NAME

deductionguidesforstd::span - deductionguidesforstd::span

Synopsis

Defined in header <span>
template <class It, class EndOrSize>
span(It, EndOrSize) -> (1)
span<std::remove_reference_t<std::iter_reference_t<It>>>;
template<class T, std::size_t N> (2)
span(T (&)[N]) -> span<T, N>;
template<class T, std::size_t N> (3)
span(std::array<T, N>&) -> span<T, N>;
template<class T, std::size_t N> (4)
span(const std::array<T, N>&) -> span<const T, N>;
template<class R> (5)
span(R&&) -> span<std::remove_reference_t<std::ranges::range_reference_t<R>>>;

The following deduction guides are provided for span.

(1) allow the element type to be deduced from the iterator-sentinel pair. This
overload participates in overload resolution only if It satisfies
contiguous_iterator

(2-4) allow the static extent to be deduced from built-in arrays and std::array.

(5) allow 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);

std::span s3 {a}; // guide (2)
print("s3", sizeof s3, s3.extent);

std::span<int> s4 {a}; // does not use a guide, makes a dynamic span
print("s4", sizeof s4, s4.extent);

std::array arr {6, 7, 8};
std::span s5 {arr}; // guide (3)
print("s5", sizeof s5, s5.extent);
s5[0] = 42; // OK, element_type is 'int'

const std::array arr2 {9, 10, 11};
std::span s6 {arr2}; // guide (4)
print("s6", sizeof s6, s6.extent);
// s6[0] = 42; // Error: element_type is 'const int'

std::vector v {66, 69, 99};
std::span s7 {v}; // guide (5)
print("s7", sizeof s7, s7.extent);
}

Output:

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