Re: std::launder and its usage in Boost

From: Niall Douglas (s_sourceforge_at_[hidden])
Date: 2022-02-18 18:01:31

On 18/02/2022 10:33, Mikhail Kremniov via Boost wrote:

> I wonder, what is the Boost community's opinion of std::launder? In 
> particular, of its necessity when accessing an object that was created 
> via placement-new in an aligned_storage. As I can see, it's used by 
> Boost.Beast in its implementation of the variant type, but not in other 
> parts of Boost.

Beast likely doesn't need to use it. He probably added it back when it 
was thought it would be needed, but CWG realised very very late on in 
the 17 release cycle that it was overly dangerous to existing code and 
they undid the need for it for the 17 release.

> So, can switching from -std=c++14 to -std=c++17 be a breaking change 
> when using Boost? The fact that Boost.Variant and Boost.Optional don't 
> use std::launder - is it an oversight or a conscious decision?

In practice, you only need to launder const types. Otherwise you don't 
need to, in practice (except for corner cases, which don't matter for 
anything Optional or Variant or Outcome do).

You also only need to launder const types if and only if you ever mutate 
them. So now the subset where laundering is needed is if and only if 
your type is const, and you mutate it.

One solution is to not store your type as const, but otherwise treat it 
as const. Then laundering is not necessary.

Another solution is don't allow mutation without enclosing type lifetime 
change. This is what Outcome does, and almost certainly what Variant and 
Optional also do.

So tl;dr; I think you're safe unless it is YOU mutating const types 
stored in Boost objects. Or you're in a corner case, which are 
exceedingly rare and probably never will occur in your professional career.

If anybody is about to ask me what those corner cases are, I'd suggest 
go ask Richard Smith. I know the only one I care about is vptr reload 
elision, and it's just about the only place where launder really is 
needed and there is no way of avoiding it.

Niall