2023-11 Kona ISO C++ Committee Trip Report — Second C++26 meeting!🌴
This week was the C++ Committee meeting, in Kona, Hawaii, USA 🇺🇸 in which we got a lot of new C++26 features!
The features voted on will be added gradually to the working draft, and will likely be officially released on the next version (C++26), barring any subsequent changes.
We shipped quite a few features. This is not the first time the meeting has taken place in Kona, and the meeting organizers always run the meeting well, and everything works as expected. We appreciate that greatly!
This was the fourth hybrid C++ Committee meeting. We had high participation by remote attendees. We had around 20% remote participation. We plan to keep operating hybrid meetings going forward.
Main C++26 Features forwarded in Kona: 🎉
- P2267R0: Library Evolution Policies (sets the framework) (Forwarded from LEWG, still requires official approval)
- P1673R13: A free function linear algebra interface based on the BLAS
- P2760R0: A Plan for C++26 Ranges (schedule and priorities) (Forwarded from LEWG)
- P2662R3: Pack Indexing
- P2546R5 : Debugging Support
- Support for runtime format strings (P2905R2 + P2918R2)
Language Progress
Evolution Working Group (EWG) Progress
Language Evolution met for most of the week, spending one day on undefined behavior. We ran out of content for the last half-day.
In our report, we use the following emoji-aided language:
- 👎 — consensus to stop work (unless new information comes, warranting revisiting)
- 👍 — advance the paper to Core (or other groups), targeting C++26
- ♻️ — consensus to continue work in EWG
- 🚫 — the issue is not a defect
- ⬆️ — see above (contextual keyword, see above where it appears, not here)
C alignments, bug fixes, deprecation
Make C++ hurt less.
- 👎 — P2958R0: typeof and typeof_unqual (came to us from C23 but did not have consensus for addition on C++. That said, it is expected that implementations will support these new keywords in C++ mode to enable interoperability with C headers, it is therefore likely that existing practice warrants adding these to C++ in the future)
- 👎 — P2984R0: Reconsider Redeclaring static constexpr Data Members (We agreed to neither un-deprecate, nor remove the feature entirely)
- 👍 — P2865R3: Remove Deprecated Array Comparisons from C++26 (Removes surprising code)
- 👍 — P2864R1: Remove Deprecated Arithmetic Conversion on Enumerations From C++26 (Removes surprising code)
- 👍 — P2795R3: Erroneous behaviour for uninitialized reads (Erroneous behaviour is a promising way to make UB hurt less. That said, there are significant wording issues in Core. This paper introduces the concept of EB, and uses it in one location. It is a different way of addressing the issue from P2723r1, which the committee wants to address)
- ♻️ — P2973R0: Erroneous behaviour for missing return from assignment (The second application of EB. There was discussion on making missing returns ill-formed instead, but the committee was interested in using EB for this instead)
- 👍 — P2809R2: Trivial infinite loops are not Undefined Behavior (The paper proceeded to Core, with option #2 (the simpler definition of trivial infinite loops) being chosen)
- ♻️ — P0901R11: Size feedback in operator new (Interest in seeing this paper again, feedback was given to the authors)
- 👎 — P2971R1: Implication for C++ (There was mild opposition and many neutrals, but also significant interest from a small but vocal set of people. It is expected that this paper will come back with stronger justification to increase consensus)
Issue processing
Remove sharp edges.
- 👍 — CWG2561: Conversion to function pointer for lambda with explicit object parameter (Proceeded with its option #1)
- ♻️ — CWG2804: Lookup for determining rewrite targets (Lookup is difficult. We’ll need a paper to address the issue in its entirety)
- 🚫 — CWG2548: Array prvalues and additive operators (Not a defect, but a paper could be written to justify this change)
- 🚫 — CWG2776: Substitution failure and implementation limits (Not a defect, but a paper could be written to justify this change)
- ♻️ — CWG2784: Unclear definition of member-designator for offsetof (Interested in pursuing, needs a paper)
- 👍 — CWG2733: Applying maybe_unused to a label (Accept the proposed wording)
- 🚫 — CWG2751: Order of destruction for parameters for operator functions (Not a defect, implementations can choose the strategy)
- 👍 — CWG2560: Parameter type determination in a requirement-parameter-list (Accept the proposed resolution, and apply it as a Defect Report to prior versions of C++)
- ♻️ — CWG2726: Alternative tokens appearing as attribute-tokens (A paper is forthcoming)
- ♻️ — CWG1699: Does befriending a class befriend its friends? (A paper is forthcoming)
- ♻️ — CWG2669: Lifetime extension for aggregate initialization (A paper is forthcoming)
- ♻️ — CWG2797: Meaning of "corresponds" for rewritten operator candidates (Related to CWG2804)
- 👍 — CWG2825: Range-based for statement using a braced-init-list (Accept the proposed resolution, and apply it as a Defect Report to prior versions of C++)
Features
Make C++ nicer.
