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4537. Improve define_static_array

Section: 21.4.3 [meta.define.static] Status: New Submitter: Hewill Kang Opened: 2026-03-09 Last modified: 2026-03-09

Priority: Not Prioritized

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Discussion:

define_static_array can transform any input range, even a non-contiguous one, into a span.

However, the returned span always has a dynamic extent. If the size of the input range is known at compile time, returning a static span seems reasonable and may offer slight runtime efficiency.

Proposed resolution:

This wording is relative to N5032.

1. Modify 21.4.1 [meta.syn], header <meta> synopsis, as indicated:

   #include <compare>              // see 17.12.1 [compare.syn]
   #include <initializer_list>     // see 17.11.2 [initializer.list.syn]
   
   namespace std {
     […]
     // 21.4.3 [meta.define.static], promoting to static storage
     […]
     template<ranges::input_range R>
       consteval span<const ranges::range_value_t<R>, see below> define_static_array(R&& r);
     […]
   }
   

2. Modify 21.4.3 [meta.define.static] as indicated:

   template<ranges::input_range R>
     consteval span<const ranges::range_value_t<R>, see below> define_static_array(R&& r);
   

   -16- Effects: Equivalent to:

   using T = ranges::range_value_t<R>;
   meta::info array = meta::reflect_constant_array(r);
   if (meta::is_array_type(meta::type_of(array))) {
     return span<const T, see below>(meta::extract<const T*>(array), meta::extent(meta::type_of(array)));
   } else {
     return span<const T, see below>();
   }
   

   -?- Remarks: The second template argument of the returned span type is static_cast<size_t>(ranges::size(r)) if ranges::size(r) is a constant expression, and dynamic_extent otherwise.