P2299R2
mdspan and CTAD

1. Introduction

[P0009R11] proposes adding non-owning multidimensional span abstractions to the C++ Standard Library; basic_mdspan, which is fully generic and can represent any kind of multidimensional data, and mdspan, a convenience template alias for simpler use cases.

template <class ElementType, class Extents, class LayoutPolicy = layout_right,
          class AccessorPolicy = accessor_basic<ElementType>>
struct basic_mdspan;

template <class ElementType, size_t... Extents>
using mdspan = basic_mdspan<ElementType, extents<Extents...>>;

In the basic_mdspan/span interface, extents can be either static, e.g. expressed at compile time:

mdspan<double, 64, 64> a(data);

or dynamic, e.g. expressed at run time:

mdspan<double, dynamic_extent, dynamic_extent> a(data, 64, 64);

You can also use a mix of the two styles:

mdspan<double, 64, dynamic_extent> a(data, 64);

While basic_mdspan and mdspan are powerful, the spelling of instantiations can be verbose, especially for the common case where all extents are dynamic.

Using class template argument deduction (introduced in C++17) and alias template argument deduction (introduced in C++20), we can make it a easier to use mdspan with all dynamic extents:

To make this work, we need to add a deduction guide for basic_mdspan. Through the power of alias template argument deduction, mdspan will be able to use said deduction guide as well.

Here’s an example implementation of such a deduction guide for basic_mdspan.

In earlier versions of this paper, it was unclear whether such a deduction guide would work for the alias template mdspan, as attempts to construct one that functioned as intended had failed. However, we have since learned that the this is due to bugs in the two implementations that currently support alias template deduction, MSVC and GCC. Those bugs have been reported to the respective compilers and hopefully will be fixed soon.

Here’s an example of the current implementation bugs that prevent the deduction guide from working for mdspan.

2. Wording

The following changes are relative to the mdspan proposal ([P0009R11]).

The � character is used to denote a placeholder number which shall be selected by the editor.

Modify the header synopsis for <mdspan> in [mdspan.syn] as follows:

22.7.� Header <mdspan> synopsis [mdspan.syn]

namespace std {
  // [mdspan.extents], class template extents
  template
    class extents;

  // [mdspan.layout], Layout mapping policies
  class layout_left;
  class layout_right;
  class layout_stride;

  // [mdspan.accessor.basic]
  template<class ElementType>
    class default_accessor;

  // [mdspan.basic], class template mdspan
  template<class ElementType, class Extents, class LayoutPolicy = layout_right,
           class AccessorPolicy = default_accessor<ElementType>>
    class basic_mdspan;

  template <class ElementType, class... IndexTypes>
  explicit basic_mdspan(ElementType*, IndexTypes...)
    -> basic_mdspan<ElementType, extents<[] (auto) constexpr
                                         { return dynamic_extent; }
                                         (identity<IndexTypes>{})...>>;
  template<class T, size_t... Extents>
    using mdspan = basic_mdspan<T, extents<Extents...>>;

  // [mdspan.submdspan]
  template<class ElementType, class Extents, class LayoutPolicy,
           class AccessorPolicy, class... SliceSpecifiers>
    constexpr basic_mdspan<see below>
    submdspan(const basic_mdspan<ElementType, Extents, LayoutPolicy, AccessorPolicy>& m,
              SliceSpecifiers... specs) noexcept;

  // tag supporting submdspan
  struct full_extent_t { explicit full_extent_t() = default; };
  inline constexpr full_extent_t full_extent = full_extent_t{};
}