[Boost-users] Re: [variant] is_a/can_be

From: Eric Friedman (ebf_at_[hidden])
Date: 2005-02-02 16:32:40

Hi,

I can imagine cases where the current behavior might be preferable 
(e.g., uses of variant types in generic contexts).  But I think for the 
vast majority of cases, and especially in non-generic contexts, checking 
the type as you propose is a good idea.

I will determine a way to work this into the library, either by 
modifying the semantics of boost::get or by introducing a new accessor 
function.  Thanks for the input.

Eric Friedman
Co-author of Variant library

Yuval Ronen wrote:
>>>Hi.
>>>I encountered the need to query a boost::variant to which type it
>>>currently contain. I'm talking about something like:
>>>
>>>variant<int, string> v(3);
>>>assert(v.is_a<int>());
>>>assert(!v.is_a<string>());
>>
>>Take a look at get(), it's almost what you need.  Unfortunately, it
>>compiles even when the type cannot be inside the variant.  This can be
>>fixed with mpl::contains:
>>
>>template<typename T, typename Variant>
>>bool variant_is_a(const Variant& v)
>>{
>>      BOOST_MPL_ASSERT(( boost::mpl::contains<typename
> 
> Variant::types,T> ));
> 
>>      return boost::get<T>(&v) != 0;
>>}
>>
>>There should be a version of get that works as variant_is_a above.  IMHO,
>>the current behaviour is more dangerous than that of variant_is_a, so
>>get() should be changed and maybe a new function be created that keeps the
>>current behaviour.  Some code may break with this change with a compile
>>error that would go away if the other version of get is used.
> 
> 
> Your code works beautifully. I really hope this feature will find its way to
> the variant library.