[boost] Re: Proposal --- A type-safe union

From: Itay Maman (itay_maman_at_[hidden])
Date: 2002-06-14 03:18:06

Andrei Alexandrescu wrote:
> "Douglas Gregor" <gregod_at_[hidden]> wrote in message
> news:200206132337.43800.gregod_at_cs.rpi.edu...
> 
>>On Thursday 13 June 2002 07:54 pm, Itay Maman wrote:
>>
>>>2. Is direct recursion (i.e: variant<int, rec>) allowed?
>>>I think we should not allow instantiations like this:
>>>variant<int, rec> v1;
>>
>>Direct recursion seems to cause more headaches than it fixes
> 
> problems.
> 
> Direct recursion would be *very* desirable. I could enable it in a
> couple of limited instances, but unfortunately not in the general
> case.
> 
> The classic example is when you want to define a tree. A binary tree
> node is either a leaf or a pair of tree nodes:
> 
> typedef Variant<TYPELIST_2(Leaf, std::pair<Self*, Self*>)>
>     TreeNode;
> 
> 
Your example is not what I call 'direct recursion'.
If 'rec' is the pseudo keyword for recursion, then I would like to 
disallow instantiations where 'rec' appears as-is in the type-list:
        typedef Variant<rec, string, char*> Concrete_variant;

-Itay