std::dynamic_extent - cppreference.com (original) (raw)

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 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)

[edit] Note

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

See integral conversions.

[edit] Example

#include #include #include #include #include #include #include   int main() { auto print = 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

[edit] See also

| | a non-owning view over a contiguous sequence of objects (class template) [edit] | | ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | | a descriptor of a multidimensional index space of some rank (class template) [edit] |