NAME¶

std::span::span - std::span::span

Synopsis¶

constexpr span() noexcept; (1) (since C++20)
template< class It >

explicit(extent != std::dynamic_extent) (2) (since C++20)

constexpr span( It first, size_type count );
template< class It, class End >

explicit(extent != std::dynamic_extent) (3) (since C++20)

constexpr span( It first, End last );
template< std::size_t N >
constexpr span( std::type_identity_t<element_type> (&arr)[N] ) (4) (since C++20)
noexcept;
template< class U, std::size_t N > (5) (since C++20)
constexpr span( std::array<U, N>& arr ) noexcept;
template< class U, std::size_t N > (6) (since C++20)
constexpr span( const std::array<U, N>& arr ) noexcept;
template< class R >

explicit(extent != std::dynamic_extent) (7) (since C++20)

constexpr span( R&& range );
explicit(extent != std::dynamic_extent) (8) (since C++26)
constexpr span( std::initializer_list<value_type> il ) noexcept;
template< class U, std::size_t N >

explicit(extent != std::dynamic_extent && N == (9) (since C++20)
std::dynamic_extent)

constexpr span( const std::span<U, N>& source ) noexcept;
constexpr span( const span& other ) noexcept = default; (10) (since C++20)

Constructs a span.

1) Constructs an empty span whose data() == nullptr and size() == 0.
* This overload participates in overload resolution only if extent == 0 || extent
== std::dynamic_extent.
2) Constructs a span that is a view over the range [first, first + count); the
resulting span has data() == std::to_address(first) and size() == count.
* The behavior is undefined if [first, first + count) is not a valid range, if It
does not actually model contiguous_iterator, or if extent != std::dynamic_extent
&& count != extent.
* This overload participates in overload resolution only if

* It satisfies contiguous_iterator,
* the conversion from std::iter_reference_t<It> to element_type is at most a
qualification conversion.
3) Constructs a span that is a view over the range [first, last); the resulting span
has data() == std::to_address(first) and size() == last-first.
* The behavior is undefined if [first, last) is not a valid range, if It does not
actually model contiguous_iterator, if End does not actually model
sized_sentinel_for for It, or if extent != std::dynamic_extent && last-first !=
extent.
* This overload participates in overload resolution only if

* It satisfies contiguous_iterator,
* End satisfies sized_sentinel_for for It,
* the conversion from std::iter_reference_t<It> to element_type is at most a
qualification conversion, and
* std::is_convertible_v<End, std::size_t> is false.
4-6) Constructs a span that is a view over the array arr; the resulting span has
size() == N and data() == std::data(arr).
* These overloads participate in overload resolution only if extent ==
std::dynamic_extent || N == extent is true and the conversion from
std::remove_pointer_t<decltype(data(arr))> to element_type is at most a
qualification conversion.
7) Constructs a span that is a view over the range range; the resulting span has
size() == std::ranges::size(range) and data() == std::ranges::data(range).
* The behavior is undefined if R does not actually model contiguous_range and
sized_range or if R does not model borrowed_range while element_type is
non-const or both extent != dynamic_extent and std::ranges::size(range) !=
extent are true.
* This overload participates in overload resolution only if

* R satisfies contiguous_range and sized_range,
* either R satisfies borrowed_range or std::is_const_v<element_type> is true,
* std::remove_cvref_t<R> is not a specialization of std::span,
* std::remove_cvref_t<R> is not a specialization of std::array,
* std::is_array_v<std::remove_cvref_t<R>> is false, and
* the conversion from std::ranges::range_reference_t<R> to element_type is at most
a qualification conversion.
8) Constructs a span that is a view over the initializer list il; the resulting span
has size() == il.size() and data() == il.begin().
* The behavior is undefined if both extent != dynamic_extent and il.size() !=
extent are true.
* This overload participates in overload resolution only if
std::is_const_v<element_type> is true.
9) Converting constructor from another span source; the resulting span has size() ==
source.size() and data() == source.data().
* The behavior is undefined if both extent != dynamic_extent and source.size() !=
extent are true.
* This overload participates in overload resolution only if at least one of extent
== std::dynamic_extent, N == std::dynamic_extent and N == extent is true and the
conversion from U to element_type is at most a qualification conversion.
10) Defaulted copy constructor copies the size and data pointer; the resulting span
has size() == other.size() and data() == other.data().

Parameters¶

first - iterator to the first element of the sequence
count - number of elements in the sequence
last - iterator past the last element of the sequence or another sentinel
arr - array to construct a view for
range - range to construct a view for
source - another span to convert from
other - another span to copy from

Exceptions¶

2) Throws nothing.
3) Throws what and when last - first throws.
7) Throws what and when std::ranges::size(r) and std::ranges::data(r) throw.

Notes¶

Feature-test macro Value Std Feature
__cpp_lib_span_initializer_list 202311L (C++26) Constructing std::span from a
std::initializer_list, (8)

Example¶

// Run this code

#include <array>
#include <iostream>
#include <span>
#include <vector>

void print_span(std::span<const int> s)
{
for (int n : s)
std::cout << n << ' ';
std::cout << '\n';
}

int main()
{
int c[]{1, 2, 3};
print_span(c); // constructs from array

std::array a{4, 5, 6};
print_span(a); // constructs from std::array

std::vector v{7, 8, 9};
print_span(v); // constructs from std::vector

#if __cpp_lib_span_initializer_list
print_span({0, 1, 2}); // constructs from initializer_list
#else
print_span({{0, 1, 2}}); // ditto, a workaround
#endif
}

Output:¶

1 2 3
4 5 6
7 8 9
0 1 2

See also¶

data direct access to the underlying contiguous storage
(public member function)
size returns the number of elements
(public member function)
operator= assigns a span
(public member function)
size
ssize returns the size of a container or array
(C++17) (function template)
(C++20)
data obtains the pointer to the underlying array
(C++17) (function template)