std::span

< cpp‎ | container
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
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

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

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.

Example

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

#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) {
        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})
        && starts_with(std::span{a+1, 4}, std::span{a+1,3})
        && !starts_with(std::span{a}, std::span{b})
        && !starts_with(std::span{a,8}, std::span{a+1,3})
        && ends_with(std::span{a}, std::span{a+6,3})
        && !ends_with(std::span{a}, std::span{a+6,2})
        && contains(std::span{a}, std::span{a+1,4})
        && !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++20)
 combines an iterator-sentinel pair into a view
(class template)