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Say that dereferencing a pointer to a ZST is no longer undefined #467

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traviscross merged 2 commits into from
Apr 15, 2025
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Say that dereferencing a pointer to a ZST is no longer undefined #467

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Say that dereferencing a pointer to a ZST is no longer undefined
mattheww Oct 15, 2024
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Zero Sized Types: stop saying that "dereferencing" a non-ZST is UB
mattheww Oct 27, 2024
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6 changes: 3 additions & 3 deletions src/exotic-sizes.md
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Expand Up @@ -106,9 +106,9 @@ consequence of types with no size. In particular, pointer offsets are no-ops,
and allocators typically [require a non-zero size][alloc].

Note that references to ZSTs (including empty slices), just like all other
references, must be non-null and suitably aligned. Dereferencing a null or
unaligned pointer to a ZST is [undefined behavior][ub], just like for any other
type.
references, must be non-null and suitably aligned. However, loading or storing
through a null pointer to a ZST is not [undefined behavior][ub], unlike
pointers to other types.

@RalfJung RalfJung Oct 28, 2024

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I think it would be less confusing to say something like "loading or storing through a null pointer is UB, unless the load/store has size 0". Let's talk about the common case first.

@mattheww mattheww Oct 28, 2024

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The subject of this section is ZSTs, so "loading or storing through a null pointer to a ZST is not undefined behavior" is the information it's supplying, and "unlike pointers to other types" is an aside.

@RalfJung RalfJung Oct 29, 2024

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Ah, fair.


[alloc]: ../std/alloc/trait.GlobalAlloc.html#tymethod.alloc
[ub]: what-unsafe-does.html
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