Featured Song: Octavarium

I mentioned in my last Featured Song post that I’ve been dabbling in progressive metal. That dabbling led to my (re-)discovery of Dream Theater, one of the genre’s defining bands.

I say re-discovery, because I had listened to some Dream Theatre over a decade ago, including to some of their most popular songs at the time, “Pull Me Under” and “Metropolis” (both from their 1992 album Images and Words). I somewhat liked them, but also found them somewhat tedious / boring, and ultimately wasn’t motivated to check out more of their work.

More recently, however, a comment on the “Elysium” video I featured last time prompted me to check out some of their more recent albums, such as Octavarium (2005) and Systematic Chaos (2007), and, thanks to the evolution of both their style and my tastes over time, my impression was quite different: this stuff is great!

The title track of Octavarium, in particular, captivated me immediately, and that is what I’m featuring today.

At 24 minutes (26 in the live performance I’m linking to), this is the longest song I’ve featured to date (I promise, they will get shorter going forward!), and yet I do not find this song tedious at all – each part of it is different and interesting in its own right, and contributes to a very satisfying whole.

One particularly notable passage from this song is the extended fingerboard intro. While I’m not generally a huge fan of synthetic sounds, this passage in this piece is really well placed, and sets the atmosphere for the rest of the song perfectly.

My favourite part of the song, though, is “Intervals” (beginning at around 16:42 in the linked video) – the slow build-up of tension that leads to the screamed “TRAPPED INSIDE THIS OCTAVARIUM” lines – and the climactic sequence / dénouement that follows and takes you to the end of the song.

(What is an “octavarium”, you ask? The only prior use of the term that I could find was in the name of a liturgical book, but the root word is “octave“, and if you listen to the lyrics, the notion of cycles and the end being the beginning (a property which musical octaves have) comes up repeatedly – so I interpret “trapped inside this octavarium” as meaning “trapped in a cycle you can’t break out of”.)

My one complaint about progressive metal is that some of the extended keyboard / guitar solos (such as, in this song, the ones in the 2-3 minutes leading up to the “Intervals” section), while being technically challenging and intricate, lack some of the “interestingness” (for lack of a better word) of similar solos in power metal. For example, DragonForce‘s guitar solos, while being every bit as fast and technically intricate as Dream Theater’s, also have a sense of “movement” that the latter seem to lack. I think this is what I disliked about older Dream Theater songs like “Pull Me Under”, and I like “Octavarium” so much because it covers a lot of other stylistic ground.

Without further ado, I invite to you enjoy this live performance of “Octavarium”:



As one might expect from a song of this length, there is a lot of speculation / discussion of exactly what meaning it intends to convey. If you’re interested in that, or just want to see the lyrics, check out its SongMeanings page.

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Control Flow Visualizer (CFViz): an rr / gdb plugin

rr (short for “record and replay”) is a very powerful debugging tool for C++ programs, or programs written in other compiled languages like Rust1. It’s essentially a reverse debugger, which allows you to record the execution of a program, and then replay it in the debugger, moving forwards or backwards in the replay.

I’ve been using rr for Firefox development at Mozilla, and have found it to be enormously useful.

One task that comes up very often while debugging is figuring out why a function produced a particular value. In rr, this is often done by going back to the beginning of the function, and then stepping through it line by line.

This can be tedious, particularly for long functions. To help automate this task, I wrote – in collaboration with my friend Derek Berger, who is learning Rust – a small rr plugin called Control Flow Visualizer, or CFViz for short.

To illustrate CFViz, consider this example function foo() and a call site for it:

example code

With the CFViz plugin loaded into rr, if you invoke the command cfviz while broken anywhere in the call to foo() during a replay, you get the following output:

example output

Basically, the plugin illustrates what path control flow took through the function, by coloring each line of code based on whether and how often it was executed. This way, you can tell at a glance things like:

  • which of several return statements produced the function’s return value
  • which conditional branches were taken during the execution
  • which loops inside the function are hot (were executed many times)

saving you the trouble of having to step through the function to determine this information.

CFViz’s implementation strategy is simple: it uses gdb’s Python API to step through the function of interest and see which lines were executed in what order. In then passes that information to a small Rust program which handles the formatting and colorization of the output.

While designed with rr in mind, CFViz also works with vanilla gdb, with the limitation that it will only visualize the rest of the function’s execution from the point where it was invoked (since, without rr, it cannot go backwards to the function’s starting point).

I’ve found CFViz to be quite useful for debugging Firefox’s C++ code. Hope you find it useful too!

CFViz is open source. Bug reports, patches, and other contributions are welcome!

Footnotes

1. rr also has a few important limitations: it only runs on Intel CPUs, and only on Linux (although there is a similar tool called Time-Travel Debugging for Windows)

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A response to “Net Neutrality. No big deal.”

I recently watched this video titled “Net Neturality. No big deal.” by Bryan Lunduke.

I watched this video because, while I am in favour of Net Neutrality, and concerned about the impending repeal of Net Neutrality regulations in the United States, I consciously try to avoid being stuck in an echo chamber of similar views, and try to expose myself to opposing viewpoints as well; particularly when such opposing viewpoints are held by a figure in a community that I respect and identify with (in Bryan’s case, the Linux and free software community).

I found the video interesting and well-presented, but I didn’t find Bryan’s arguments convincing. I decided to write this post to respond to two of the arguments Bryan makes in particular.

The first argument I wanted to address was about the fact that some of the largest companies that are lobbying to keep Net Neutrality rules in place – Google, Netflix, and Microsoft – are also supporters of DRM. Bryan argues, that since these companies support DRM, which is a threat to a free and open internet, we should not take their support for Net Neutrality (which they claim to also be motivated by a desire for a free and open internet) at face value; rather, they only support Net Neutrality regulations because they have a financial incentive for doing so (namely, they run high-bandwidth streaming and similar services that are likely to be first in line to be throttled in a world without Net Neutrality protections).

I don’t dispute that companies like Google, Netflix, and Microsoft support Net Neutrality for selfish reasons. Yes, the regulations affect their bottom line, at least in the short term. But that doesn’t mean there aren’t also good reasons for supporting Net Neutrality. Many organizations – like the Electronic Frontier Foundation, whom I have no reason to suspect to be beholden to the pocketbooks of large tech companies – have argued that Net Neutrality is, in fact, important for a free and open internet. That Netflix supports it for a different reason, doesn’t make that any less the case.

I also think that comparing DRM and the lack of Net Neutrality in this way confuses the issue. Yes, both are threats to a free and open internet, but I think they are qualitatively very different.