- ♻️ — P1046R2: Automatically Generate More Operators (The author wrote a new draft paper P3039R0 Automatically Generate
operator->which narrows the scope of change and has strong support) - ♻️ — P1045R1: constexpr Function Parameters (This paper was discussed as a potential language solution for the following paper. We would need an updated paper)
- ♻️ — P2781R3:
std::constexpr_t(LEWG was interested in a language feature to resolve this usecase. EWG agrees, but needs papers to advance this topic) - ♻️ — P3009R0: Injected class name in the base specifier list (Consensus was not particularly strong, but there was mild interest)
- ♻️ — P2994R0: On the Naming of Packs (Interest, needs an update)
- 👍 — P2662R2: Pack Indexing (Core pointed out a potential design oversight, EWG was OK with the potential closed design door)
- ♻️ — P2963R0: Ordering of constraints involving fold expressions (The author was encouraged to perform further work)
- ♻️ — P2686R2: constexpr structured bindings and references to constexpr variables (There is still strong interest in allowing structured bindings in constexpr, but doing so requires consideration around adding references that potentially dangle as constexpr. The room was not sure the design was comprehensible, further work was encouraged)
- 👍 — P2893R1: Variadic Friends (Forward to Core for C++26)
- 👍 — P2748R2: Disallow Binding a Returned Glvalue to a Temporary (Forward to Core for C++26)
- ♻️ — P2991R0: Stop Forcing std::move to Pessimize (Weak consensus, needs more work)
- 👍 — P2927R0: Observing exceptions stored in exception_ptr (EWG approves the direction, LEWG may proceed)
- 👎 — P0342R2: pessimize_hint (No consensus)
- ♻️ — P2989R0: A Simple Approach to Universal Template Parameters (Strong interest, the paper should be seen again. The precise syntax was not chosen yet, we will do so later)
- ♻️ — P2841R1: Concept Template Parameters (Feedback was provided)
- ♻️ — P2979R0: The Need for Design Policies in WG21 (We are interested in defining principles (or rules of thumb) and policies for the design and evolution of C++, with the goal of documenting our common understanding and simplifying our ongoing work, but without creating a bureaucracy)
Undefined behavior
Remove it, or at least document it.
- ♻️ — P2843R0: Preprocessing is never undefined (The paper surveys all UB in the preprocessor, and implementation behavior. It suggests solutions to each, making them not UB. There’s strong support for this work. Alignment with the C committee has started)
- ⬆️ — CWG2577: UB for preprocessing directives in macro arguments
- ⬆️ — CWG2581: UB for predefined macros
- ⬆️ — CWG2580: UB with #line
- ⬆️ — CWG2579: UB when token pasting does not create a preprocessing token
- ⬆️ — CWG2578: UB when creating an invalid string literal via stringizing
- ⬆️ — CWG2576: UB with macro-expanded #include directives
- ⬆️ — CWG2575: UB when macro-replacing "defined" operator
- ♻️ — CWG2514: Modifying const subjects (Strong interest to resolve, but needs a paper)
- 👍 — P1705R1: Enumerating Core Undefined Behavior (This and the next paper will make their way to the standard. The editor can add them editorially. However, there is substantial wording in the explanation and examples, Core is therefore interested in reviewing the work. Collaboration is welcome. Note that this paper only documents existing explicit Core UB. It does not change what it UB, nor does it document implicit UB, not library UB. It’s a start to allow us to revisit what is UB)
- 👍 — P2234R1: Consider a UB and IF-NDR Audit (As above, this will add an index of IF-NDR, or an appendix with justification and examples)
What will EWG do in Tokyo?
