1 Introduction

This paper proposes element access with bounds checking to std::mdspan via at() member functions.

2 Revision history

2.1 R1

Update wording to align with [P2821R5]:

• Bump __cpp_lib_mdspan instead of adding __cpp_lib_mdspan_at
• Mark at() as freestanding deleted and adjust freestanding comments of header and class

2.2 R0

Initial version

3 Motivation

3.1 Safety

The new at() member functions provide memory-safe element access to std::mdspan, and thus have defined behavior. Out-of-bound access can be caught by catching the std::out_of_range exception.

3.2 Consistency

In [P2821R5], element access with bounds checking via at() has been added to std::span. One of the main motivations for this change was consistency with other containers that have element access with bounds checking via at(). Similarly, such element access should be added to std::mdspan.

4 Impact On the Standard

The impact of this proposal on the standard is low. The proposed function signatures for the at() member functions are identical to the function signatures for the subscript operators as proposed in [P0009R18].

One consideration is that the at() method has previously not been used with multi-dimensional arguments. However, this was also true for the subscript operator before the possibility was introduced in [P2128R6].

5 Wording

The wording is relative to [N4993].

In 17.3.2 ([version.syn]), adjust the value of __cpp_lib_mdspan to the date of this proposal’s adoption.

In 23.7.3.2 ([mdspan.syn]), change the following lines:

- // all freestanding
+ // mostly freestanding
namespace std {

// 23.7.3.6, class template mdspan
template<class ElementType, class Extents, class LayoutPolicy = layout_right,
         class AccessorPolicy = default_accessor<ElementType>>
-  class mdspan;
+  class mdspan;                                                                // partially freestanding

In 23.7.3.6.1 ([mdspan.mdspan.overview]), add the following immediately after the subscript operators:

In 23.7.3.6.3 ([mdspan.mdspan.members]), add the following immediately after the subscript operators:

6 Reference Implementation

The at() member functions have been implemented in the std::mdspan reference implementation from the Kokkos project at Sandia National Laboratories [kokkos/mdspan], see [kokkos/mdspan#302].

7 References