To explain why, let’s model an instance of communication over the internet as being between two parties: a sender or producer of the communication, and its receiver or consumer. DRM exists to give the producer control over how the communication is consumed. There are many problems with DRM, but at least it is not intended to interfere with communication in cases where the producer and consumer agree on the terms (e.g. the price, or lack thereof) of the exchange1.

By contrast, in a world without Net Neutrality rules, an intermediary (such as an ISP) can interfere with (such as by throttling) communication between two parties even when the two parties agree on the terms of the communication. This potentially opens the door to all manner of censorship, such as interfering with the communications of political activists. I see this as being a much greater threat to free communication than DRM.

(I also find it curious that Bryan seems to focus particularly on the standardization of DRM on the Web as being objectionable, rather than DRM itself. Given that DRM exists regardless of whether or not it’s standardized on the Web, the fact that it is standardized on the Web is a good thing, because it enables the proprietary software that implements the DRM to be confined to a low-privilege sandbox in the user’s browser, rather than having “full run of the system” as pre-standardization implementations of DRM like Adobe Flash did. See this article for more on that topic.)

The second argument Bryan makes that I wanted to address was that Net Neutrality rules mean the U.S. government being more involved in internet communications, such as by monitoring communications to enforce the rules.

I don’t buy this argument for two reasons. First, having Net Neutrality rules in place does not mean that internet communications need to be proactively monitored to enforce the rules. The role of the government could very well be limited to investigating and corrections violations identified and reported by users (or organizations acting on behalf of users).

But even we assume there will be active monitoring of internet communications to enforce the rules, I don’t see that as concerning. Let’s not kid ourselves: the U.S. government already monitors all internet communications it can get its hands on; axing Net Neutrality rules won’t cause them to stop. Moreover, users already have a way to protect the content of their communications (and, if desired, even the metadata, using tools like Tor) from being monitored: encryption. Net Neutrality rules don’t change that in any way.

In sum, I enjoyed watching Bryan’s video and I always appreciate opposing viewpoints, but I didn’t find the arguments that Net Neutrality is not a big deal convincing. For the time being, I continue to believe that the impending rollback of U.S. Net Neutrality rules is a big deal.

Footnotes

1. I am thinking here of cases where the content being communicated is original content, that is, content originated by the producer. I am, of course, aware, that DRM can and does interfere with the ability of two parties to communicate content owned by a third party, such as sending a movie to a friend. To be pedantic, DRM can even interfere with communication of original content in cases where such content is mistakenly identified as belonging to a third party. I’m not saying DRM is a good thing – I’m just saying it doesn’t rise to being the same level of threat to free communication as not having Net Neutrality protections does.

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Trip Report: C++ Standards Meeting in Albuquerque, November 2017

Summary / TL;DR

Project What’s in it? Status
C++17 See below Publication imminent
Library Fundamentals TS v2 source code information capture and various utilities Published!
Concepts TS Constrained templates Merged into C++20 with some modifications
Parallelism TS v2 Task blocks, library vector types and algorithms and more Nearing feature-completion; expect PDTS ballot at next meeting
Transactional Memory TS Transaction support Published! Not headed towards C++20
Concurrency TS v1 future.then(), latches and barriers, atomic smart pointers Published! Parts of it headed for C++20
Concurrency TS v2 See below Under active development
Networking TS Sockets library based on Boost.ASIO Publication imminent
Ranges TS Range-based algorithms and views Publication imminent
Coroutines TS Resumable functions, based on Microsoft’s await design Publication imminent
Modules TS A component system to supersede the textual header file inclusion model Resolution of comments on Proposed Draft in progress
Numerics TS Various numerical facilities Under active development; no new progress
Graphics TS 2D drawing API Under active design review; no new progress
Reflection Code introspection and (later) reification mechanisms Introspection proposal awaiting wording review. Targeting a Reflection TS.
Contracts Preconditions, postconditions, and assertions Proposal under wording review

Some of the links in this blog post may not resolve until the committee’s post-meeting mailing is published (expected within a few days of November 27, 2017). If you encounter such a link, please check back in a few days.

Introduction

A couple of weeks ago I attended a meeting of the ISO C++ Standards Committee (also known as WG21) in Albuquerque, New Mexico. This was the third committee meeting in 2017; you can find my reports on previous meetings here (February 2017, Kona) and here (July 2017, Toronto). These reports, particularly the Toronto one, provide useful context for this post.

With the final C++17 International Standard (IS) having been voted for publication, this meeting was focused on C++20, and the various Technical Specifications (TS) we have in flight, most notably Modules.

What’s the status of C++17?

The final C++17 International Standard (IS) has been sent off for publication in September. The final document is based on the Draft International Standard (DIS), with only minor editorial changes (nothing normative) to address comments on the DIS ballot; it is now in ISO’s hands, and official publication is imminent.

In terms of implementation status, the latest versions of GCC and Clang both have complete support for C++17, modulo bugs. MSVC is said to be on track to be C++17 feature-complete by March 2018; if that ends up being the case, C++17 will be quickest standard version to date to be supported by these three major compilers.

C++20

This is the second meeting that the C++20 Working Draft has been open for changes. (To use a development analogy, think of the current Working Draft as “trunk”; it was opened for changes as soon as C++17 “branched” earlier this year). Here, I list the changes that have been voted into the Working Draft at this meeting. For a list of changes voted in at the previous meeting, see my Toronto report.

Technical Specifications

In addition to the C++ International Standard, the committee publishes Technical Specifications (TS) which can be thought of “feature branches” (to continue the development analogy from above), where provisional specifications for new language or library features are published and the C++ community is invited to try them out and provide feedback before final standardization.

At the last meeting, we published three TSes: Coroutines, Ranges, and Networking. The next steps for these features is to wait for a while (usually at least a year) to give users and implementers a chance to try them out and provide feedback. Once we’re confident the features are ripe for final standardization, they will be merged into a future version of the International Standard (possibly C++20).

Modules TS

The Modules TS made significant progress at the last meeting: its Proposed Draft (PDTS) was published and circulated for balloting, a process where national standards bodies evaluate, vote on, and submit comments on a proposed document. The ballot passed, but numerous technical comments were submitted that the committee intends to address before final publication.

A lot of time at this meeting was spent working through those comments. Significant progress was made, but not enough to vote out the final published TS at the end of the meeting. The Core Working Group (CWG) intends to hold a teleconference in the coming months to continue reviewing comment resolutions. If they get through them all, a publication vote may happen shortly thereafter (also by teleconference); otherwise, the work will be finished, and the publication vote held, at the next meeting in Jacksonville.

