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Subject: Re: [boost] [variant2] documentation request
From: Andrzej Krzemienski (akrzemi1_at_[hidden])
Date: 2019-03-06 15:08:36

sob., 2 mar 2019 o 13:18 Andrzej Krzemienski <akrzemi1_at_[hidden]>

> Hi Peter,
> Given that the never-empty guarantee appears to be at the heart of the
> motivation for boost::variant2, I would suggest to:
> 1. put in the documentation some motivation behind pursuing the
> never-empty guarantee.
> 2. Make it clear that never-empty guarantee is not a strong
> exception-safety guarantee.
> 3. Describe the algorithm for achieving the guarantee.

Somewhat related to that. When I try to study the documentation to see what
the assignment guarantees, I get this:

constexpr variant& operator=( const variant& r )
  noexcept( mp_all<std::is_nothrow_copy_constructible<T>...,
    std::is_nothrow_copy_assignable<T>...>::value );


   Let j be r.index().

      If index() == j, assigns the value contained in r to the value
      contained in *this.

      Otherwise, equivalent to emplace<j>(get<j>(r)).

I am interested in the latter ("Otherwise") case, so, going to emplace:

template<size_t I, class... A>
  constexpr variant_alternative_t<I, variant<T...>>&
    emplace( A&&... a );

   - Requires:

   I < sizeof(T…).

   Destroys the currently contained value, then initializes a new contained
   value as if using the expression Ti(std::forward<A>(a)…).

This doesn't mention any double buffering. No tricks, no "monostate". It
reads as if the variant is left with no contained object whatsoever.


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