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 non-owning view into a contiguous sequence of objects that reinterprets it as a multidimensional array.

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).
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.
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

Data members

Typical implementations of mdspan hold only three non-static data member:

  • An accessor of type accessor_type (shown here as acc_ for exposition only).
  • A layout mapping of type mapping_type (shown here as map_ for exposition only).
  • A underlying data handle of type data_handle_type (shown here as ptr_ for exposition only).

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
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 indexes 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 indexes)
(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)

Helper types and templates

(C++23)
 a descriptor of a multidimensional index space of some rank
(class template)
(C++23)
 convenience alias for an all-dynamic std::extents
(class template)
(C++23)
 row-major multidimensional array layout mapping; rightmost extent has stride 1
(class)
(C++23)
 column-major multidimensional array layout mapping; leftmost extent has stride 1
(class)
(C++23)
 a layout mapping with user-defined strides
(class)
(C++23)
 a type for indexed access to elements of mdspan
(class template)

Deduction guides(c++23)

Notes

Feature-test macro Value Std Comment
__cpp_lib_mdspan 202207L (C++23) std::mdspan

Example

can be previewed at https://godbolt.org/z/PK7bccGr3

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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