Freestanding Library: inout expected span

Changes from previous revisions

Changes from R0

• Reworked wording based on changes in P2407R5
• Made feature test macros // freestanding
• Discuss P2821 span::at merge conflict

Introduction

This proposal is part of a group of papers aimed at improving the state of freestanding. It adds facilities that are largely unrelated to each other, but didn't fit into a better category. Most of these facilities were added in C++23, and missed the last sweep of freestanding utilities.

At a high level, this proposal adds the following facilities:

• out_ptr
• inout_ptr
• Most of expected
• span
• mdspan

Motivation and Scope

On hosted implementations, the included facilities don't directly require heap allocations, don't use system calls, and don't require exceptions. The facilities are generally useful. This combination keeps the burden low for standard library implementations to single source these facilities as both hosted and freestanding.

Design

All overloads of expected::value are deleted, as they can throw exceptions. This follows the precedent from P2407 "Freestanding Library: Partial Classes".

The declaration of shared_ptr is included, but the template class is left undefined. This allows out_ptr and inout_ptr to disallow certain shared_ptr usages in the same way in hosted and freestanding. This approach is less specification work and less implementation work, while still providing reasonable diagnostics to users if they mistakenly use shared_ptr on a minimal freestanding implementation. This approach still allows non-minimal freestanding implementations to provide a shared_ptr implementation that will interact with out_ptr and inout_ptr the same as they do on hosted implementations.

This paper mostly follows the guide lines as specified in [P2198R6] "Freestanding Feature-Test Macros and Implementation-Defined Extensions". However, __cpp_lib_freestanding_mdspan is added instead of bumping __cpp_lib_mdspan because I suspect the mdspan authors will be bumping the __cpp_lib_mdspan macro frequently in the C++26 cycle. When multiple orthogonal papers bump the same feature test macro, it becomes difficult for users to test for the specific features they care about.

The in flight paper [P2821] "span.at()" paper adds at() to span. The at() function originates exceptions, so it will not be required to be present in a freestanding implementation. This follows the precedent set by array in P2407 "Freestanding Library: Partial Classes".

Experience

[P0323R12 std::expected] cites usage experience on bare metal systems without exceptions enabled. [P0009R18 MDSPAN] has a reference implementation that is usable with CUDA. mdspan uses span in the implementation and interface, so the CUDA experience extends to span as well.

The author has prototyped the necessary out_ptr and inout_ptr changes with MSVC's STL. This involved #if'ing out weak_ptr, as well as the definitions of shared_ptr facilities. The tests worked as is, except for the ones specifically testing shared_ptr.

Wording

Change in [freestanding.item]

?

Class template declarations followed by a comment that include freestanding-deleted are freestanding deleted class template declarations.

On freestanding implementations, it is implementation-defined whether each entity introduced by a freestanding deleted class template declaration is undefined or whether the requirements are the same as the corresponding requirements for a hosted implementation.

[ Note: An instantiation of an undefined template yields an incomplete class type (temp.inst#2.sentence-4). -end note]

[ Example:

template<class T> class shared_ptr; // freestanding-deleted

-end example]

Changes in [compliance]

Changes in [version.syn]

#define __cpp_lib_freestanding_expected             new-val // freestanding, also in <expected>
#define __cpp_lib_freestanding_mdspan               new-val // freestanding, also in <mdspan>
...
#define __cpp_lib_out_ptr                           202106Lnew-val // freestanding, also in <memory>
...
#define __cpp_lib_span                              202002Lnew-val // freestanding, also in <span>

Changes in [memory.syn]

Please append a // freestanding-deleted comment to the following declaration:

• shared_ptr

• out_ptr_t
• out_ptr
• inout_ptr_t
• inout_ptr

Changes in [expected.syn]

Please insert a // mostly freestanding comment at the beginning of the [expected.syn] synopsis.

Please append a // partially freestanding comment to every expected declaration in the synopsis.

Changes in [expected.object.general]

Please append a // freestanding-deleted comment to every value overload in the class synopsis.

Changes in [expected.void.general]

Please append a // freestanding-deleted comment to every value overload in the class synopsis.

Changes in [span.syn]

Changes assuming [P2821] "span.at()" has NOT been merged

Please insert an // all freestanding comment at the beginning of the [span.syn] synopsis.

Changes assuming [P2821] "span.at()" has been merged

Please insert a // mostly freestanding comment at the beginning of the [span.syn] synopsis.

Please append a // partially freestanding comment to the span declaration.

Changes in [views.span]

Drafting note: Only apply this change if [P2821] "span.at()" has been merged.

Please append a // freestanding-deleted comment to every overload of at.

Changes in [mdspan.syn]

Please insert an // all freestanding comment at the beginning of the [mdspan.syn] synopsis.

References