NAME¶

deductionguidesforstd::span - deductionguidesforstd::span

Synopsis¶

Defined in header <span>
template< class It, class EndOrSize > (since
span( It, EndOrSize ) -> C++20)
span<std::remove_reference_t<std::iter_reference_t<It>>>; (until
C++26)
template< class It, class EndOrSize >

span( It, EndOrSize ) -> (since
span<std::remove_reference_t<std::iter_reference_t<It>>, C++26)

/*maybe-static-ext*/<EndOrSize>>; (1)
template< class T, std::size_t N > (2) (since
span( T (&)[N] ) -> span<T, N>; C++20)
template< class T, std::size_t N > (3) (since
span( std::array<T, N>& ) -> span<T, N>; C++20)
template< class T, std::size_t N > (4) (since
span( const std::array<T, N>& ) -> span<const T, N>; C++20)
template< class R > (since
span( R&& ) -> (5) C++20)
span<std::remove_reference_t<std::ranges::range_reference_t<R>>>;

The following deduction guides are provided for span.

1) Allows the element type to be deduced from the iterator-sentinel pair.
It also allows the static extent to be deduced if EndOrSize satisfies
integral-constant-like.
(since C++26). This overload participates in overload resolution only if It
satisfies contiguous_iterator.
2-4) Allows the static extent to be deduced from built-in arrays and std::array.
5) Allows the element type to be deduced from ranges. This overload participates in
overload resolution only if R satisfies contiguous_range.

Example¶

// Run this code

#include <array>
#include <cstddef>
#include <iomanip>
#include <iostream>
#include <span>
#include <string_view>
#include <vector>

void print(std::string_view rem = "", std::size_t size_of = 0, std::size_t extent = 0)
{
if (rem.empty())
{
std::cout << "name │ sizeof │ extent\n"
"─────┼────────┼────────\n";
return;
}
std::cout << std::setw(4) << rem << " │ " << std::setw(6) << size_of << " │ ";
if (extent == std::dynamic_extent)
std::cout << "dynamic";
else
std::cout << extent;
std::cout << '\n';
}

int main()
{
int a[]{1, 2, 3, 4, 5};

print();
std::span s1{std::begin(a), std::end(a)}; // guide (1)
print("s1", sizeof s1, s1.extent);

std::span s2{std::begin(a), 3}; // guide (1)
print("s2", sizeof s2, s2.extent);

#if __cplusplus > 202302L
std::span s3{std::begin(a), std::integral_constant<std::size_t, 2>{}}; // guide (1)
print("s3", sizeof s3, s3.extent);
#endif // C++26

std::span s4{a}; // guide (2)
print("s4", sizeof s4, s4.extent);

std::span<int> s5{a}; // does not use a guide, makes a dynamic span
print("s5", sizeof s5, s5.extent);

std::array arr{6, 7, 8};
std::span s6{arr}; // guide (3)
print("s6", sizeof s6, s6.extent);
s6[0] = 42; // OK, element_type is 'int'

const std::array arr2{9, 10, 11};
std::span s7{arr2}; // guide (4)
print("s7", sizeof s7, s7.extent);
// s7[0] = 42; // Error: element_type is 'const int'

std::vector v{66, 69, 99};
std::span s8{v}; // guide (5)
print("s8", sizeof s8, s8.extent);
}

Possible output:¶

name │ sizeof │ extent
─────┼────────┼────────
s1 │ 16 │ dynamic
s2 │ 16 │ dynamic
s3 │ 8 │ 2
s4 │ 8 │ 5
s5 │ 16 │ dynamic
s6 │ 8 │ 3
s7 │ 8 │ 3
s8 │ 16 │ dynamic