std::mdspan

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C++
 
Containers library
 
std::mdspan
 
Defined in header <mdspan>
template<

    class T,
    class Extents,
    class LayoutPolicy = std::layout_right,
    class AccessorPolicy = std::default_accessor<T>

> class mdspan;
 (since C++23)

std::mdspan is a multidimensional array view that maps a multidimensional index to an element of the array. The mapping and element access policies are configurable, and the underlying array need not be contiguous or even exist in memory at all.

Each specialization MDS of mdspan models copyable and satisfies:

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). Must satisfy the requirements of LayoutMappingPolicy.
AccessorPolicy - specifies how to convert underlying 1D index to a reference to T. Must satisfy the constraint that std::is_same_v<T, typename AccessorPolicy​::​element_type> is true. Must satisfy the requirements of AccessorPolicy.

Member types

Member 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

Data members

Member Description
accessor_type acc_ (private) the accessor
(exposition-only member object*)
mapping_type map_ (private) the layout mapping
(exposition-only member object*)
data_handle_type ptr_ (private) the underlying data handle
(exposition-only member object*)

Member functions

 constructs an mdspan
(public member function)
assigns an mdspan
(public member function)
Element access
accesses an element at the specified multidimensional index
(public member function)
Observers
[static]
 returns the rank of a mdspan
(public static member function)
[static]
 returns the dynamic rank of a mdspan
(public static member function)
[static]
 returns the static extent size of a mdspan at a given rank index
(public static member function)
returns the extent of a mdspan at a given rank index
(public member function)
returns the size of the multidimensional index space
(public member function)
checks if the size of the index space is zero
(public member function)
obtains the stride along the specified dimension
(public member function)
obtains the extents object
(public member function)
obtains the pointer to the underlying 1D sequence
(public member function)
obtains the mapping object
(public member function)
obtains the accessor policy object
(public member function)
determines if this mdspan's mapping is unique (every combination of indices maps to a different underlying element)
(public member function)
determines if this mdspan's mapping is exhaustive (every underlying element can be accessed with some combination of indices)
(public member function)
determines if this mdspan's mapping is strided (in each dimension, incrementing an index jumps over the same number of underlying elements every time)
(public member function)
[static]
 determines if this mdspan's layout mapping is always unique
(public static member function)
[static]
 determines if this mdspan's layout mapping is always exhaustive
(public static member function)
[static]
 determines if this mdspan's layout mapping is always strided
(public static member function)

Non-member functions

(C++23)
 specializes the std::swap algorithm for mdspan
(function template)
Subviews
(C++26)
 returns a view of a subset of an existing mdspan
(function template)
(C++26)
 creates new extents from the existing extents and slice specifiers
(function template)

Helper types and templates

(C++23)
 a descriptor of a multidimensional index space of some rank
(class template)
(C++23)(C++26)
 convenience alias template for an all-dynamic std::extents
(alias template)
(C++23)
 a type for indexed access to elements of mdspan
(class template)
(C++26)
 a type for aligned access to elements of mdspan
(class template)
Layout mapping policies
(C++23)
 column-major multidimensional array layout mapping policy; leftmost extent has stride 1
(class)
(C++23)
 row-major multidimensional array layout mapping policy; rightmost extent has stride 1
(class)
(C++23)
 a layout mapping policy with user-defined strides
(class)
(C++26)
 column-major layout mapping policy with padding stride that can be greater than or equal to the leftmost extent
(class template)
(C++26)
 row-major layout mapping policy with padding stride that can be greater than or equal to the rightmost extent
(class template)
Subviews helpers
(C++26)
 a slice specifier tag describing full range of indices in the specified extent.
(tag)
(C++26)
 a slice specifier representing a set of regularly spaced indices as indicated by an offset, an extent, and a stride
(class template)
(C++26)
 a return type of the overloads of submdspan_mapping
(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
202403L (C++26) std::mdspan padded layouts
__cpp_lib_aligned_accessor 202411L (C++26) std::aligned_accessor

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

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