東京は何をしましょうか。
- Continue improving C++.
- Focus on reflection.
- Prepare for focusing on contracts.
Given the light open workload, EWG will not host any telecons between now and Tokyo.
Evolution Working Group Incubator Study Group (SG17) Progress
EWGI met for 2 afternoon sessions in Kona and was able to see 8 papers. EWGI is concentrated on giving feedback on the paper and feature’s readiness, preparedness, and presentability, and typically avoids being opinionated on features themselves (other than implementability, completeness, etc).
Papers
EWGI saw 8 papers, 5 of which were forwarded to EWG:
- P2992R0: Attribute
[[discard]]` and attributes on expressions - P2971R1: Implication for C++
- P2573R1:
delete(“should have a reason”); - P2826R1: Replacement Functions
Additionally, EWGI gave extensive feedback to 3 additional papers, and expects to see them again in the future: - P2946R0: A flexible solution to the problems of noexcept - P2986R0: Generic Function Pointer - P2830R1: constexpr type comparison
Core Working Group (CWG) Progress
As always, Core met during the week and reviewed a lot of issues and papers. The following were approved into the working draft.
Approved
- P3046R0: Core Language Working Group "ready" Issues for the November, 2023 meeting (+ Defect Report)
- P2308R1: Template parameter initialization (+ Defect Report)
- P2662R3: Pack Indexing
- P2864R2: Remove Deprecated Arithmetic Conversion on Enumerations From C++26
Library Progress
Library Evolution Working Group (LEWG) Progress
Library Evolution Work Group continues to hold weekly telecons in order to handle the workload in between in-person WG21 meetings.
The Kona meeting has brought together a lot of our experienced and long-time experts. This was very beneficial for moving forward with large features.
During the Kona meeting, we also discussed a new framework to set Library Evolution policies for the standard library.
A “policy” is a rule that should generally be followed for every new proposal library that targets the standard library.
The framework contains the requirements from a policy paper, as well as the process for setting, modifying, and removing a Library Evolution policy. The framework also defines the requirement needed for a paper to bypass (avoid applying) the policy.
As in previous meetings, Library Evolution focused on large features targeting C++26. NOTE: Our LEWG process requires an electronic poll (then plenary approval, for the WG21 process), the status is not final
Features approved
- P2267R0: Library Evolution Policies
- P0447R24: Introduction of std::hive to the standard library (D0447R25 to be published)
- P2999R2: Sender Algorithm Customization (require LEWG wording review)
- P3019R2: Vocabulary Types for Composite Class Design (require LEWG wording review)
- P2019R4: Thread attributes
- P2845R4: Formatting of std::filesystem::path
- P2971R1: Implication for C++ (library part tentatively approved)
- P2663R4: Proposal to support interleaved complex values in std::simd
- P2933R0: std::simd overloads for standard C++ <bit> header
- P1928R8: std::simd - Merge data-parallel types from the Parallelism TS 2
- P2642R4: Padded mdspan layouts
- P1684R5: mdarray: An Owning Multidimensional Array Analog of mdspan (requires review of the requested changes)
- P2760R0: A Plan for C++26 Ranges (schedule and priorities)
Small fixes and deprecations
- P2862R1: text_encoding::name() should never return null values
- P2300R7: std::execution (small fixes)
- P2809R2: Trivial infinite loops are not Undefined Behavior (naming discussion on the library part)
- P2845R4: Formatting of std::filesystem::path
- P2918R1: Runtime format strings II
- P2944R2: Comparisons for reference_wrapper
- P2863R2: Review Annex D for C++26
- P2869R2: Remove Deprecated
shared_ptrAtomic Access APIs From C++26 - P2866R1: Remove Deprecated Volatile Features From C++26
Not supported or needs-revision
- P3022R0: A Boring Thread Attributes Interface
- P3001R0: std::hive and containers like it are not a good fit for the standard library
- D3024R0: Interface direction for std::simd
- P3014R0: Customizing std::expected's exception
- P2664R4: Proposal to extend std::simd with permutation API
- P2927R0: Observing exceptions stored in exception_ptr
Requires more review
- P1028R5: SG14 status_code and standard error object
- P2855R0: Member customization points for Senders and Receivers
- P2170R0: Feedback on implementing the proposed std::error type (feedback should be discussed with the author of P1028)
- P2781R3: std::constexpr_t (support for a language feature)
Info papers
- P2979R0: The Need for Design Policies in WG21
- P3011R0: Supporting document for Hive proposal #1: outreach for evidence of container-style use in industry
- P3012R0: Supporting document for Hive proposal #2: use of std::list in open source codebases
- P2980R0: A motivation, scope, and plan for a physical quantities and units library
Evening Sessions
LEWG was also organizing two evening sessions during Kona:
- Tuesday: SIMD API
- Thursday: P3023R0: C++ Should Be C++
The evening sessions encouraged discussion and were beneficial for increasing consensus and making informed decisions. LEWG will arrange additional evening sessions on similar topics when required (the next evening session is tentatively devoted to “Units and Quantities for the standard library”).
