std::span<T,Extent>::subspan

From cppreference.com
< cpp‎ | container‎ | span
template< std::size_t Offset,

          std::size_t Count = std::dynamic_extent >

constexpr std::span<element_type, E /* see below */> subspan() const;
(1)
constexpr std::span<element_type, std::dynamic_extent>

    subspan( size_type Offset,

             size_type Count = std::dynamic_extent ) const;
(2)

Obtains a span that is a view over the Count elements of this span starting at offset Offset. If Count is std::dynamic_extent, the number of elements in the subspan is size() - offset (i.e., it ends at the end of *this.).

(1) is ill-formed if

  • Offset is greater than Extent, or
  • Count is not std::dynamic_extent and Count is greater than Extent - Offset.

The behavior is undefined if either Offset or Count is out of range. This happens if

  • Offset is greater than size(), or
  • Count is not std::dynamic_extent and Count is greater than size() - Offset.

The extent E of the span returned by (1) is determined as follows:

  • If Count is not std::dynamic_extent, Count;
  • Otherwise, if Extent is not std::dynamic_extent, Extent - Offset;
  • Otherwise, std::dynamic_extent.

Return value

The requested subspan r, such that r.data() == this->data() + Offset. If Count is std::dynamic_extent, r.size() == this->size() - Offset; otherwise r.size() == Count.

Example

#include <algorithm>
#include <cstdio>
#include <numeric>
#include <ranges>
#include <span>
 
void display(std::span<const char> abc)
{
    const auto columns{ 20U };
    const auto rows{ abc.size() - columns + 1 };
 
    for (auto offset{ 0U }; offset < rows; ++offset) {
        std::ranges::for_each(
            abc.subspan(offset, columns), 
            std::putchar
        );
        std::putchar('\n');
    }
}
 
int main()
{
    char abc[26];
    std::iota(std::begin(abc), std::end(abc), 'A');
    display(abc);
}

Output:

ABCDEFGHIJKLMNOPQRST
BCDEFGHIJKLMNOPQRSTU
CDEFGHIJKLMNOPQRSTUV
DEFGHIJKLMNOPQRSTUVW
EFGHIJKLMNOPQRSTUVWX
FGHIJKLMNOPQRSTUVWXY
GHIJKLMNOPQRSTUVWXYZ

See also

obtains a subspan consisting of the first N elements of the sequence
(public member function)
obtains a subspan consisting of the last N elements of the sequence
(public member function)