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4275. std::dynamic_extent should also be defined in <mdspan>

Section: 23.7.3.2 [mdspan.syn] Status: Tentatively Ready Submitter: Aiden Grossman Opened: 2025-06-06 Last modified: 2025-10-17

Priority: 3

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Discussion:

std::dynamic_extent can be used in certain circumstances in std::mdspan, such as with padded layouts. However, std::dynamic_extent is currently only defined in <span> which necessitates including <span> solely for the std::dynamic_extent definition.

Previous resolution [SUPERSEDED]:

This wording is relative to N5008.

1. Modify 23.7.3.2 [mdspan.syn], header <span> synopsis, as indicated:

   // all freestanding
   namespace std {
     // constants
     inline constexpr size_t dynamic_extent = numeric_limits<size_t>::max();
     
     // 23.7.3.3 [mdspan.extents], class template extents
     template<class IndexType, size_t... Extents>
     class extents;
   
     […]
   }
   

[2025-06-10; Jonathan provides improved wording]

[2025-10-15; Reflector poll]

Set priority to 3 after reflector poll.

[2025-10-17; Reflector poll.]

Set status to Tentatively Ready after eight votes in favour during reflector poll.

Proposed resolution:

This wording is relative to N5008.

1. Modify 23.7.1 [views.general] as indicated:

   The header <span> (23.7.2.1 [span.syn]) defines the view span. The header <mdspan> (23.7.3.2 [mdspan.syn]) defines the class template mdspan and other facilities for interacting with these multidimensional views.

   -?- In addition to being available via inclusion of the <span> header, dynamic_extent is available when the header <mdspan> is included.