We will continue our weekly telecons until Tokyo.
Library Evolution Working Group Incubator Study Group (SG18) Progress
LEWGI did not meet during the week, and the chairs were assisting in managing the workload at LEWG.
Library Working Group (LWG) Progress
LWG was focused on large topics such as SIMD and std::execution. Note that LWG also conducts small group telecons in between sessions to review P2300.
Approved
- P1673R13: A free function linear algebra interface based on the BLAS
- P0543R3: Saturation arithmetic
- P2407R5: Freestanding Library: Partial Classes
- P2546R5: Debugging Support
- P0952R2: A new specification for std::generate_canonical
- P2447R6: std::span over an initializer list
- P2821R5: span.at()
- P2868R3: Remove Deprecated std::allocator Typedef From C++26
- P2870R3: Remove basic_string::reserve() From C++26
- P2871R3: Remove Deprecated Unicode Conversion Facets from C++26
- P2819R2: Add tuple protocol to complex
- P2264R7: Make assert() macro user friendly for C and C++
- P2937R0: Freestanding: Remove strtok
- P2833R2: Freestanding Library: inout expected span
- P2918R2: Runtime format strings II
- P2905R2: Runtime format strings (+ Defect Report)
- P2909R4: Fix formatting of code units as integers (Dude, where’s my char?) (+ Defect Report)
- P2836R1 : std::basic_const_iterator should follow its underlying type’s convertibility (+ Defect Report)
- LWG2432: initializer_list assignability
Additional Papers Reviewed
- P1928R8: std::simd - Merge data-parallel types from the Parallelism TS 2 (ongoing)
- P2300R7: std::execution (ongoing)
- P2527R2: std::variant_alternative_index and std::tuple_element_index (ongoing)
- P1068R9: Vector API for random number generation (ongoing)
- P0876: fiber_context - fibers without scheduler (forwarded to CWG)
LWG will continue running small group reviews on P2300 between the in-person meetings.
Study Group Progress
Concurrency and Parallelism Study Group (SG1) Progress
Another short meeting for SG1, about a dozen papers were seen. There’s renewed interest over the reflector in solving the OOTA problem; considering a 2-day working meeting on this topic either at a future WG21 meeting or as a one-off.
Papers
- P2956R0: Add saturating library support to std::simd
- P2933R0: std::simd overloads for bit header
- P2929R0: simd_invoke
- P2921R0: Exploring std::expected based API alternatives for buffer_queue
- P2912R0: Concurrent queues and sender/receivers
- P3008R0: Atomic floating-point min/max
- LWG3941: §[atomics.order] inadvertently prohibits widespread implementation techniques
- CWG2298: Actions and expression evaluation
- P2774R1: Concurrent object pool (was: Scoped thread-local storage)
- P2633R0: thread_local_inherit: Enhancing thread-local storage
- P2500R2: C++ parallel algorithms and P2300
- P2902R0: constexpr 'Parallel' Algorithms
Networking (SG4) Progress
SG4 met for one session after lunch on Wednesday to review the latest revision of “P2762R2: Sender/Receiver Interface For Networking”. The session was mainly focused on providing feedback to the author for future development, such as what features we need to keep or add for an initial version of C++ networking. Notably, the group had a consensus in favor of supporting only the Senders/Receivers model for asynchronous operations and dropping the executors model from the Networking TS.
