deductionguidesforstd::span(3)

NAME
deductionguidesforstd::span - deductionguidesforstd::span

Synopsis
   Defined in header <span>
   template< class It, class EndOrSize > (since
   span( It, EndOrSize ) -> C++20)
   span<std::remove_reference_t<std::iter_reference_t<It>>>; (until
                        C++26)
   template< class It, class EndOrSize >
   span( It, EndOrSize ) -> (since
   span<std::remove_reference_t<std::iter_reference_t<It>>, C++26)
                        /*maybe-static-ext*/<EndOrSize>>; (1)
   template< class T, std::size_t N > (2) (since
   span( T (&)[N] ) -> span<T, N>; C++20)
   template< class T, std::size_t N > (3) (since
   span( std::array<T, N>& ) -> span<T, N>; C++20)
   template< class T, std::size_t N > (4) (since
   span( const std::array<T, N>& ) -> span<const T, N>; C++20)
   template< class R > (since
   span( R&& ) -> (5) C++20)
   span<std::remove_reference_t<std::ranges::range_reference_t<R>>>;
   The following deduction guides are provided for span.
   1) Allows the element type to be deduced from the iterator-sentinel pair.
   It also allows the static extent to be deduced if EndOrSize satisfies
   integral-constant-like.
   (since C++26). This overload participates in overload resolution only if It
   satisfies contiguous_iterator.
   2-4) Allows the static extent to be deduced from built-in arrays and std::array.
   5) Allows the element type to be deduced from ranges. This overload participates in
   overload resolution only if R satisfies contiguous_range.

Example
// Run this code
 #include <array>
 #include <cstddef>
 #include <iomanip>
 #include <iostream>
 #include <span>
 #include <string_view>
 #include <vector>
 void print(std::string_view rem = "", std::size_t size_of = 0, std::size_t extent = 0)
 {
     if (rem.empty())
     {
         std::cout << "name │ sizeof │ extent\n"
                      "─────┼────────┼────────\n";
         return;
     }
     std::cout << std::setw(4) << rem << " │ " << std::setw(6) << size_of << " │ ";
     if (extent == std::dynamic_extent)
         std::cout << "dynamic";
     else
         std::cout << extent;
     std::cout << '\n';
 }
 int main()
 {
     int a[]{1, 2, 3, 4, 5};
     print();
     std::span s1{std::begin(a), std::end(a)}; // guide (1)
     print("s1", sizeof s1, s1.extent);
     std::span s2{std::begin(a), 3}; // guide (1)
     print("s2", sizeof s2, s2.extent);
 #if __cplusplus > 202302L
     std::span s3{std::begin(a), std::integral_constant<std::size_t, 2>{}}; // guide (1)
     print("s3", sizeof s3, s3.extent);
 #endif // C++26
     std::span s4{a}; // guide (2)
     print("s4", sizeof s4, s4.extent);
     std::span<int> s5{a}; // does not use a guide, makes a dynamic span
     print("s5", sizeof s5, s5.extent);
     std::array arr{6, 7, 8};
     std::span s6{arr}; // guide (3)
     print("s6", sizeof s6, s6.extent);
     s6[0] = 42; // OK, element_type is 'int'
     const std::array arr2{9, 10, 11};
     std::span s7{arr2}; // guide (4)
     print("s7", sizeof s7, s7.extent);
     // s7[0] = 42; // Error: element_type is 'const int'
     std::vector v{66, 69, 99};
     std::span s8{v}; // guide (5)
     print("s8", sizeof s8, s8.extent);
 }

Possible output:
 name │ sizeof │ extent
 ─────┼────────┼────────
   s1 │ 16 │ dynamic
   s2 │ 16 │ dynamic
   s3 │ 8 │ 2
   s4 │ 8 │ 5
   s5 │ 16 │ dynamic
   s6 │ 8 │ 3
   s7 │ 8 │ 3
   s8 │ 16 │ dynamic