[boost] Re: variant - is there an easy way to tell if an object is a variantat compile time?

From: Eric Friedman (ebf_at_[hidden])
Date: 2004-02-05 20:56:02

Brian McNamara wrote:

> On Thu, Feb 05, 2004 at 06:28:27PM -0600, Brock Peabody wrote:
> 
>>Is there an easy way to write:
>>
>>   template <typename T> struct is_variant {
>>
>>      typedef ??? type;
>>   };
>>
>>So far the best I can come up with is:
>>
>>1) T has a member type "types" and 
>>2) T is convertible to make_variant_over<T::types>::type and
>>3) make_variant_over<T::types>::type is convertible to T
>>
>>Does anyone have a better idea?
> 
> 
> Perhaps I'm missing something, but why not just something like
> 
>    template <typename T> struct is_variant {
>       typedef false_ type;
>    };
>    template <typename T1, typename T2, ... /*however many*/> 
>    struct is_variant<variant<T1,T2,...> > {
>       typedef true_ type;
>    };
> 
> ?

Precisely,

   template <typename T> struct is_variant
     : mpl::false_ {};

   template <BOOST_VARIANT_ENUM_PARAMS(typename T)>
   struct is_variant< variant<BOOST_VARIANT_ENUM_PARAMS(T)> >
     : mpl::true_ {};

should do the trick. If this is deemed useful, I can include it in the 
next release. For now, BOOST_VARIANT_ENUM_PARAMS should cover you.

Eric