Numerics Study Group (SG6) Progress
The Numerics study group met for a whole day on Tuesday. "Low-Level Integer Arithmetic" was forwarded with the expectation of becoming available in C++26, providing the building blocks for user-defined numeric types. The discussion on floating-point rounding modes and explicitly rounding operations seems to be converging. We also looked at a language issue related to GCC 13 implementing excess precision for floating-point literals. There was no consensus at this point. The group will likely need a paper listing all the possibilities and implications.
Papers
- P3003R0: The design of a library of number concepts
- P2980R0: A motivation, scope, and plan for a physical quantities and units library
- P2982R0:
std::quantityas a numeric type - P2746R3: Deprecate and Replace Fenv Rounding Modes
- CWG2752: Excess-precision floating-point literals
- P3018R0: Low-Level Integer Arithmetic
Compile-time Programming Study Group (SG7) Progress
We heard and discussed the experience of two paper authors using existing reflection facilities (from previous papers) to implement JavaScript and Python bindings. Then we forwarded Static Reflection paper to EWG aiming for C++26. Current design contains std::meta::info opaque handlers and we ask authors to make sure that no future changes will close the door in pursuing different types of handlers based on reflected entities.
Papers
- P3010R0: Using Reflection to Replace a Metalanguage for Generating JS Bindings
- P2911R1: Python Bindings with Value-Based Reflection
- P2996R1: Reflection for C++26
Ranges Study Group (SG9) Progress
SG9 met on Monday and saw the following issues and papers
Papers
- P2760R0: A Plan for C++26 Ranges
- P1030R6: std::filesystem::path_view
- P2997R0: Removing the common reference requirement from the indirectly invocable concepts
- P1729R3: Text Parsing
Issues
- LWG3913: ranges::const_iterator_t<range R> fails to accept arrays of unknown bound
- LWG3988: Should as_const_view and basic_const_iterator provide base()?
Low Latency Study Group (SG14) Progress
SG14 met in CppCon 2023, and so they did not meet this time.
Tooling Study Group (SG15) Progress
The tooling study group met on Thursday and Friday. We reviewed the following papers and included some of them into the planned Tooling Ecosystem IS draft (a new standard, focused on C++ toolchain).
Papers
- P2947R0: Contracts must avoid disclosing sensitive information (Forwarded by SG23)
- P2717R4: Tool Introspection (merged into Tooling Ecosystem IS draft)
- D2977R0: Module configuration files
- P2962R0: Communicating the Baseline Compile Command for C++ Modules support
- P2978R0: A New Approach For Compiling C++
- P2990R0: C++ Modules Roadmap
- P3034R0: Module Declarations Shouldn’t be Macros
- D3041R0: Transitioning from #include World to Modules
Text and Unicode Study Group (SG16) Progress
SG16 did not meet this week but continues to meet remotely twice monthly to review and forward papers. The following papers are ones that were adopted into C++26 this week that SG16 previously contributed to.
Papers
- P2909R4: Fix formatting of code units as integers (Dude, where’s my char?)
- P2871R3: Remove Deprecated Unicode Conversion Facets from C++26
Issues
The following LWG issues that SG16 previously contributed to were also resolved this week. - LWG3965: Incorrect example in [format.string.escaped] p3 for formatting of combining characters
Contracts Study Group (SG21) Progress
SG21 was sitting Tuesday, Thursday, and Friday morning in Kona and we had a very productive meeting.
Papers
We saw the following papers:
- P2935R4: An Attribute-Like Syntax for Contracts
- P2961R2: A natural syntax for Contracts
- P2038R0: An Overview of Syntax Choices for Contracts
- P2896R0: Outstanding design questions for the Contracts MVP
- D2932R2: A Principled Approach to Open Design Questions for Contracts
- P2957R0: Contracts and coroutines
We achieved strong consensus on a new syntax for contract checks called the "natural syntax" that no longer uses double square brackets:
int f(const int x)
pre (x != 1)
post (r : r != 2)
{
contract_assert (x != 3);
return x;
}
We also decided that
- Precondition and postcondition checks on functions defaulted on their first declaration are ill-formed
- Precondition and postcondition checks on coroutines are ill-formed
We're on track with our roadmap P2695R1 to get Contracts ready in time for C++26. At this point, we only have a handful of unresolved design questions left:
- Contracts and implicit lambda captures
- Constant evaluation of contracts (constexpr/consteval)
- Contracts and virtual functions
- Contracts on multiple declarations of the same function
- Contracts and deduced exception specifications
We scheduled six SG21 telecons between now and Tokyo to deal with the above. By Tokyo (the next committee meeting in March 2024) we are aiming to have a complete Contracts working paper P2900R2 and to forward it to EWG, LEWG, and SG21 for review.
