Re: std::launder and its usage in Boost

From: Peter Dimov (pdimov_at_[hidden])
Date: 2022-02-18 19:01:20

> The third issue we're having is with our aligned_storage. Its address() member
> function doesn't return the address of the unsigned char[] array, but the
> address of the aligned_storage object itself (or rather, to its
> aligned_storage_impl base, which contains the char array as its first member.
> Inside a union.)
> 
> This means that we aren't in the clear with respect to the "provides storage"
> wording in https://eel.is/c++draft/basic.memobj#intro.object-3. I'm not
> entirely
> sure that what we're doing is undefined, but it looks like we can avoid this
> issue
> by just returning the address of the char array in address().
> 
> Note that this potential source of UB can't be fixed with launder.

Actually, Boost.Optional uses its own aligned_storage:

https://github.com/boostorg/optional/blob/develop/include/boost/optional/detail/optional_aligned_storage.hpp

which does return the address of the char[] buffer, but only if the compiler
doesn't have the may_alias attribute, in which case the address of the union
is returned, but the union is marked with may_alias.

What effect this has is anyone's guess. I suppose it's effectively equivalent
to laundering the address each time, but as the attribute is nonstandard,
there's no way to know.