[boost] Re: unions

From: Eric Friedman (ebf_at_[hidden])
Date: 2003-10-07 20:59:50

E. Gladyshev wrote:

> --- Eric Friedman <ebf_at_[hidden]> wrote:
>>In the context of my_class, Variant is an "unknown" variant type. 
>>Nonetheless, objects of Variant (e.g., var_) operate according to rules 
>>that remain the same regardless of the types bounded by the variant. In 
>>the context of my_class, objects of Variant behave as any other 
>>variant<...> might. In short, I don't see any unpredictability here.
> 
> 
> See my previous post on why it is unpredictable.
> 
> Another requirement is that I'd like to make sure that
>     template <typename Variant>
>     class my_class
>     {
>       Variant var_;
>     };
> 
>   OR
> 
>     template <typename Variant, typename A = std::allocator<Variant> >
>     class my_class
>     {
>       std::vector<Variant, A> var_;
>     };
> 
> never touches the heap.
> 
> Eugene

In general it is impossible to avoid the heap, unless we go back to 
double-storage. (I suppose a policy might be workable here, but I'd like 
to stay away from that option for the time being.)

Eric

P.S. You might try this workaround (sorta hackish):

   template <typename Variant>
   class my_class
   {
     typedef typename mpl::push_front<
         typename Variant::types
       , boost::empty
       >::type var_t;

     var_t var_;
   };