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Subject: [std-proposals] Allowing access to object representations
From: Tjernstrom, Staffan (Staffan.Tjernstrom_at_[hidden])
Date: 2019-08-22 08:08:23


>I believe we should clarify the wording such that it becomes unambiguous that ch points to a *byte* of x. We should also clarify that this is not UB, even if 100000 does not fit into a char, as this ?fitting the whole object into a char" is >not at all what should be going on. We should also fix the wording to allow to access the other bytes of x, not just the first one, without causing UB (by allowing pointer arithmetic and looking at the other addresses inside the char >array that provides storage for x).
>
>This would give us a portable way of looking at the bytes of an object (which ought to be possible anyway ? this is why we have the ?char and std::byte? can alias any type? rule in the first place).
>
>I think if we would have such a fix, then together with Richard?s P0593 this would be a comprehensive plug for this gaping hole in the language.
>
>What do y?all think?
>
>Cheers,
>Timur

+1

Whilst we're at it, P0593 would be greater if it applied the bless for the duration of the storage, that is to say

. get buffer
. bless buffer
. read bytes into buffer
. reinterpret buffer as type_1
. switch on deferenced pointer value
. reinterpret buffer as type_2, type_3, ...., type_n

would be defined / well-formed behaviour.

But I regretfully accept the reality of that kind of "bless allows aliasing" never gaining consensus.

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