Re: [boost] Re: Another safe_bool alternative?

From: Rob Stewart (stewart_at_[hidden])
Date: 2003-12-03 14:00:31

From: Douglas Paul Gregor <gregod_at_[hidden]>
> 
>   struct some_safe_bool_nested_class {
>     int my_data_member;
>   };
> 
>   typedef int some_safe_bool_nested_class::*safe_bool_type;
> 
> You don't ever need an instance of some_safe_bool_nested_class.

Here's the context for what you're proposing.  It's code that
Dave posted in an earlier message.

   template <class T>
   struct bool_testable
   {
   private:
      struct impl { int x; };
      typedef int impl::*safe_bool;
   public:
      operator safe_bool() const
      {
         return !*static_cast<T const*>(this) ? 0 : &impl::x;
      }
   };

I'm clearly missing something.  How can there be an address for
impl::x if there's no instance of impl?

Were impl::x declared static, there could be an instance --
though you'd need to define it somewhere -- so you could take its
address.  Lacking that, "impl::x" must refer to something and so
it must either come from a dummy dm in bool_testable or from the
derived class (through a template parameter).

-- 
Rob Stewart                           stewart_at_[hidden]
Software Engineer                     http://www.sig.com
Susquehanna International Group, LLP  using std::disclaimer;