Re: [Boost-users] [utility] question about enable_if

From: Christian Henning (chhenning_at_[hidden])
Date: 2008-02-15 13:18:58

Thanks Steven, that worked.

Christian

On Fri, Feb 15, 2008 at 1:05 PM, Steven Watanabe <watanabesj_at_[hidden]> wrote:
>
> Christian Henning wrote:
> > Hi there, I'm having a problem with using enable_if. For the sake of
> > simplicity I have created the following test code which compiles fine.
> >
> > #include <string>
> >
> > #include <boost/mpl/and.hpp>
> > #include <boost/utility/enable_if.hpp>
> >
> > using namespace std;
> > using namespace boost;
> >
> > struct reader {};
> >
> > template< typename String > struct is_string : public mpl::false_ {};
> > template<> struct is_string<std::string> : public mpl::true_ {};
> >
> > template< typename Device > struct is_device : public mpl::false_ {};
> > template<> struct is_device<reader> : public mpl::true_ {};
> >
> > template< typename String>
> > struct enable_if_helper
> > {
> > typedef typename enable_if< typename is_string< String >::type >::type type;
> > };
> >
> > template<typename Device >
> > void
> > foo( typename enable_if< typename is_device<Device>::type >::type* ptr = 0 )
> > {}
> >
> > template<typename String>
> > void foo( typename enable_if< typename is_string<String>::type
> >
> >> ::type* ptr = 0 )
> >>
> > { foo<reader>(); }
> >
> >
> > int main(int argc, _TCHAR* argv[])
> > {
> >    foo<std::string>();
> >       return 0;
> > }
> >
> >
> > If I changed the string foo() interface to use the enable_if_helper
> > the code doesn't compile anymore. Why is that?
> >
> > I've changed the foo ( with the string template parameter ) to the following:
> >
> > template<typename String>
> > void foo( typename enable_if_helper< String >::type* ptr = 0 )
> > { foo<reader>(); }
> >
> >
> > Anyone can help me out here? I'm using Visual Studio 8.
> >
>
> Basically, enable_if_helper is instantiated and fails and SFINAE doesn't
> protect you. Use the following:
>
> template< typename String>
> struct enable_if_helper
>  : enable_if< is_string< String > >
> {
> };
>
>
> In Christ,
> Steven Watanabe
>
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