I summarize some of the technical discussion about Modules that took place at this meeting below.

The state of Modules implementation is also progressing: in addition to Clang and MSVC, Facebook has been contributing to a GCC implementation.

Parallelism TS v2

The Parallelism TS v2 is feature-complete, with one final feature, a template library for parallel for loops voted in at this meeting. A vote to send it out for its PDTS ballot is expected at the next meeting.

Concurrency TS v2

The Concurrency TS v2 (no working draft yet) continues to be under active development. Three new features targeting it have received design approval at this meeting: std::cell, a facility for deferred reclamation; apply() for synchronized_value; and atomic_ref. An initial working draft that consolidates the various features slated for the TS into a single document is expected at the next meeting.

Executors, slated for a separate TS, are making progress: the Concurrency Study Group approved the design of the unified executors proposal, thereby breaking the lockdown that has been holding the feature up for a number of years.

Stackful coroutines continue to be a unique beast of their own. I’ve previously reported them to be slated for the Concurrency TS v2; I’m not sure whether that’s still the case. They change the semantics of code in ways that impacts the core language, and thus need to be reviewed by the Evolution Working Group; one potential concern is that the proposal may not be implementable on all platforms (iOS came up as a concrete example during informal discussion). For the time being, the proposal is still being looked at by the Concurrency Working Group, where there continues to be strong interest in standardizing them in some form, but the details remain to be nailed down; I believe the latest development is that an older API proposal may end up being preferred over the latest call/cc one.

Future Technical Specifications

There are some planned future Technical Specifications that don’t have an official project or working draft yet:

Reflection

The static introspection / “reflexpr” proposal (see its summary, design, and specification for details), headed for a Reflection TS, has been approved by the Evolution and Library Evolution Working Groups, and is awaiting wording review. The Reflection Study Group (recently renamed to “Compile-Time Programming Study Group”) approved an extension to it, concerning reflection over functions, at this meeting.

There are more reflection features to come beyond what will be in the static introspection TS. One proposal that has been drawing a lot of attention is metaclasses, an updated version of which was reviewed at this meeting (details below).

Graphics

I’m not aware of much new progress on the planned Graphics TS (containing 2D graphics primitives inspired by cairo) since the last meeting. The latest draft spec can be found here, and is still on the Library Evolution Working Group’s plate.

Numerics

Nothing particularly new to report here either; the Numerics Study Group did not meet this week. The high-level plan for the TS remains as outlined previously. There are concrete proposals for several of the listed topics, but not working draft for the TS yet.

Other major features

Concepts

As I related in my previous report, Concepts was merged into C++20, minus abbreviated function templates (AFTs) and related features which remain controversial.

I also mentioned that there will likely be future proposals to get back AFTs in some modified form, that address the main objection to them (that knowing whether a function is a template or not requires knowing whether the identifiers in its signature name types or concepts). Two such proposals were submitted in advance of this paper; interestingly, both of them proposed a very similar design: an adjective syntax where in an AFT, a concept name would act as an adjective tacked onto the thing it’s constraining – most commonly, for a type concept, typename or auto. So instead of void sort(Sortable& s);, you’d have void sort(Sortable& auto s);, and that makes it clear that a template is being defined.

These proposals were not discussed at this meeting, because some of the authors of the original Concepts design could not make it to the meeting. I expect a lively discussion in Jacksonville.

Now that Concepts are in the language, the question of whether new library proposals should make use of them naturally arose. The Library Evolution Working Group’s initial guidance is “not yet”. The reason is that most libraries require some foundational concepts to build their more specific concepts on top of, and we don’t want different library proposals to duplicate each other / reinvent the wheel in that respect. Rather, we should start by adding a well-designed set of foundational concepts, and libraries can then start building on top of those. The Ranges TS is considered a leading candidate for providing that initial set of foundational concepts.

Operator Dot

I last talked about overloading operator dot a year ago, when I mentioned that there are two proposals for this: the original one, and an alternative approach that achieves a similar effect via inheritance-like semantics.

There hasn’t been much activity on those proposals since then. I think that’s for two reasons. First, the relevant people have been occupied with Concepts. Second, as the reflection proposals develop, people are increasingly starting to see them as a more general mechanism to satisfy operator dot’s use cases. The downside, of course, is that reflection will take longer to arrive in C++, while one of the above two proposals could plausibly have been in C++20.

Evolution Working Group

I’ll now write in a bit more detail about the technical discussions that took place in the Evolution Working Group, the subgroup that I sat in for the duration of the week.

All proposals discussed in EWG at this meeting were targeting C++20 (except for Modules, where we discussed some changes targeting the Modules TS). I’ve categorized them into the usual “accepted”, “further work encouraged”, and “rejected” categories:

Accepted proposals:

