Re: [boost] Variant2 review

Subject: Re: [boost] Variant2 review
From: Peter Dimov (pdimov_at_[hidden])
Date: 2019-04-08 17:53:46

Andrzej Krzemienski wrote:

> Such thing could be achieved if the behavior of variant's assignment and 
> emplacement was controlled by a type trait. The default implementation for 
> unfriendly types would be compile-time error; but users would be able to 
> specialize it for their instantiations of variant in order to assign 
> different semantics: double buffering, or setting the 
> valueless_by_exception state.

This can't work in general. If you have

template<class... T> class lib1::X
{
private:

    variant<T...> state_;
};

and

template<class... T> class lib2::Y
{
private:

    variant<T...> state_;
};

it's not possible for lib1 and lib2 to choose their preferred variant 
behaviors independently of one another.