std::span
| Defined in header <span>
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||
| template< class T, |
(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 |
(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
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Member types
| Member type | Definition |
element_type
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T
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value_type
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std::remove_cv_t<T> |
size_type
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std::size_t |
difference_type
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std::ptrdiff_t |
pointer
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T*
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const_pointer
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const T*
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reference
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T&
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const_reference
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const T&
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iterator
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implementation-defined LegacyRandomAccessIterator, ConstexprIterator, and contiguous_iterator whose value_type is value_type
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reverse_iterator
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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; |
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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; |
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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; |
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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.
#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 extents were not view
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they are as default_initializable is not required
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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) |