std::span::span(3)

NAME
std::span::span - std::span::span

Synopsis
   constexpr span() noexcept; (1) (since C++20)
   template< class It >
   explicit(extent != std::dynamic_extent) (2) (since C++20)
   constexpr span( It first, size_type count );
   template< class It, class End >
   explicit(extent != std::dynamic_extent) (3) (since C++20)
   constexpr span( It first, End last );
   template< std::size_t N >
   constexpr span( std::type_identity_t<element_type> (&arr)[N] ) (4) (since C++20)
   noexcept;
   template< class U, std::size_t N > (5) (since C++20)
   constexpr span( std::array<U, N>& arr ) noexcept;
   template< class U, std::size_t N > (6) (since C++20)
   constexpr span( const std::array<U, N>& arr ) noexcept;
   template< class R >
   explicit(extent != std::dynamic_extent) (7) (since C++20)
   constexpr span( R&& range );
   explicit(extent != std::dynamic_extent) (8) (since C++26)
   constexpr span( std::initializer_list<value_type> il ) noexcept;
   template< class U, std::size_t N >
   explicit(extent != std::dynamic_extent && N == (9) (since C++20)
   std::dynamic_extent)
   constexpr span( const std::span<U, N>& source ) noexcept;
   constexpr span( const span& other ) noexcept = default; (10) (since C++20)
   Constructs a span.
   1) Constructs an empty span whose data() == nullptr and size() == 0.
     * This overload participates in overload resolution only if extent == 0 || extent
       == std::dynamic_extent.
   2) Constructs a span that is a view over the range [first, first + count); the
   resulting span has data() == std::to_address(first) and size() == count.
     * The behavior is undefined if [first, first + count) is not a valid range, if It
       does not actually model contiguous_iterator, or if extent != std::dynamic_extent
       && count != extent.
     * This overload participates in overload resolution only if
     * It satisfies contiguous_iterator,
     * the conversion from std::iter_reference_t<It> to element_type is at most a
       qualification conversion.
   3) Constructs a span that is a view over the range [first, last); the resulting span
   has data() == std::to_address(first) and size() == last-first.
     * The behavior is undefined if [first, last) is not a valid range, if It does not
       actually model contiguous_iterator, if End does not actually model
       sized_sentinel_for for It, or if extent != std::dynamic_extent && last-first !=
       extent.
     * This overload participates in overload resolution only if
     * It satisfies contiguous_iterator,
     * End satisfies sized_sentinel_for for It,
     * the conversion from std::iter_reference_t<It> to element_type is at most a
       qualification conversion, and
     * std::is_convertible_v<End, std::size_t> is false.
   4-6) Constructs a span that is a view over the array arr; the resulting span has
   size() == N and data() == std::data(arr).
     * These overloads participate in overload resolution only if extent ==
       std::dynamic_extent || N == extent is true and the conversion from
       std::remove_pointer_t<decltype(data(arr))> to element_type is at most a
       qualification conversion.
   7) Constructs a span that is a view over the range range; the resulting span has
   size() == std::ranges::size(range) and data() == std::ranges::data(range).
     * The behavior is undefined if R does not actually model contiguous_range and
       sized_range or if R does not model borrowed_range while element_type is
       non-const or both extent != dynamic_extent and std::ranges::size(range) !=
       extent are true.
     * This overload participates in overload resolution only if
     * R satisfies contiguous_range and sized_range,
     * either R satisfies borrowed_range or std::is_const_v<element_type> is true,
     * std::remove_cvref_t<R> is not a specialization of std::span,
     * std::remove_cvref_t<R> is not a specialization of std::array,
     * std::is_array_v<std::remove_cvref_t<R>> is false, and
     * the conversion from std::ranges::range_reference_t<R> to element_type is at most
       a qualification conversion.
   8) Constructs a span that is a view over the initializer list il; the resulting span
   has size() == il.size() and data() == il.begin().
     * The behavior is undefined if both extent != dynamic_extent and il.size() !=
       extent are true.
     * This overload participates in overload resolution only if
       std::is_const_v<element_type> is true.
   9) Converting constructor from another span source; the resulting span has size() ==
   source.size() and data() == source.data().
     * The behavior is undefined if both extent != dynamic_extent and source.size() !=
       extent are true.
     * This overload participates in overload resolution only if at least one of extent
       == std::dynamic_extent, N == std::dynamic_extent and N == extent is true and the
       conversion from U to element_type is at most a qualification conversion.
   10) Defaulted copy constructor copies the size and data pointer; the resulting span
   has size() == other.size() and data() == other.data().

Parameters
   first - iterator to the first element of the sequence
   count - number of elements in the sequence
   last - iterator past the last element of the sequence or another sentinel
   arr - array to construct a view for
   range - range to construct a view for
   source - another span to convert from
   other - another span to copy from

Exceptions
   2) Throws nothing.
   3) Throws what and when last - first throws.
   7) Throws what and when std::ranges::size(r) and std::ranges::data(r) throw.

Notes
         Feature-test macro Value Std Feature
   __cpp_lib_span_initializer_list 202311L (C++26) Constructing std::span from a
                        std::initializer_list, (8)

Example
// Run this code
 #include <array>
 #include <iostream>
 #include <span>
 #include <vector>
 void print_span(std::span<const int> s)
 {
     for (int n : s)
         std::cout << n << ' ';
     std::cout << '\n';
 }
 int main()
 {
     int c[]{1, 2, 3};
     print_span(c); // constructs from array
     std::array a{4, 5, 6};
     print_span(a); // constructs from std::array
     std::vector v{7, 8, 9};
     print_span(v); // constructs from std::vector
 #if __cpp_lib_span_initializer_list
     print_span({0, 1, 2}); // constructs from initializer_list
 #else
     print_span({{0, 1, 2}}); // ditto, a workaround
 #endif
 }

Output:
 1 2 3
 4 5 6
 7 8 9
 0 1 2

See also
   data direct access to the underlying contiguous storage
             (public member function)
   size returns the number of elements
             (public member function)
   operator= assigns a span
             (public member function)
   size
   ssize returns the size of a container or array
   (C++17) (function template)
   (C++20)
   data obtains the pointer to the underlying array
   (C++17) (function template)