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4243. as_bytes/as_writable_bytes is broken with span<volatile T>

Section: 23.7.2.3 [span.objectrep] Status: New Submitter: Hewill Kang Opened: 2025-04-12 Last modified: 2025-04-13

Priority: Not Prioritized

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Discussion:

They both use reinterpret_cast to cast the underlying pointer type of span to const byte* and byte* respectively, which leads to a hard error when the element type is volatile-qualified (demo):

#include <span>

int main() {
  std::span<volatile int> span;
  auto bytes = as_bytes(span);                   // hard error
  auto writable_bytes = as_writable_bytes(span); // hard error
}

Proposed resolution:

This wording is relative to N5008.

1. Modify 23.7.2.1 [span.syn], header <span> synopsis, as indicated:

   […]
   namespace std {
     […]
     // 23.7.2.3 [span.objectrep], views of object representation
     template<class ElementType, size_t Extent>
       span<const conditional_t<is_volatile_v<ElementType>, volatile byte, byte>, 
            Extent == dynamic_extent ? dynamic_extent : sizeof(ElementType) * Extent>
         as_bytes(span<ElementType, Extent> s) noexcept;
   
     template<class ElementType, size_t Extent>
       span<conditional_t<is_volatile_v<ElementType>, volatile byte, byte>, 
            Extent == dynamic_extent ? dynamic_extent : sizeof(ElementType) * Extent>
         as_writable_bytes(span<ElementType, Extent> s) noexcept;
   }
   

2. Modify 23.7.2.3 [span.objectrep] as indicated:

   template<class ElementType, size_t Extent>
     span<const conditional_t<is_volatile_v<ElementType>, volatile byte, byte>,
          Extent == dynamic_extent ? dynamic_extent : sizeof(ElementType) * Extent>
       as_bytes(span<ElementType, Extent> s) noexcept;
   

   -1- Effects: Equivalent to: return R{reinterpret_cast<R::pointerconst byte*>(s.data()), s.size_bytes()};
   where R is the return type.

   template<class ElementType, size_t Extent>
     span<conditional_t<is_volatile_v<ElementType>, volatile byte, byte>,
          Extent == dynamic_extent ? dynamic_extent : sizeof(ElementType) * Extent>
       as_writable_bytes(span<ElementType, Extent> s) noexcept;
   

   -2- Constraints: is_const_v<ElementType> is false.

   -3- Effects: Equivalent to: return R{reinterpret_cast<R::pointerbyte*>(s.data()), s.size_bytes()};
   where R is the return type.