NAME

std::span - std::span

Synopsis

Defined in header <span>
template<

class T, (since C++20)
std::size_t Extent = std::dynamic_extent

> class span;

The class template span describes an object that can refer to a contiguous sequence
of objects with the first element of the sequence at position zero. A span can
either have a static extent, in which case the number of elements in the sequence is
known at compile-time and encoded in the type, or a dynamic extent.

If a span has dynamic extent a typical implementation holds two members: a pointer
to T and a size. A span with static extent may have only one member: a pointer to T.

Every specialization of std::span is a TriviallyCopyable type. (since C++23)

Template parameters

T - element type; must be a complete object type that is not an abstract class
type
Extent - the number of elements in the sequence, or std::dynamic_extent if dynamic

Member types

Member type Definition
element_type T
value_type std::remove_cv_t<T>
size_type std::size_t
difference_type std::ptrdiff_t
pointer T*
const_pointer const T*
reference T&
const_reference const T&
implementation-defined LegacyRandomAccessIterator,
iterator ConstexprIterator, and contiguous_iterator whose value_type is
value_type
reverse_iterator std::reverse_iterator<iterator>

Note: iterator is a mutable iterator if T is not const-qualified.

All requirements on the iterator types of a Container apply to the iterator type of
span as well.

Member constant

static constexpr std::size_t extent = Extent;

Member functions

constructor constructs a span
(public member function)
operator= assigns a span
(public member function)

Iterators

begin returns an iterator to the beginning
(C++20) (public member function)
end returns an iterator to the end
(C++20) (public member function)
rbegin returns a reverse iterator to the beginning
(C++20) (public member function)
rend returns a reverse iterator to the end
(C++20) (public member function)

Element access

front access the first element
(C++20) (public member function)
back access the last element
(C++20) (public member function)
operator[] accesses an element of the sequence
(public member function)
data returns a pointer to the beginning of the sequence of elements
(public member function)

Observers

size returns the number of elements in the sequence
(public member function)
size_bytes returns the size of the sequence in bytes
(public member function)
empty checks if the sequence is empty
(public member function)
Subviews
first obtains a subspan consisting of the first N elements of the sequence
(public member function)
last obtains a subspan consisting of the last N elements of the sequence
(public member function)
subspan obtains a subspan
(public member function)

Non-member functions

as_bytes converts a span into a view of its underlying bytes
as_writable_bytes (function template)
(C++20)

Non-member constant

dynamic_extent a constant of type size_t signifying that the span has dynamic extent
(C++20) (constant)

Helper templates

template<class T, std::size_t Extent>
inline constexpr bool ranges::enable_borrowed_range<std::span<T, Extent>> = true;

This specialization of ranges::enable_borrowed_range makes span satisfy
borrowed_range.

template<class T, std::size_t Extent>
inline constexpr bool ranges::enable_view<std::span<T, Extent>> = true;

This specialization of ranges::enable_view makes span satisfy view.

Deduction guides

Notes

Specializations of std::span are already trivially copyable types in all existing
implementations, even before the formal requirement introduced in C++23.

Feature-test macro: __cpp_lib_span

Example

The example uses std::span to implement some algorithms on contiguous ranges.

// Run this code

#include <algorithm>
#include <cstddef>
#include <iostream>
#include <span>

template<class T, std::size_t N> [[nodiscard]]
constexpr auto slide(std::span<T,N> s, std::size_t offset, std::size_t width) {
return s.subspan(offset, offset + width <= s.size() ? width : 0U);
}

template<class T, std::size_t N, std::size_t M> [[nodiscard]]
constexpr bool starts_with(std::span<T,N> data, std::span<T,M> prefix) {
return data.size() >= prefix.size()
&& std::equal(prefix.begin(), prefix.end(), data.begin());
}

template<class T, std::size_t N, std::size_t M> [[nodiscard]]
constexpr bool ends_with(std::span<T,N> data, std::span<T,M> suffix) {
return data.size() >= suffix.size()
&& std::equal(data.end() - suffix.size(), data.end(),
suffix.end() - suffix.size());
}

template<class T, std::size_t N, std::size_t M> [[nodiscard]]
constexpr bool contains(std::span<T,N> span, std::span<T,M> sub) {
return std::search(span.begin(), span.end(), sub.begin(), sub.end()) != span.end();
// return std::ranges::search(span, sub).begin() != span.end();
}

void print(const auto& seq) {
for (const auto& elem : seq) std::cout << elem << ' ';
std::cout << '\n';
}

int main()
{
constexpr int a[] { 0, 1, 2, 3, 4, 5, 6, 7, 8 };
constexpr int b[] { 8, 7, 6 };

for (std::size_t offset{}; ; ++offset) {
static constexpr std::size_t width{6};
auto s = slide(std::span{a}, offset, width);
if (s.empty())
break;
print(s);
}

static_assert(
starts_with( std::span{a}, std::span{a, 4} ) and
starts_with( std::span{a + 1, 4}, std::span{a + 1, 3} ) and
! starts_with( std::span{a}, std::span{b} ) and
! starts_with( std::span{a, 8}, std::span{a + 1, 3} ) and
ends_with( std::span{a}, std::span{a + 6, 3} ) and
! ends_with( std::span{a}, std::span{a + 6, 2} ) and
contains( std::span{a}, std::span{a + 1, 4} ) and
! contains( std::span{a, 8}, std::span{a, 9} )
);
}

Output:

0 1 2 3 4 5
1 2 3 4 5 6
2 3 4 5 6 7
3 4 5 6 7 8

Defect reports

The following behavior-changing defect reports were applied retroactively to
previously published C++ standards.

DR Applied to Behavior as published Correct behavior
P2325R3 C++20 span of non-zero static they are as default_initializable is not
extents were not view required