  • Standardizing feature test macros (and another paper effectively asking for the same thing). Feature test macros are macros like __cpp_lambdas that tell you whether your compiler or standard library supports a particular feature without having to resort to the more indirect approach of having a version check for each of your supported compilers. The committee maintains a list of them, but they’re not an official part of the standard, and this has led some implementations to refuse to support them, thus significantly undermining their usefulness. To rectify this, it was proposed that they are made part of the official standard. This was first proposed at the last meeting, but failed to gain consensus at that time. It appears that people have since been convinced (possibly by the arguments laid out in the linked papers), as this time around EWG approved the proposal.
  • Bit-casting object representations. This is a library proposal, but EWG was asked for guidance regarding making this function constexpr, which requires compiler support. EWG decided that it could be made constexpr for all types except a few categories – unions, pointers, pointers-to-members, and references – for which that would have been tricky to implement.
    • As a humorous side-note about this proposal, since it could only apply to “plain old data” types (more precisely, trivially copyable types; as mentioned above, “plain old data” was deprecated as a term of art), one of the potential names the authors proposed for the library function was pod_cast. Sadly, this was voted down in favour of bit_cast.
  • Language support for empty objects. This addresses some of the limitations of the empty base optimization (such as not being able to employ it with types that are final or otherwise cannot be derived from) by allowing data members to opt out of the rule that requires them to occupy at least 1 byte using an attribute, [[no_unique_address]]. The resulting technique is called the “empty member optimization”.
  • Efficient sized delete for variable-sized classes. I gave some background on this in my previous post. The authors returned with sign-off from all relevant implementers, and a clearer syntax (the “destroying delete” operator is now identified by a tag type, as in operator delete(Type*, std::destroying_delete_t), and the proposal was approved.
  • Attributes for likely and unlikely statements. This proposal has been updated as per previous EWG feedback to allow placing the attribute on all statements. It was approved with one modification: placing the attribute on a declaration statement was forbidden, because other attributes on declaration statements consistently apply to the entity being declared, not the statement itself.
  • Deprecate implicit capture of *this. Only the implicit capture of *this via [=] was deprecated; EWG felt that disallowing implicit capture via [&] would break too much idiomatic code.
  • Allow pack expansions in lambda init-capture. There was no compelling reason to disallow this, and the workaround of constructing a tuple to store the arguments and then unpacking it is inefficient.
  • String literals as template parameters. This fixes a longstanding limitation in C++ where there was previously no way to do compile-time processing of strings in such a way that the value of the string could affect the type of the result (as an example, think of a compile-time regex parsing library where the resulting type defines an efficient matcher (DFA) for the regex). The syntax is very simple: template <auto& String>; the auto then gets deduced as const char[N] (or const char16_t[N] etc. depending on the type of the string literal passed as argument) where N is the length of the string. (You can also write template <const char (&String)[N]> if you know N, but you can’t write template <size_t N, const char (&String)[N]> and have both N and String deduced from a single string literal template argument, because EWG did not want to create a precedent for a single template argument matching two template parameters. That’s not a big deal, though: using the auto form, you can easily recover N via traits, and even constrain the length or the character type using a requires-clause.)
  • A tweak to the Contracts proposal. An issue came up during CWG review of the proposal regarding inline functions with assertion checks inside them: what should happen if the function is called from two translation units, one of which is compiled with assertion checks enabled and one of them not? EWG’s answer was that, as with NDEBUG today, this is technically an ODR (one definition rule) violation. The behaviour in practice is fairly well understood: the linker will pick one version or the other, and that version will be used by both translation units. (There are some potential issues with this: what if, while compiling a caller in one of the translation units, the optimizer assumed that the assertion was checked, but the linker picks the version where the assertion isn’t checked? That can result in miscompilation. The topic remains under discussion.)

There were also a few that, after being accepted by EWG, were reviewed by CWG and merged into the C++20 working draft the same week, and thus I already mentioned them in the C++20 section above:

  • Fixing small-ish functionality gaps in concepts. This consisted of three parts, two of which were accepted:
    • requires-clauses in lambdas. This was accepted.
    • requires-clauses in template template parameters. Also accepted.
    • auto as a parameter type in regular (non-lambda) functions. This was mildly controversial due to the similarity to AFTs, whose design is still under discussion, so it was deferred to be dealt with together with AFTs.
  • Access specifiers and specializations.
  • Deprecating “plain old data” (POD).
  • Default constructible and assignable stateless lambdas.

  • Proposals for which further work is encouraged:

    • Standard containers and constexpr. This is the latest version of an ongoing effort by compiler implementers and others to get dynamic memory allocation working in a constexpr context. The current proposal allows most forms of dynamic allocation and related constructs during constant evaluation: non-trivial destructors, new and delete expressions, placement new, and use of std::allocator; this allows reusing a lot of regular code, including code that uses std::vector, in a constexpr context. Direct use of operator new is not allowed, because that returns void*, and constant evaluation needs to track the type of dynamically allocated objects. There is also a provision to allow memory that is dynamically allocated during constant evaluation to survive to runtime, at which point it’s treated as static storage. EWG liked the direction (and particularly the fact that compiler writers were on the same page regarding its implementability) and encouraged development of a more concrete proposal along these lines.
    • Supporting offsetof for stable-layout classes. “Stable-layout” is a new proposed category of types, broader than “standard-layout”, for which offsetof could be implemented. EWG observed that the definition of “standard-layout” itself could be broadened a bit to include most of the desired use cases, and expressed a preference for doing that instead of introducing a new category. There was also talk of potentially supporting offsetof for all types, which may be proposed separately as a follow-up.
    • short float. This proposal for a 16-bit floating-point type was approved by EWG earlier this year, but came back for some reason. There was some re-hashing of previous discussions about whether the standard should mandate the size (16 bits) and IEEE behaviour.
    • Adding alias declarations to concepts. This paper proposed three potential enhancements to concept declarations to make writing concepts easier. EWG was not particularly convinced about the need for this, but believed at least the first proposal could be entertained given stronger motivation.
    • [[uninitialized]] attribute. This attribute is intended to suppress compiler warnings about variables that are declared but not initialized in cases where this is done intentionally, thus facilitating the use of such warnings in a codebase to catch unintentional cases. EWG pointed out that most compiler these days warn not about uninitialized declarations, but uninitialized uses. There was also a desire to address the broader use case of allocating dynamic memory that is purposely uninitialized (e.g. std::vector<char> buffer(N) currently zero-initializes the allocated memory).
    • Relaxed incomplete multidimensional array type declaration. This is a companion proposal to the std::mdspan library proposal, which is a multi-dimensional array view. It would allow writing things like std::mdspan<double[][][]> to denote a three-dimensional array where the size in each dimension is determined at runtime. Note that you still would not be able to create an object of type double[][][]; you could only use it in contexts that do not require creating an object, like a template argument. Basically, mdspan is trying to (ab)use array types as a mini-DSL to describe its dimensions, similar to how std::function uses function types as a mini-DSL to describe its signature. This proposal was presented before, when mdspan was earlier in its design stage, and EWG did not find it sufficiently motivating. Now that the mdspan is going forward, the authors tried again. EWG was open to entertaining the idea, but only if technical issues such as the interaction with template argument deduction are ironed out.
    • Class types in non-type template parameters. This has been proposed before, but EWG was stuck on the question of how to determine equivalence (something you need to be able to do for template arguments) for values of class types. Now, operator<=> has given us a way to move forward on this question, basically by requiring that class types used in non-type template parameters have a defaulted operator<=>. It was observed that there is some overlap with the proposal to allow string literals as template parameters (since one way to pass a character array as a template parameter would be to wrap it in a struct), but it seemed like they also each have their own use cases and there may be room for both in the language.
    • Dynamic library loading. The C++ standard does not talk about dynamic libraries, but some people would find it useful to have a standardized library interface for dealing with them anyways. EWG was asked for input on whether it would be acceptable to standardize a library interface without saying too much about its semantics (since specifying the semantics would require that the C++ standard start talking about dynamic libraries, and specifying their behaviour in relation to exceptions, thread-local storage, the One Definition Rule, and so on). EWG was open to this direction, but suggested that the library interface be made much more general, as in its current incarnation it seemed to be geared towards certain platforms and unimplementable on others.
    • Various proposed extensions to the Modules TS, which I talk about below.

