std::span subspan() method

since C++20

// (1) Const version only
template< std::size_t Offset,
          std::size_t Count = std::dynamic_extent >
constexpr std::span<element_type, E /* see below */> subspan() const;

// (2) Const version only
constexpr std::span<element_type, std::dynamic_extent>
    subspan( size_type Offset,
             size_type Count = std::dynamic_extent ) const;

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).

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.

important

(1) is ill-formed

if:

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

Undefined Behaviour

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

Parameters

• (2)
  • Count - the number of the elements to make a span of
  • Offset - offset to start from

Return value

A span s that is a view over the subspan Count elements of *this, with the following properties:

• s.data() == this->data() + Offset
  and
• If Count is std::dynamic_extent:
  • s.size() == this->size() - Offset
• Otherwise
  • s.size() == Count

Complexity

Constant - O(1).

Exceptions

(none)

Example

Main.cpp

#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