LEWG Kona 11/2023 mdarray

Super Brief mdarray introduction

Why we want mdarray

Creating a simple 2D array with mdspan requires multiple steps because mdspan is non-owning.

// Create a mapping:
layout_left map(extents(N,M));

// Create an allocation
vector<int> data(map.required_span_size());

// Create mdspan:
mdspan a(data.data(), map);

mdarray is a multidimensional array with value-semantics and makes the above simpler

// default layout
mdarray a(N,M);
// other layout
mdarray b(layout_left(extents(N,M))); 

What aspects of mdspan does mdarray have

mdarray has the

• element_type,
• extents and
• layout policy aspects of mdspan

The accessor policy is replaced by a container type.

template<class ElementType, class Extents, class Layout, class Accessor>
class mdspan {
   public:
   ...
     template<class ... Indices>
     reference operator [] (Indices ... idx) const { return acc_.access(ptr_, map_(idx...)); }
   private:
     typename Accessor::data_handle_type ptr_;
     Layout::mapping<Extents> map_;
     Accessor acc_;
};

template<class ElementType, class Extents, class Layout, class Container>
class mdarray {
   public:
   ...
     template<class ... Indices>
     reference operator [] (Indices ... idx) { return ctr_[map_(idx...)]); }

   private:
     Layout::mapping<Extents> map_;
     Container ctr_;
};

This means mdarray does not have the kind of access modality customization point functionality of mdspan, but that functionality is anyway intended for local specialization of accesses, and thus more suited for the view-like class.

Constructors

• have constructors similar to mdspan
  • indicies
  • extents
  • mapping
• Combine those with copy from container, and move from container
• Combine all of those with add optional allocator

Move behavior

• relies on containers move behavior
• preconditions on problematic functions to check that container size is larger or equal to required_span_size()

The issue contains an mdarray design sketch but names no repository file, test, or entry point. Start by reviewing the stated relationship to mdspan, then turn the listed constructors, container behavior, layouts, and move requirements into a scoped proposal; the issue does not define a concrete completion criterion.