    There was also a proposal for recursive lambdas that wasn’t discussed because its author realized it needed some more work first.

    Rejected proposals:

    • A proposed trait has_padding_bits, the need for which came up during review of an atomics-related proposal by the Concurrency Study Group. EWG expressed a preference for an alternative approach that removed the need for the trait by putting the burden on compiler implementers to make things work correctly.
    • Attributes for structured bindings. This was proposed previously and rejected on the basis of insufficient motivation. The author came back with additional motivation: thread-safety attributes such as [[guarded_by]] or [[locks_held]]. However, it was pointed out that the individual bindings are just aliases to fields of an (unnamed) object, so it doesn’t make sense to apply attributes to them; attributes can be applied to the deconstructed object as a whole, or to one of its fields at the point of the field’s declaration.
    • Keeping the alias syntax extendable. This proposed reverting the part of the down with typename! proposal, approved at the last meeting, that allowed omitting the typename in using alias = typename T::type; where T was a dependent type. The rationale was that even though today only a type is allowed in that position (thus making the typename disambiguator redundant), this prevents us from reusing the same syntax for expression aliases in the future. EWG already considered this, and didn’t find it compelling: the preference was to make the “land grab” for a syntax that is widely used today, instead of keeping it in reserve for a hypothetical future feature.
    • Forward without forward.The idea here is to abbreviate the std::forward<decltype(x)>(x) boilerplate that often occurs in generic code, to >>x (i.e. a unary >> operator applied to x). EWG sympathized with the desire to eliminate this boilerplate, but felt that >>, or indeed any other unary operator, would be too confusing of a syntax, especially when occuring after an = in a lambda init-capture (e.g. [foo=>>foo](...){ ... }). EWG was willing to entertain a keyword instead, but the best people could come up with was fwdexpr and that didn’t have consensus; as a result, the future of this proposal is uncertain.
    • Relaxing the rules about invoking an explicit constructor with a braced-init-list. This would have allowed , among a few other changes, writing return {...}; instead of return T{...}; in a function whose declared return type is T, even if the invoked constructor was explicit. This has been proposed before, but rejected on the basis that it makes it easy to introduce bugs (see e.g. this response). The author proposed addressing those concerns by introducing some new rules to limit the cases in which this was allowed, but EWG did not find the motivation sufficiently compelling to further complicate C++’s already complex initialization rules.
    • Another attempt at standardizing arrays of runtime bound (ARBs, a pared-down version of C’s variable-length arrays), and a C++ wrapper class for them, stack_array. ARBs and a wrapper class called dynarray were previously headed for standardization in the form of an Array Extensions TS, before the project was scrapped because dynarray was found to be unimplementable. This proposal would solve the implementability concerns by restricting the usage of stack_array (e.g. it couldn’t be used as a class member). EWG was concerned that the restrictions would result in a type that’s not very usable. (It was pointed out that a design to make such a type more composable was proposed previously, but the author didn’t have time to pursue it further.) Ultimately, EWG didn’t feel that this proposal had a better chance of succeeding than the last time standardization of ARBs was attempted. However, a future direction that might be more promising was outlined: introducing a core language “allocation expression” that allocates a unnamed (and runtime-sized) stack array and returns a non-owning wrapper, such as a std::span, to access it.
    • A modern C++ signature for main(). This would have introduced a new signature for main() (alongside the existing allowed signatures) that exposed the command-line arguments using an iterable modern C++ type rather than raw pointers (the specific proposal was int main(std::initializer_list<std::string_view>). EWG was not convinced that such a thing would be easier to use and learn than int main(int argc, char*[] argv);. It was suggested that instead, a trivial library facility that took argc and argv as inputs and exposed an iterable interface could be provided; alternatively (or in addition), a way to access command-line arguments from anywhere in the program (similar to Rust’s std::env::args()) could be explored.
    • Abbreviated lambdas for fun and profit. This proposal would introduce a new abbreviated syntax for single-expression lambdas; a previous version of it was presented and largely rejected in Kona. Not much has changed to sway EWG’s opinion since then; if anything, additional technical issues were discovered.

      For example, one of the features of the abbreviated syntax is “automatic SFINAE”. That is, [x] => expr would mean [x] -> decltype(expr) { return expr; }; the appearance of expr in the return type rather than just the body would mean that a substitution failure in expr wouldn’t be a hard error, it would just remove the function overload being considered from the overload set (see the paper for an example). However, it was pointed out that in e.g. [x] -> decltype(x) { return x; }, the x in the decltype and the x in the body refer to two different entities: the first refers to the variable in the enclosing scope that is captured, and the second to the captured copy. If we try to make [x] => x “expand to” that, then we get into a situation where the x in the abbreviated form refers to two different entities for two different purposes, which would be rather confusing. Alternatively, we could say in the abbreviated form, x refers to the captured copy for both purposes, but then we are applying SFINAE in new scenarios, and some implementers are strongly opposed to that.

      It was also pointed out that the abbreviated form’s proposed return semantics were “return by reference”, while regular lambdas are “return by value” by default. EWG felt it would be confusing to have two different defaults like this.
    • Making the lambda capture syntax more liberal in what it accepts. C++ currently requires that in a lambda capture list, the capture-default, if present, come before any explicit captures. This proposal would have allowed them to be written in any order; in addition, it would have allowed repeating variables that are covered by the capture-default as explicit captures for emphasis. EWG didn’t find the motivation for either of these changes compelling.
    • Lifting overload sets into objects. This is a resurrection of an earlier proposal to allow passing around overload sets as objects. It addressed previous concerns with that proposal by making the syntax more explicit: you’d pass []f rather than just f, where f was the name of the overloaded function. There were also provisions for passing around operators, and functions that performed member access. EWG’s feedback was that this proposal seems to be confused between two possible sets of desired semantics:
      1. a way to build super-terse lambdas, which essentially amounts to packaging up a name; the overload set itself isn’t formed at the time you create the lambda, only later when you instantiate it
      2. a way to package and pass around overload sets themselves, which would be formed at the time you package them

      EWG didn’t have much of an appetite for #1 (possibly because it had just rejected another terse-lambda proposal), and argued that #2 could be achieved using reflection.

    Discussion papers

    There were also a few papers submitted to EWG that weren’t proposals per se, just discussion papers.

    These included a paper arguing that Concepts does not significantly improve upon C++17, and a response paper arguing that it in fact does. The main issue was whether Concepts delivers on its promise of making template error messages better; EWG’s consensus was that they do when compared to unconstrainted templates, but perhaps not as much as one would hope when compared to C++17 techniques for constraining templates, like enable_if. There may be room for implementations (to date there is just the one in GCC) to do a better job here. (Of course, Concepts are also preferable over enable_if in other ways, such as being much easier to read.)

