# std::dynamic_extent

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< cpp‎ | container‎ | span

Defined in header `<span>` |  |
inline constexpr std::size_t dynamic_extent = std::numeric_limits < std::size_t >::max(); |  | (since C++20)

`std::dynamic_extent` is a constant of type std::size_t that is generally used to indicate that any type using `std::dynamic_extent` will dynamically store its value (e.g., size) rather than having the value statically known in the type.

It is being used in several contexts:

- To differentiate `std::span` of static and dynamic extent.

- To indicate that the extent at a certain rank index will be stored dynamically in `std::extents`.

(since C++23)

- To indicate that the padded layouts for `std::mdspan` will dynamically store its padding value.

(since C++26)

### Note

Since std::size_t is an unsigned type, an equivalent definition is:

    inline constexpr std::size_t dynamic_extent = -1;

See integral conversions.

### Example

Run this code

    #include <array>
    #include <cassert>
    #include <cstddef>
    #include <iostream>
    #include <span>
    #include <string_view>
    #include <vector>

    int main()
    {
        auto print = [](std::string_view const name, std::size_t ex)
        {
            std::cout << name << ", ";
            if (std::dynamic_extent == ex)
                std::cout << "dynamic extent\n";
            else
                std::cout << "static extent = " << ex << '\n';
        };

        int a[]{1, 2, 3, 4, 5};

        std::span span1{a};
        print("span1", span1.extent);

        std::span<int, std::dynamic_extent> span2{a};
        print("span2", span2.extent);

        std::array ar{1, 2, 3, 4, 5};
        std::span span3{ar};
        print("span3", span3.extent);

        std::vector v{1, 2, 3, 4, 5};
        std::span span4{v};
        print("span4", span4.extent);
    }

Output:

    span1, static extent = 5
    span2, dynamic extent
    span3, static extent = 5
    span4, dynamic extent

### See also

span (C++20) | a non-owning view over a contiguous sequence of objects (class template)
extents (C++23) | a descriptor of a multidimensional index space of some rank (class template)

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