On Sat, Nov 6, 2021 at 10:21 AM connor horman via Std-Proposals
<std-proposals_at_[hidden]> wrote:
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> On Fri, 5 Nov 2021 at 20:27, Lénárd Szolnoki via Std-Proposals <std-proposals_at_[hidden]> wrote:
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>> Hi,
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>> I don't think a non-UB implementation is possible here. I have the impression that the pointer to the parent type is intentionally unreachable and in theory compilers are allowed to optimize based on that.
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> If it's a standard library function, there's always an implementation: __builtin_parent_of_member
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> That being said, isn't this undefined behaviour anyways for anything other than the first member of a standard-layout type? Either that or the definition of reachability would have to be extended to include pointers to any subobject.

I don't really see how it matters within the context of a standard
library *proposal*. The function does what it says it does: it returns
a reference to the parent type of a class given a pointer-to-member
object of that class and a reference to a member object instance of
that class.

How it does this is sophistry.