# deduction guides for `std::span`

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< cpp‎ | container‎ | span

Defined in header `<span>` |  |
template< class It, class EndOrSize > span( It, EndOrSize ) -> span<std::remove_reference_t<std::iter_reference_t<It>>>; | (1) | (since C++20)
template< class T, std::size_t N > span( T (&)[N] ) -> span<T, N>; | (2) | (since C++20)
template< class T, std::size_t N > span( std::array<T, N>& ) -> span<T, N>; | (3) | (since C++20)
template< class T, std::size_t N > span( const std::array<T, N>& ) -> span<const T, N>; | (4) | (since C++20)
template< class R > span( R&& ) -> span<std::remove_reference_t<std::ranges::range_reference_t<R>>>; | (5) | (since C++20)

The following deduction guides are provided for `span`.

1) Allows the element type to be deduced from the iterator-sentinel pair. 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);

        std::span s3 {a}; // guide (2)
        print("s3", sizeof s3, s3.extent);

        std::span<int> s4 {a}; // does not use a guide, makes a dynamic span
        print("s4", sizeof s4, s4.extent);

        std::array arr {6, 7, 8};
        std::span s5 {arr}; // guide (3)
        print("s5", sizeof s5, s5.extent);
        s5[0] = 42; // OK, element_type is 'int'

        const std::array arr2 {9, 10, 11};
        std::span s6 {arr2}; // guide (4)
        print("s6", sizeof s6, s6.extent);
        // s6[0] = 42; // Error: element_type is 'const int'

        std::vector v {66, 69, 99};
        std::span s7 {v}; // guide (5)
        print("s7", sizeof s7, s7.extent);
    }

Output:

    name │ sizeof │ extent
    ─────┼────────┼────────
      s1 │     16 │ dynamic
      s2 │     16 │ dynamic
      s3 │      8 │ 5
      s4 │     16 │ dynamic
      s5 │      8 │ 3
      s6 │      8 │ 3
      s7 │     16 │ dynamic

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