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std::span

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Revision as of 14:28, 21 December 2023 by Space Mission (Talk | contribs)

 
C++
 
Containers library
 
std::span
 
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)

Contents

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

 constructs a span
(public member function) [edit]
assigns a span
(public member function) [edit]
(destructor)
(implicitly declared)
 destructs a span
(public member function)
Iterators
(C++23)
 returns an iterator to the beginning
(public member function) [edit]
(C++23)
 returns an iterator to the end
(public member function) [edit]
(C++23)
 returns a reverse iterator to the beginning
(public member function) [edit]
(C++23)
 returns a reverse iterator to the end
(public member function) [edit]
Element access
access the first element
(public member function) [edit]
access the last element
(public member function) [edit]
(C++26)
 access specified element with bounds checking
(public member function) [edit]
access specified element
(public member function) [edit]
direct access to the underlying contiguous storage
(public member function) [edit]
Observers
returns the number of elements
(public member function) [edit]
returns the size of the sequence in bytes
(public member function) [edit]
checks if the sequence is empty
(public member function) [edit]
Subviews
obtains a subspan consisting of the first N elements of the sequence
(public member function) [edit]
obtains a subspan consisting of the last N elements of the sequence
(public member function) [edit]
obtains a subspan
(public member function) [edit]

Non-member functions

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

Non-member constant

(C++20)
 a constant of type std::size_t signifying that the span has dynamic extent
(constant) [edit]

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 Value Std Feature
__cpp_lib_span 202002L (C++20) std::span
__cpp_lib_span_at 202311L (C++26) std::span::at

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::ranges::search(span, sub).begin() != span.end();
}
 
void println(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};
    constexpr static std::size_t width{6};
 
    for (std::size_t offset{}; ; ++offset)
        if (auto s = slide(std::span{a}, offset, width); !s.empty())
            println(s);
        else
            break;
 
    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
LWG 3903 C++20 the declaration of span's destructor was unnucessary removed the declaration
P2325R3 C++20 a span of non-zero static extents was not a view any span is a view

See also

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