    There was also a paper describing the experiences of the author teaching Concepts online. One of the takeaways here is that students don’t tend to find the variety of concept declaration syntaxes confusing; they tend to mix them freely, and they tend to like the abbreviated function template (AFT) syntax.

    Modules

    I mentioned above that a significant focus of the meeting was to address the national body comments on the Modules PDTS, and hopefully get to a publication vote on the final Modules TS.

    EWG looked at Modules on two occasions: first to deal with PDTS comments that had language design implications, and second to look at new proposals concerning Modules. The latter were all categorized as “post-TS”: they would not target the Modules TS, but rather “Modules v2”, the next iteration of Modules (for which the ship vehicle has not yet been decided).

    Modules TS

    The first task, dealing with PDTS comments in EWG, was a short affair. Any comment that proposed a non-trivial design change, or even remotely had the potential to delay the publication of the Modules TS, was summarily rejected (with the intention that the concern could be addressed in Modules v2 instead). It was clear that the committee leadership was intent on shipping the Modules TS by the end of the meeting, and would not let it get derailed for any reason.

    “That’s a good thing, right?” you ask. After all, the sooner we ship the Modules TS, the sooner people can start trying it out and providing feedback, and thus the sooner we can get a refined proposal into the official standard, right? I think the reality is a bit more nuanced than that. As always, it’s a tradeoff: if we ship too soon, we can risk shipping a TS that’s not sufficiently polished for people to reasonably implement and use it; then we don’t get much feedback and we effectively waste a TS cycle. In this case, I personally feel like EWG could have erred a bit more on the side of shipping a slightly more polished TS, even if that meant delaying the publication by a meeting (it ended up being delayed by at least a couple of months anyways). That said, I can also sympathize with the viewpoint that Modules has been in the making for a very long time and we need to ship something already.

    Anyways, for this reason, most PDTS comments that were routed to EWG were rejected. (Again, I should emphasize that this means “rejected for the TS“, not “rejected forever”.) The only non-rejection response that EWG gave was to comment US 041, where EWG confirmed that the intent was that argument-dependent lookup could find some non-exported entities in some situations.

    Of course, there were other PDTS comments that weren’t routed to EWG because they weren’t design issues; these were routed to CWG, and CWG spent much of the week looking at them. At one point towards the end of the week, CWG did consult EWG about a design issue that came up. The question concerned whether a translation unit that imports a module sees a class type declared in that module as complete or incomplete in various situations. Some of the possibilities that have to be considered here are whether the module exports the class’s forward declaration, its definition, or both; whether the module interface unit contains a definition of the class (exported or not) at all; and whether the class appears in the signature of an exported entity (such as a function) without itself being exported.

    There are various use cases that need to be considered when deciding the behaviour here. For example, a module may want to export functions that return or take as parameters pointers or references to a type that’s “opaque” to the module’s consumer, i.e. the module’s consumer can’t create an instance of such a class or access its fields; that’s a use case for exporting a type as incomplete. At the same time, the module author may want to avoid splitting her module into separate interface and implementation units at all, and thus wants to define the type in the interface unit while still exporting it as incomplete.

    The issue that CWG got held up on was that the rules as currently specified seemed to imply that in a consumer translation unit, an imported type could be complete and incomplete at the same time, depending on how it was named (e.g. directly vs. via decltype(f()) where it was the return type of a function f). Some implementers indicated that this would be a significant challenge to implement, as it would require a more sophisticated implementation model for types (where completeness was a property of “views of types” rather than of types themselves) that no existing language feature currently requires.

    Several alternatives were proposed which avoided these implementation challenges. While EWG was favourable to some of them, there was also opposition to making what some saw as a design change to the Modules TS at this late stage, so it was decided that the TS would go ahead with the current design, possibly annotated as “we know there’s a potential problem here”, and it would be fixed up in v2.

    I find the implications of this choice a bit unfortunate. It sounded like the implementers that described this model as being a significant challenge to implement, are not planning to implement it (after all, it’s going to be fixed in v2; why redesign your compiler’s type system if ultimately you won’t need it). Other implementers may or may not implement this model. Either way, we’ll either have implementation divergence, or all implementations will agree on a de facto model that’s different from what the spec says. This is one of those cases where I feel like waiting to polish the spec a bit more, so that it’s not shipped in a known-to-be-broken state, may have been advised.

    I mentioned in my previous report that I thought the various Modules implementers didn’t talk to each other enough about their respective implementation strategies. I still feel like that’s very much the case. I feel like discussing each other’s implementation approaches in more depth would have unearthed this issue, and allowed it to be dealt with, sooner.

    Modules v2

    Now moving on to the proposals targeting Modules v2 that EWG reviewed:

    • Two of them (module interface imports and namespace pervasiveness and modules) it turned out were already addressed in the Modules TS by changes made in response to PDTS comments.
    • Placement of module declarations. Currently, if a module unit contains declarations in the global module, the module declaration (which effectively “starts” the module) needs to go after those global declarations. However, this makes it more difficult for both humans and tools to find the module declaration. This paper proposes a syntax that allows having the module declaration be the first declaration in the file, while still having a way to place declarations in the global module. It was observed that this proposal would make it easier to make module a context-sensitive keyword, which has also been requested. EWG encouraged continued exploration in this direction.
    • Module partitions. This iterates on the previous module partitions proposal (found in this paper), with a new syntax: module basename : partition; (unlike in the previous version, partition here is not a keyword, it’s the partition’s name). EWG liked this approach as well. Module partitions also make proclaimed-ownership-declarations unnecessary, so those can be axed.
    • Making module names strings. Currently, module names are identifier sequences separated by dots (e.g. foo.bar.baz), with the dots not necessarily implying a hierarchical relationship; they are mapped onto files in an implementation-defined manner. Making them strings instead would allow mapping onto the filesystem more explicitly. There was no consensus for this change in EWG.
    • Making module a context-sensitive keyword. As always, making a common word like module a hard keyword breaks someone. In this case, it shows up as an identifier in many mature APIs like Vulkan, CUDA, Direct X 9, and others, and in some of these cases (like Vulkan) the name is enshrined into a published specification. In some cases, the problem can be solved by making the keyword context-sensitive, and that’s the case for module (especially if the proposal about the placement of module declarations is accepted). EWG agreed to make the keyword context-sensitive. The authors of this paper asked if this could be done for the TS rather than for Modules v2; that request was rejected, but implementers indicated that they would implement it as context-sensitive ASAP, thus avoiding problems in practice.
    • Modules TS does not support intended use case. The bulk of the concerns here were addressed in the Modules TS while addressing PDTS comments, except for a proposed extension to allow using-declarations with an unqualified name. EWG indicated it was open to such an extension for v2.
    • Two papers about support for exporting macros, which remains one of the most controversial questions about Modules. The first was a “rumination” paper, which was mostly arguing that we need a published TS and deployment experience before we can settle the question; the second argued that having deployed modules (clang’s pre-TS implementation) in a large codebase (Apple’s), it’s clear that macro support is necessary. A number of options for preserving hygiene, such as only exporting and importing individual macros, were discussed. EWG expressed a lukewarm preference to continuing to explore macro support, particularly with such fine-grained control for hygiene.

