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

 

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std::span

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span::beginspan::cbegin (C++20)(C++23)

span::endspan::cend (C++20)(C++23)

span::rbeginspan::crbegin (C++20)(C++23)

span::rendspan::crend (C++20)(C++23)

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span::empty (C++20)

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span::size_bytes (C++20)

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span::first (C++20)

span::last (C++20)

span::subspan (C++20)

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as_bytesas_writable_bytes (C++20)(C++20)

Non-member constant

dynamic_extent (C++20)

Deduction guides(C++20)

 

template< std::size_t Offset,           std::size_t Count = std::dynamic_extent > constexpr std::span<element_type, E /* see below */>     subspan() const;	(1)	(since C++20)
constexpr std::span<element_type, std::dynamic_extent>     subspan( size_type Offset,              size_type Count = std::dynamic_extent ) const;	(2)	(since C++20)

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

Run this code

#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

first (C++20)	obtains a subspan consisting of the first N elements of the sequence (public member function)
last (C++20)	obtains a subspan consisting of the last N elements of the sequence (public member function)