std::mdspan(3)

NAME
std::mdspan - std::mdspan

Synopsis
   Defined in header <mdspan>
   template<
       class T,
       class Extents, (since C++23)
       class LayoutPolicy = std::layout_right,
       class AccessorPolicy = std::default_accessor<T>
   > class mdspan;
   std::mdspan is a view into a contiguous sequence of objects that reinterprets it as
   a multidimensional array.
   Each specialization MDS of mdspan models copyable and satisfies:
     * std::is_nothrow_move_constructible_v<MDS> is true,
     * std::is_nothrow_move_assignable_v<MDS> is true, and
     * std::is_nothrow_swappable_v<MDS> is true.
   A specialization of mdspan is a TriviallyCopyable type if its accessor_type,
   mapping_type and data_handle_type are TriviallyCopyable types.

Template parameters
   T - element type; a complete object type that is neither an abstract
                    class type nor an array type.
   Extents - specifies number of dimensions, their sizes, and which are known at
                    compile time. Must be a specialization of std::extents.
   LayoutPolicy - specifies how to convert multidimensional index to underlying 1D
                    index (column-major 3D array, symmetric triangular 2D matrix, etc).
                    specifies how to convert underlying 1D index to a reference to T.
   AccessorPolicy - Must satisfy the constraint that std::is_same_v<T, typename
                    AccessorPolicy::element_type> is true.
    This section is incomplete
    Reason: add the requirement of layout policy and accessor policy.

Member types
   Member type Definition
   extents_type Extents
   layout_type LayoutPolicy
   accessor_type AccessorPolicy
   mapping_type LayoutPolicy::mapping<Extents>
   element_type T
   value_type std::remove_cv_t<T>
   index_type Extents::index_type
   size_type Extents::size_type
   rank_type Extents::rank_type
   data_handle_type AccessorPolicy::data_handle_type
   reference AccessorPolicy::reference

Member objects
   Member name Definition
   acc_ (private) The accessor of type accessor_type
                  (exposition-only member object*)
   map_ (private) The layout mapping of type mapping_type
                  (exposition-only member object*)
   ptr_ (private) The underlying data handle of type data_handle_type
                  (exposition-only member object*)

Member functions
   constructor constructs an mdspan
                        (public member function)
   operator= assigns an mdspan
                        (public member function)

Element access
   operator[] accesses an element at the specified multidimensional index
                        (public member function)

Observers
   size returns the size of the multidimensional index space
                        (public member function)
   empty checks if the size of the index space is zero
                        (public member function)
   stride obtains the stride along the specified dimension
                        (public member function)
   extents obtains the extents object
                        (public member function)
   data_handle obtains the pointer to the underlying 1D sequence
                        (public member function)
   mapping obtains the mapping object
                        (public member function)
   accessor obtains the accessor policy object
                        (public member function)
                        determines if this mdspan's mapping is unique (every
   is_unique combination of indices maps to a different underlying element)
                        (public member function)
                        determines if this mdspan's mapping is exhaustive (every
   is_exhaustive underlying element can be accessed with some combination of
                        indices)
                        (public member function)
                        determines if this mdspan's mapping is strided (in each
   is_strided dimension, incrementing an index jumps over the same number of
                        underlying elements every time)
                        (public member function)
   is_always_unique determines if this mdspan's layout mapping is always unique
   [static] (public static member function)
   is_always_exhaustive determines if this mdspan's layout mapping is always exhaustive
   [static] (public static member function)
   is_always_strided determines if this mdspan's layout mapping is always strided
   [static] (public static member function)

Non-member functions
   std::swap(std::mdspan) specializes the std::swap algorithm for mdspan
   (C++23) (function template)
         Subviews
   submdspan_extents creates new extents from the existing extents and slice
   (C++26) specifiers
                        (function template)
   submdspan returns a view of a subset of an existing mdspan
   (C++26) (function template)
   Helper types and templates
   extents a descriptor of a multidimensional index space of some rank
   (C++23) (class template)
   dextents convenience alias template for an all-dynamic std::extents
   (C++23) (alias template)
   layout_right row-major multidimensional array layout mapping policy;
   (C++23) rightmost extent has stride 1
                        (class)
   layout_left column-major multidimensional array layout mapping policy;
   (C++23) leftmost extent has stride 1
                        (class)
   layout_stride a layout mapping policy with user-defined strides
   (C++23) (class)
   default_accessor a type for indexed access to elements of mdspan
   (C++23) (class template)
         Subviews helpers
   full_extent a slice specifier tag describing full range of indices in
   full_extent_t the specified extent
   (C++26) (tag)
   strided_slice a slice specifier representing a set of regularly spaced
   (C++26) indices as indicated by an offset, an extent, and a stride
                        (class template)
   submdspan_mapping_result a return type of the overloads of submdspan_mapping
   (C++26) (class template)
   Deduction guides

Notes
   Feature-test macro Value Std Feature
   __cpp_lib_mdspan 202207L (C++23) std::mdspan
   __cpp_lib_submdspan 202306L (C++26) std::submdspan

Example
   Can be previewed on Compiler Explorer. // Run this code
 #include <cstddef>
 #include <mdspan>
 #include <print>
 #include <vector>
 int main()
 {
     std::vector v{1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12};
     // View data as contiguous memory representing 2 rows of 6 ints each
     auto ms2 = std::mdspan(v.data(), 2, 6);
     // View the same data as a 3D array 2 x 3 x 2
     auto ms3 = std::mdspan(v.data(), 2, 3, 2);
     // Write data using 2D view
     for (std::size_t i = 0; i != ms2.extent(0); i++)
         for (std::size_t j = 0; j != ms2.extent(1); j++)
             ms2[i, j] = i * 1000 + j;
     // Read back using 3D view
     for (std::size_t i = 0; i != ms3.extent(0); i++)
     {
         std::println("slice @ i = {}", i);
         for (std::size_t j = 0; j != ms3.extent(1); j++)
         {
             for (std::size_t k = 0; k != ms3.extent(2); k++)
                 std::print("{} ", ms3[i, j, k]);
             std::println("");
         }
     }
 }

Output:
 slice @ i = 0
 0 1
 2 3
 4 5
 slice @ i = 1
 1000 1001
 1002 1003
 1004 1005

See also
   span a non-owning view over a contiguous sequence of objects
   (C++20) (class template)
   valarray numeric arrays, array masks and array slices
            (class template)

Category:
     * Todo with reason