Re: [boost] Re: Another safe_bool alternative?

From: Douglas Paul Gregor (gregod_at_[hidden])
Date: 2003-12-03 14:25:08

On Wed, 3 Dec 2003, Rob Stewart wrote:
> From: Douglas Paul Gregor <gregod_at_[hidden]>
> 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?

Member data pointers are nothing more than glorified offsets. &impl::x
essentially represents the offset of "x" from the start of any given
"impl" object.

        Doug