Re: [boost] [variant] Maintainer

Subject: Re: [boost] [variant] Maintainer
From: Peter Dimov (lists_at_[hidden])
Date: 2015-07-14 14:33:13

Larry Evans wrote:

> Thanks Peter.
>
> To be explicit:
>
>   tuple<> t0;
>
> would be allowed, but:
>
>   variant<> v0;
>
> wouldn't compile because there are "zero valid values" in a variant<>?

Yes; you can use the type variant<> but you can't declare a variable of it. 
N4542 says

variant<>
A variant without alternatives cannot be constructed; it is otherwise an 
allowed
type. It is easier to allow it than to forbid it.

which is consistent with variant being a sum type without an extra empty 
state (same as boost::variant).

For a variant that has an extra empty state, one would expect variant<> to 
be instantiable and always empty.