Re: Mother of all Variants

From: Niall Douglas (s_sourceforge_at_[hidden])
Date: 2019-06-27 20:53:24

> But bugs aside, this is done by testing is_nothrow_swappable_v<T>. Or,
> before C++17, with a construction similar to this:
> 
> Â  template< typename T >
> Â  T& make_lvalue() noexcept;
> 
> Â  constexpr bool is_nothrow_swappable =
> Â Â Â  noexcept(swap(make_lvalue<T>(), make_lvalue<T>()));

Outcome did this in v2.1.0. It didn't work right, swap() wasn't always
instanceable at the point of parsing basic_result. I replaced it with an
exact replica of std::is_nothrow_swappable, and now it works properly in
v2.1.1.

>> In other words, how can it make sense that swap<T> be noexcept without
>> considering the specific type T?

Robert, you may find studying Outcome's strong_swap() of interest.

Ref page: https://ned14.github.io/outcome/reference/functions/strong_swap/

Implementation:
https://github.com/ned14/outcome/blob/develop/include/outcome/detail/value_storage.hpp#L32

tldr; implementing the strong guarantee is much harder than it looks,
and actually offers surprisingly little benefit in practice. It's also a
pain to implement without making constexpr puke, at least in C++ 14. All
in all, the weak guarantee is likely plenty good for almost everybody
almost all of the time.

Niall