    Other Working Groups

    The Library Evolution Working Group, as usual, reviewed a decent amount of proposed new library features. While I can’t give a complete listing of the proposals discussed and their outcomes (having been in EWG all week), I’ll mention a few highlights of accepted proposals:

    Targeting C++20:

    std::span (formerly called array_view) is also targeting C++20, but has not quite gotten final approval from LEWG yet.

    Targeting the Library Fundamentals TS v3:

    • mdspan, a multi-dimensional array view. (How can a multi-dimensional array view be approved sooner than a single-dimensional one, you ask? It’s because mdspan is targeting a TS, but span is targeting the standard directly, so span needs to meet a higher bar for approval.)
    • std::expected<T>, a “value or error” variant type very similar to Rust’s Result

    Targeting the Ranges TS:

    • Range adaptors (“views”) and utilities. Range views are ranges that lazily consume elements from an underlying range, while performing an additional operation like transforming the elements or filtering them. This finally gives C++ a standard facility that’s comparable to C#’s LINQ (sans the SQL syntax), Java 8’s streams, or Rust’s iterators. C++11 versions of the facilities proposed here are available today in the range-v3 library (which was in turn inspired by Boost.Range).

    There was an evening session to discuss the future of text handling in C++. There was general agreement that it’s desirable to have a text handling library that has notions of code units, code points, and grapheme clusters; for many everyday text processing algorithms (like toupper), operating at the level of grapheme clusters makes the most sense. Regarding error handling, different people have different needs (safety vs. performance), and a policy-based approach to control error handling may be advisable. Some of the challenges include standard library implementations having to ship a database of Unicode character classifications, or hook into the OS’s database. The notion of whether we should have a separate character type to represent UTF-8 encoded text, or just use char for that purpose, remains contentious.

    SG 7 (Compile-Time Programming)

    SG 7, the Compile-Time Programming (previously Reflection) Study Group met for an evening session.

    An updated version of a proposed extension to the static reflection proposal to allow reflecting over functions was briefly reviewed and sent onwards for review in EWG and LEWG at future meetings.

    The rest of the evening was spent discussing an updated version of the metaclasses proposal. To recap, a metaclass defines a compile-time transformation on a class, and can be applied to a class to produce a transformed class (possibly among other things, like helper classes / functions). The discussion focused on one particular dimension of the design space here: how the transformation should be defined. Three options were given:

    1. The metaclass operates on a mutable input class, and makes changes to it to produce the transformed class. This is how it worked in the original proposal.
    2. Like #1, but the metaclass operates on an immutable input class, and builds the transformed class from the ground up as its output.
    3. Like #2, but the metaclass code operates on the “meta level”, where the representation of the input and output types is an ordinary object of type meta::type. This dispenses with most of the special syntax of the first two approaches, making the metaclass look a lot like a normal constexpr function.

    SG 7 liked the third approach the best, noting that it not only dispenses with the need for the $ syntax (which couldn’t have been the final syntax anyways, it would have needed to be something uglier), but makes the proposal more general (opening up more avenues for how and where you can invoke/apply the metaclass), and more in line with the preference the group previously expressed to have reflection facilities operate on a homogeneous value representation of the program’s entities.

    Discussion of other dimensions of the design space, such as what the invocation syntax for metaclasses should look like (i.e. how you apply them to a class) was deferred to future meetings.

    SG 12 (Undefined Behaviour and Vulnerabilities)

    SG 12, the Undefined Behaviour Study Group, recently had its scope expanded to also cover documenting vulnerabilities in the C++ language, and ways to avoid them.

    This latter task is a joint effort between SG 12 and WG 23, a sister committee of the C++ Standards Committee that deals with vulnerabilities in programming languages in general. WG 23 produces a language-agnostic document that catalogues vulnerabilities without being specific to a language, and then language-specific annexes for a number of programming languages. For the last couple of meetings, WG 23 has been collaborating with our SG 12 to produce a C++ annex; the two groups met for that purpose for two days during this meeting. The C++ annex is at a pretty early stage, but over time it has the potential to grow to be a comprehensive document outlining many interesting types of vulnerabilities that C++ programmers can run into, and how to avoid them.

    SG 12 also had a meeting of its own, where it looked at a proposal to make certain low-level code patterns that are widely used but technically have undefined behaviour, have defined behaviour instead. This proposal was reviewed favourably.

    C++ Stability and Velocity

    On Friday evening, there was a session to discuss the stability and velocity of C++.

    One of the focuses of the session was reviewing the committee’s policy on deprecating and removing old features that are known to be broken or that have been superseded by better alternatives. Several language features (e.g. dynamic exception specifications) and library facilities (e.g. std::auto_ptr) have been deprecated and removed in this way.

    One of the library facilities that were removed in C++17 was the deprecated “binders” (std::bind1st and std::bind2nd). These have been superseded by the C++11 std::bind, but, unlike say auto_ptr, they aren’t problematic or dangerous in any way. It was argued that the committee should not deprecate features like that, because it causes unnecessary code churn and maintenance cost for codebases whose lifetime and update cycle is very long (on the order of decades); embedded software such as an elevator control system was brought up as a specific example.

    While some sympathized with this viewpoint, the general consensus was that, to be able to evolve at the speed it needs to to satisfy the needs of the majority of its users, C++ does need to be able to shed old “cruft” like this over time. Implementations often do a good job of maintaining conformance modes with older standard versions (and even “escape hatches” to allow old features that have been removed to be used together with new features that have since been added), thus allowing users to continue using removed features for quite a while in practice. (Putting bind1st and bind2nd specifically back into C++20 was polled, but didn’t have consensus.)

