std::dynamic_extent
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dynamic_extent
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.
- To indicate that the padded layouts for std::mdspan will dynamically store its padding value.
[edit] Note
Since std::size_t is an unsigned type, an equivalent definition is:
inline constexpr std::size_t dynamic_extent = -1;
See integral conversions.
[edit] 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