P2389R0
dextents Index Type Parameter

1. Background

[P0009R18] added mdspan, a non-owning multidimensional span abstraction to the C++ Standard Library. It is excellent and flexible, allowing users to customize customize data layout, access method, and index type. However, this flexibility often comes with verbosity.

The length of each dimension (the extent) of an mdspan are represented by an extents object, and each extent may be expressed either statically or dynamically:

// All static.
mdspan<int, 64, 64, 64> a(d);

// All dynamic.
mdspan<int, dynamic_extent, dynamic_extent, dynamic_extent> a(d, 64, 64, 64);

// Mixed static and dynamic.
mdspan<int, 64, dynamic_extent, 64> a(d, 64);

[P2299R3] sought to make mdspans easier to work with for one of the most common cases - when all of your extents are dynamic. It added deduction guides to make class template argument deduction (CTAD) work for mdspan and extents.

mdspan a(d, 64, 64); // All dynamic.

However, CTAD does not help in all situations. If you are declaring a member of a class or a parameter to a function, you cannot use CTAD.

struct X {
  std::mdspan<int, std::dynamic_extent, std::dynamic_extent, std::dynamic_extent> a;
};

void f(std::mdspan<int, std::dynamic_extent, std::dynamic_extent, std::dynamic_extent> a);

To simplify these cases, [P2299R3] also added dextents<N>, a template alias for an extents with N dynamic extents.

template <std::size_t N>
using dextents = /* ... */;

struct X {
  std::mdspan<int, std::dextents<3>> a;
};

void f(mdspan<int, std::dextents<3>> a);

2. Problem

Originally, mdspan and extents used a fixed index type (std::size_t). However, the signedness and size of the index type can have an impact on performance in certain cases. So, [P2553R1] parameterized the index type used by mdspan and extents.

As a part of this change, an index type template parameter was added to dextents:

template <typename IndexType, std::size_t Rank>
using dextents = /* ... */

This change has made using dextents more verbose, which is unfortunate, as the main purpose of dextents is to make common mdspan usages as simple as possible:

struct X {
  std::mdspan<int, std::dextents<std::size_t, 3>> a;
};

void f(mdspan<int, std::dextents<std::size_t, 3>> a);

Index type customization is an important feature for mdspan to support, but it is not something that most users will need to use or think about. If they do need it, they can always use the more verbose extents.

3. Proposed Changes

Remove the index type parameter from dextents and have it default to size_t.

MSVC’s STL and LLVM’s libc++ are already shipping dextents. GCC’s libstdc++ is not shipping mdspan yet.

So, modifying dextents would be a source-breaking change but would have no ABI impact as dextents is a template alias.

Alternatively and more practically, we could leave dextents alone, but add a new dims template alias that does not have an index type parameter.