[boost] Re: In-place construction for variant

From: Eric Friedman (ebf_at_[hidden])
Date: 2003-10-28 01:45:14

Eugene,

E. Gladyshev wrote:
> Also, I mentioned it to Eric that the current 
> implementation doesn't actually provide basic guarantees. 
> He was going to look into that.

Are you referring to the following problem?

   T f();

   class my_type
   {
     T t_;
   public:
     my_type() { }
     my_type(const my_type& other)
       : t_( f() )
     {
     }
   };

   variant< int, my_type > v;

   int main()
   {
     my_type m1;

     my_type m2 = m1; // ok
     v = m1; // potentially undefined: does f visit v?
   }

If so, then yes, I've devised a solution for that problem, but it incurs 
quite a costly run-time overhead (i.e., a backup copy of lhs content 
required even when heap backup of lhs content is not).

A better solution, I believe, is simply to document that accessing a 
variant *during* assignment will result in undefined behavior. That way, 
unless the user really needs to do something like the above, such (IMHO, 
excessive) overhead will be avoided.

If you're *not* referring the above problem, then I'd like to know when 
else variant does not provide the basic guarantee.

Thanks,
Eric