mdspan revision

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_;
};

Design issues left at last time

• Which constructors do we want?
• What is mdarrays move behavior
• Do we want to support moving out (extracting) the container

Constructor

• multiple options for how to design overload set
  • complicated

Overview Table

The following table gives an overview of the three approaches.
In this table we use these shorthands:

• mda_t: a specialization of mdarray
• c_t: the container_type of mda_t
• e: an extents object
• m: a layout mapping object
• me: a extents object or a layout mapping object
• c: an instance of c_t
• a: an allocator object
• v: a value (something convertible to mda_t::value_type
• mda: an mdarray object, not necessarily the same type as mda_t
• mds: an mdspan obejct, not necessarily the same as returned by mda_t::to_mdspan

No source files or tests are named. Start by reviewing the constructor overview and the listed design questions for mdarray, then identify the project’s relevant implementation and test locations. Done means reaching an agreed design for constructors, move behavior, and container extraction, rather than making a narrowly scoped edit.