std::mdspan<T,Extents,LayoutPolicy,AccessorPolicy>::operator[]
From cppreference.com
template< class... OtherIndexTypes >
constexpr reference operator[]( OtherIndexTypes... indices ) const;
|
(1) | (since C++23) |
template< class OtherIndexType >
constexpr reference
operator[]( std::span<OtherIndexType, rank()> indices ) const;
|
(2) | (since C++23) |
template< class OtherIndexType >
constexpr reference
operator[]( const std::array<OtherIndexType, rank()>& indices ) const;
|
(3) | (since C++23) |
Returns a reference to the indicesth element of the mdspan.
1) Equivalent to
return acc_ .access(ptr_, map_ (static_cast<index_type>(std::move(indices))...));. This overload participates in overload resolution only if all following values are
true:
(std::is_convertible_v<OtherIndexTypes, index_type> && ...)(std::is_nothrow_constructible_v<index_type, OtherIndexTypes> && ...)sizeof...(OtherIndexTypes) == rank()
|
If |
(until C++26) |
|
If
|
(since C++26) |
2,3) Let
P be a parameter pack such that
std::is_same_v<std::make_index_sequence<rank()>
std::index_sequence<P...>>
true, equivalent to return operator[](extents_type::index-cast (std::as_const(indices[P]))...);. This overload participates in overload resolution only if all following values are
true:
std::is_convertible_v<const OtherIndexType&, index_type>std::is_nothrow_constructible_v<index_type, const OtherIndexType&>
Parameters
| indices | - | the indices of the element to access |
Return value
A reference to the element.
Example
Run this code
#include <array>
#include <mdspan>
#include <span>
int main()
{
static constexpr std::array data // row-major 2 x 3 matrix
{
10, 11, 12, // row 0
20, 21, 22 // row 1
};
constexpr std::mdspan m{data.data(), 2, 3}; // 2D mdspan (2 rows, 3 columns)
static_assert(m[1, 2] == 22, "Overload (1)");
// Coordinates are stored in a fixed-size container
constexpr std::array<std::size_t, 2> arr_indices{0, 1};
static_assert(m[arr_indices] == 11, "Overload (3)");
static_assert(m[arr_indices] == m[0, 1]);
// Useful in dynamic contexts or interfaces that get multi-dimensional coordinates
static constexpr std::array<std::size_t, 2> raw_indices{1, 0};
constexpr std::span<const std::size_t, 2> span_indices{raw_indices};
static_assert(m[span_indices] == 20, "Overload (2)");
static_assert(m[span_indices] == m[1, 0]);
}
Defect reports
The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
| DR | Applied to | Behavior as published | Correct behavior |
|---|---|---|---|
| LWG 3974 | C++23 | overloads (2,3) did not apply extents_type::index-cast
|
applies |
See also
(C++26) |
accesses an element at the specified multidimensional index with bounds checking (public member function) |