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std::span subspan() method

// (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:

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:

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
This article originates from this CppReference page. It was likely altered for improvements or editors' preference. Click "Edit this page" to see all changes made to this document.
Hover to see the original license.

std::span subspan() method

// (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:

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:

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
This article originates from this CppReference page. It was likely altered for improvements or editors' preference. Click "Edit this page" to see all changes made to this document.
Hover to see the original license.