C / C++ Liaison Group (SG22) Progress
Liaison group did not meet during this meeting.
Safety & Security Group (SG23) Progress
The SG23 work group met on Wednesday and saw six papers.
Papers
- D3038R0: Concrete suggestions for initial Profiles
- P2947R0: Contracts must avoid disclosing sensitive information (Forwarded to SG21 - contracts group)
- P2973R0: Erroneous behaviour for missing return from assignment
- P2981R0: Improving our safety with a physical quantities and units library
- P2955R1: Safer Range Access
The following papers did not get the group’s support:
- P2951R3: Shadowing is good for safety
C++ Release Schedule
NOTE: This is a plan not a promise. Treat it as speculative and tentative.
See P1000, P0592, P2000 for the latest plan.
| Meeting | Location | Objective |
|---|---|---|
| 2023 Fall Meeting | Kona 🇺🇸 | Design major C++26 features. |
| 2024 Winter Meeting | Japan 🇯🇵 | Design major C++26 features. |
| 2024 Summer Meeting | St. Louie 🇺🇸 | Design major C++26 features. |
| 2024 Fall Meeting | Wroclaw 🇵🇱 | C++26 major language feature freeze. |
| 2025 Winter Meeting | 🗺️ | C++26 feature freeze. C++26 design is feature-complete. |
| 2025 Summer Meeting | 🗺️ | Complete C++26 CD wording. Start C++26 CD balloting ("beta testing"). |
| 2025 Fall Meeting | 🗺️ | C++26 CD ballot comment resolution ("bug fixes"). |
| 2026 Winter Meeting | 🗺️ | C++26 CD ballot comment resolution ("bug fixes"), C++26 completed. |
| 2026 Summer Meeting | 🗺️ | First meeting of C++29. |
Status of Major C++ Feature Development
NOTE: This is a plan not a promise. Treat it as speculative and tentative.
IS = International Standard. The C++ programming language. C++11, C++14, C++17, C++20, C+23, etc.
TS = Technical Specification. "Feature branches" are available on some but not all implementations. Coroutines TS v1, Modules TS v1, etc.
CD = Committee Draft. A draft of an IS/TS is sent out to national standards bodies for review and feedback ("beta testing").
Updates since the last Reddit trip report are in bold.
| Feature | Status | Depends On | Current Target (Conservative Estimate) | Current Target (Optimistic Estimate) |
|---|---|---|---|---|
| Senders | Processed by LWG | C++26 | C++26 | |
| Networking | Require rebase on Senders | Senders | C++29 | C++26 |
| Linear Algebra | Plenary approved | C++26 | C++26 | |
| SIMD | Forwarded to LWG | C++26 | C++26 | |
| Contracts | Processed on Study Group SG21 | C++29 | C++26 | |
| Reflection | Forwarded to EWG (new paper!) | C++29 | C++26 | |
| Pattern Matching | No developments | C++29 | C++29 |
Last Meeting's Reddit Trip Report.
If you have any questions, ask them in this thread!
Report issues by replying to the top-level stickied comment for issue reporting.
/u/InbalL, Library Evolution (LEWG) Chair, Israeli National Body Chair
/u/jfbastien, Evolution (EWG) Chair
/u/ErichKeane, Evolution (EWG) Vice Chair, Evolution Incubator (SG17) Chair
/u/c0r3ntin, Library Evolution (LEWG) Vice Chair
/u/FabioFracassi, Library Evolution (LEWG) Vice Chair
/u/je4d, Networking (SG4) Chair
/u/V_i_r, Numerics (SG6) Chair
/u/hanickadot, Compile-Time programming (SG7) Chair, Evolution (EWG) Vice Chair, Czech National Body Chair
/u/tahonermann, Unicode (SG16) Chair
/u/mtaf07, Contracts (SG21) Chair
/u/timur_audio, Contracts (SG21) Vice Chair
... and others ...