType_ const& key)
>         {
>             return has_key(view_, key);
>         }
>
> HTH,
>
> --
> Dave Abrahams
> Boost Consulting
> http://www.boost-consulting.com
>
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