Freestanding Library: inout expected span

Changes from previous revisions

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 P2338 "Freestanding Library: Character primitives and the C library".

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.

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 is prototyping out_ptr and inout_ptr changes with MSVC's STL. Ask about this implementation experience during the LEWG review!

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 class introduced by a freestanding deleted class template declaration is a freestanding item or undefined.

[ 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 20XXXXL //also in <expected>
#define __cpp_lib_freestanding_mdspan 20XXXXL //also in <mdspan>

• __cpp_lib_out_ptr
• __cpp_lib_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 // freestanding-partial 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]

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

Changes in [mdspan.syn]

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

References