std::span<T,Extent>::span

`constexpr span() noexcept;`	(1)	(since C++20)
`template< class It > explicit(extent != std::dynamic_extent) constexpr span( It first, size_type count );`	(2)	(since C++20)
`template< class It, class End > explicit(extent != std::dynamic_extent) constexpr span( It first, End last );`	(3)	(since C++20)
`template< std::size_t N > constexpr span( std::type_identity_t<element_type> (&arr)[N] ) noexcept;`	(4)	(since C++20)
`template< class U, std::size_t N > constexpr span( std::array<U, N>& arr ) noexcept;`	(5)	(since C++20)
`template< class U, std::size_t N > constexpr span( const std::array<U, N>& arr ) noexcept;`	(6)	(since C++20)
`template< class R > explicit(extent != std::dynamic_extent) constexpr span( R&& range );`	(7)	(since C++20)
`explicit(extent != std::dynamic_extent) constexpr span( std::initializer_list<value_type> il ) noexcept;`	(8)	(since C++26)
`template< class U, std::size_t N > explicit(extent != std::dynamic_extent && N == std::dynamic_extent) constexpr span( const std::span<U, N>& source ) noexcept;`	(9)	(since C++20)
`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

#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:

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)
sizessize (C++17)(C++20)	returns the size of a container or array (function template)
data (C++17)	obtains the pointer to the underlying array (function template)

Docs

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std::span<T,Extent>::span

Parameters

Exceptions

Notes

Example

See also