std::span

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< cpp‎ | container
 
C++
 
Containers library
 
std::span
Member functions
(C++20)
(C++20)
Element access
(C++20)
(C++20)
(C++20)
(C++20)
Iterators
(C++20)(C++23)
(C++20)(C++23)
(C++20)(C++23)
(C++20)(C++23)
Observers
(C++20)
(C++20)
(C++20)
Subviews
(C++20)
(C++20)
(C++20)
Non-member functions
(C++20)(C++20)
Non-member constant
(C++20)
Deduction guides(C++20)
 
Defined in header <span>
template<

    class T,
    std::size_t Extent = std::dynamic_extent

> class span;
 (since C++20)

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&
iterator implementation-defined LegacyRandomAccessIterator, ConstexprIterator, and contiguous_iterator whose value_type is value_type
const_iterator(C++23) std::const_iterator<iterator>
reverse_iterator std::reverse_iterator<iterator>
const_reverse_iterator(C++23) std::const_iterator<reverse_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;
 (since C++20)

Member functions

(C++20)
 constructs a span
(public member function)
(C++20)
 assigns a span
(public member function)
Iterators
(C++20)(C++23)
 returns an iterator to the beginning
(public member function)
(C++20)(C++23)
 returns an iterator to the end
(public member function)
(C++20)(C++23)
 returns a reverse iterator to the beginning
(public member function)
(C++20)(C++23)
 returns a reverse iterator to the end
(public member function)
Element access
(C++20)
 access the first element
(public member function)
(C++20)
 access the last element
(public member function)
(C++20)
 accesses an element of the sequence
(public member function)
(C++20)
 returns a pointer to the beginning of the sequence of elements
(public member function)
Observers
(C++20)
 returns the number of elements in the sequence
(public member function)
(C++20)
 returns the size of the sequence in bytes
(public member function)
(C++20)
 checks if the sequence is empty
(public member function)
Subviews
(C++20)
 obtains a subspan consisting of the first N elements of the sequence
(public member function)
(C++20)
 obtains a subspan consisting of the last N elements of the sequence
(public member function)
(C++20)
 obtains a subspan
(public member function)

Non-member functions

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

Non-member constant

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

Helper templates

template< class T, std::size_t Extent >
inline constexpr bool ranges::enable_borrowed_range<std::span<T, Extent>> = true;
 (since C++20)

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;
 (since C++20)

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

Deduction guides(C++20)

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 Value Std Comment
__cpp_lib_span 202002L (C++20) std::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>
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>
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>
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 extents were not view they are as default_initializable is not required

See also

(C++11)
 creates a temporary array in list-initialization and then references it
(class template)
(C++17)
 read-only string view
(class template)
(C++23)
 a multi-dimensional non-owning array view
(class template)
(C++20)
 combines an iterator-sentinel pair into a view
(class template)
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