[boost] Re: Re: Re: unions

From: Eric Friedman (ebf_at_[hidden])
Date: 2003-10-06 22:11:40

Eugene,

E. Gladyshev wrote:
> Eric Friedman wrote:
> > Note, too, that this code will never execute for any T that is nothrow
> > copyable or if the first bounded type of the variant is nothrow
> > default-constructible (e.g., variant<int, ...> or variant<boost::empty,
> > ...>, etc.).
>
> Is there any way to use 'variant' in a policy based library
> with a wide set of types?
> I'd like to have a memory policy in my library.
> How can I meet these constraints if my types
> are not very well predefined?

Do you mean you need to ensure allocation of some user-provided type occurs
only via some user-provided allocator? If so, variant unfortunately does not
support such functionality at this time.

However, as I noted in previous messages, allocation on the heap can be
avoided entirely for any type Ti in variant<T1, ..., TN> if T1 is nothrow
default-constructible. I would suggest then that, if none of your types are
guaranteed to have a nothrow default constructor, you work with
variant<boost::empty, T1, ..., TN>. Even if you never make direct use of
boost::empty, this will ensure all allocation will be via placement new and
never on the heap.

HTH,
Eric