Re: [boost] [variant] awkward recursion

Subject: Re: [boost] [variant] awkward recursion
From: Antony Polukhin (antoshkka_at_[hidden])
Date: 2012-10-31 12:05:37

2012/10/31 Dave Abrahams <dave_at_[hidden]>
>
>
> on Wed Oct 31 2012, "Peter Dimov" <lists-AT-pdimov.com> wrote:
>
> > Steven Watanabe wrote:
> >> I believe that I've seen tickets in trac indicating
> >> that at least one STL implementation checks that
> >> the element type is complete, so we can't even
> >> assume that it works in practice.
> >
> > The variant could perhaps define some dummy complete type X and
> > allocate space for vector<X>, betting on the fact that vector<Y> would
> > have the same size. And if it doesn't, well, the heap is it then.
>
> Yeah, I thought of that.  Very interesting problem space in the end.
> I'm slightly tempted to think about a rewrite.
>

May be it would be better to write a metafunction:

template <class T> size_forward {
    enum ENUM{value = sizeof(T)};
};

User is free to specialize it, so Boost.Variant can use it to get
sizes of types (instead of directly calling to sizeof(T))

Specialization for your case will be:

template <> struct size_forward<vector<Y>> {
enum ENUM{value = sizeof(vector<X>)};
};

I think that such metafunction could be widely used (for example in
Boost.Optional)

--
Best regards,
Antony Polukhin