deduction guides for std::span - cppreference.com (original) (raw)

Defined in header
(1)
template< class It, class EndOrSize >span( It, EndOrSize ) -> span<std::remove_reference_t<std::iter_reference_t<It>>>; (since C++20) (until C++26)
template< class It, class EndOrSize > span( It, EndOrSize ) -> span<std::remove_reference_t<std::iter_reference_t<It>>, /*maybe-static-ext*/<EndOrSize>>; (since C++26)
template< class T, std::size_t N >span( T (&)[N] ) -> span<T, N>; (2) (since C++20)
template< class T, std::size_t N >span( std::array<T, N>& ) -> span<T, N>; (3) (since C++20)
template< class T, std::size_t N >span( const std::array<T, N>& ) -> span<const T, N>; (4) (since C++20)
template< class R >span( R&& ) -> span<std::remove_reference_t<std::ranges::range_reference_t<R>>>; (5) (since C++20)

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.

  1. Allows the element type to be deduced from ranges. This overload participates in overload resolution only if R satisfies contiguous_range.

[edit] Example

#include #include #include #include #include #include #include   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 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