Issaquah (Feb 2023) P2630R2 presentation

P2630R2: submdspan

submdspan: multidimensional array slicing

• Split from P0009 after LEWG review

• P2630 expands facility to make it work for custom layouts, and adds strided_slice

R2 changes

• Added discussion choice of customization point mechanism

• Renamed strided_index_range to strided_slice

• Introduced named struct combining mapping and offset as return type for submdspan_mapping

• Removed redundant constraints, which are covered by mandates; this improves error messages for users

Customization point mechanism

• Paper: Section 2.1.3 "Pure ADL vs. CPO vs. tag invoke"

• Applies P2279's criteria to judge value of different mechanisms

• Most users will never call submdspan_mapping directly

• submdspan_mapping only needs to be implemented for custom layout mappings, a rare use case

• submdspan_mapping has no default implementation (unlike, say, std::swap)

• Name submdspan_mapping is extremely specific, making opt-in essentially explicit

• Incorrect opt-in is easy to diagnose: return type must be a layout mapping with specific extents

• This and other discussion in paper suggest no significant benefits in preferring CPOs or tag_invoke over pure ADL for submdspan_mapping. However, we would accept LEWG's preference for tag_invoke here.

The issue presents P2630R2, “submdspan,” and discusses its customization-point mechanism, but names no repository file, test, or entry point. Start by determining what presentation artifact is expected and how it should be added; the issue does not define a concrete completion check.