    The other focus of the session was the more general tension between the two pressures of stability and velocity that C++ faces as it evolves. It was argued that there is a sense in which the two are at odds with each other, and the committee needs to take a clearer stance on which is the more important goal. Two examples of cases where backwards compatibility constraints have been a drag on the language that were brought up were the keywords used for coroutines (co_await, co_yield, etc. – wouldn’t it have been nice to just be able to claim await and yield instead?), and the const-correctness issue with std::function which still remains to be fixed. A poll on which of stability or velocity is more important for the future direction of C++ revealed a wide array of positions, with somewhat of a preference for velocity.

    Conclusion

    This was a productive meeting, whose highlights included the Modules TS making good progress towards its publication; C++20 continuing to take shape as the draft standard gained the consistent comparisons feature among many other smaller ones; and range views/adaptors being standardized for the Ranges TS.

    The next meeting of the Committee will be in Jacksonville, Florida, the week of March 12th, 2018. It, too, should be an exciting meeting, as design discussion of Concepts resumes (with the future of AFTs possibly being settled), and the Modules TS is hopefully finalized (if that doesn’t already happen between meetings). Stay tuned for my report!

    Other Trip Reports

    Others have written reports about this meeting as well. Some that I’ve come across include Herb Sutter’s and Bryce Lelbach’s. I encourage you to check them out!

    Posted in c++, programming | Tagged , , | 13 Comments

    Featured Song: Elysium

    Recently, I’ve been flirting with progressive metal – or at least, symphonic / power metal with significant progressive influences. The more recent albums of Stratovarius (who I’ve featured several times before) – particularly the ones after guitarist Timo Tolkki’s 2008 departure from the band – fall into that category.

    Today’s selection is the title track of the 2011 album Elysium. At 18 minutes, I believe it’s the longest song I’ve featured to date – but then such is progressive rock/metal 🙂

    I listened to this album not long after its release, but it’s only after re-listening to it recently that I feel like I’ve come to truly appreciate it for the masterpiece that it is.

    The song has a bit of everything, from multiple intricate guitar and keyboard solos that typify progressive metal, through calm and contemplative sections that help build tension, to Stratovarius’ signature dramatic orchestral passages.

    In all honesty, I do find it a bit long – I think the first 11 minutes could have been condensed to ~5 without the song losing much value. Past the 11-minute mark though, the song really picks up stylistically, delivering a very emotional series of passages culminating in a climactic ending. So, if you give it a listen and start to get bored, I recommend skipping forward rather than giving up, lest you miss the best part!



    Lyrics can be found in the video description.

    Enjoy!

    Posted in featured song, music | Tagged | 1 Comment

    Trip Report: C++ Standards Meeting in Toronto, July 2017

    Summary / TL;DR

    Project What’s in it? Status
    C++17 See below Draft International Standard published; on track for final publication by end of 2017
    Filesystems TS Standard filesystem interface Part of C++17
    Library Fundamentals TS v1 optional, any, string_view and more Part of C++17
    Library Fundamentals TS v2 source code information capture and various utilities Published!
    Concepts TS Constrained templates Merged into C++20 with some modifications
    Parallelism TS v1 Parallel versions of STL algorithms Part of C++17
    Parallelism TS v2 Task blocks, library vector types and algorithms and more Under active development
    Transactional Memory TS Transaction support Published! Uncertain whether this is headed towards C++20
    Concurrency TS v1 future.then(), latches and barriers, atomic smart pointers Published! Parts of it headed for C++20
    Concurrency TS v2 See below Under active development
    Networking TS Sockets library based on Boost.ASIO Voted for publication!
    Ranges TS Range-based algorithms and views Voted for publication!
    Coroutines TS Resumable functions, based on Microsoft’s await design Voted for publication!
    Modules TS A component system to supersede the textual header file inclusion model Proposed Draft voted out for balloting by national standards bodies
    Numerics TS Various numerical facilities Under active development
    Graphics TS 2D drawing API Under active design review
    Reflection Code introspection and (later) reification mechanisms Introspection proposal passed core language and library design review; next stop is wording review. Targeting a Reflection TS.
    Contracts Preconditions, postconditions, and assertions Proposal passed core language and library design review; next stop is wording review.

    Some of the links in this blog post may not resolve until the committee’s post-meeting mailing is published. If you encounter such a link, please check back in a few days.

    Introduction

    A couple of weeks ago I attended a meeting of the ISO C++ Standards Committee (also known as WG21) in Toronto, Canada (which, incidentally, is where I’m based). This was the second committee meeting in 2017; you can find my reports on previous meetings here (November 2016, Issaquah) and here (February 2017, Kona). These reports, particularly the Kona one, provide useful context for this post.

    With the C++17 Draft International Standard (DIS) being published (and its balloting by national standards bodies currently in progress), this meeting was focused on C++20, and the various Technical Specifications (TS) we have in flight.

    What’s the status of C++17?

    From a technical point of view, C++17 is effectively done.

    Procedurally, the DIS ballot is still in progress, and will close in August. Assuming it’s successful (which is widely expected), we will be in a position to vote to publish the final standard, whose content would be the same as the DIS with possible editorial changes, at the next meeting in November. (In the unlikely event that the DIS ballot is unsuccessful, we would instead publish a revised document labelled “FDIS” (Final Draft International Standard) at the November meeting, which would need to go through one final round of balloting prior to publication. In this case the final publication would likely happen in calendar year 2018, but I think the term “C++17” is sufficiently entrenched by now that it would remain the colloquial name for the standard nonetheless.)

    C++20

    With C++17 at the DIS stage, C++20 now has a working draft and is “open for business”; to use a development analogy, C++17 has “branched”, and the standard’s “trunk” is open for new development. Indeed, several changes have been voted into the C++20 working draft at this meeting.

    Technical Specifications

    This meeting was a particularly productive one for in-progress Technical Specifications. In addition to Concepts (which had already been published previously) being merged into C++20, three TSes – Coroutines, Ranges, and Networking – passed a publication vote this week, and a fourth, Modules, was sent out for its PDTS ballot (a ballot process that allows national standards bodies to vote and comment on the proposed TS, allowing the committee to incorporate their feedback prior to sending out a revised document for publication).

    Coroutines TS

    The Coroutines TS – which contains a stackless coroutine design, sometimes called co_await after one of the keywords it uses – had just been sent out for its PDTS ballot at the previous meeting. The results were in before this meeting began – the ballot had passed, with some comments. The committee made it a priority to get through all the comments at this meeting and draft any resulting revisions, so that the revised TS could be voted for final publication, which happened (successfully) during the closing plenary session.

    Meanwhile, an independent proposal for stackful coroutines with a library-only interface is making its way through the Library groups. Attempts to unify the two varieties of coroutines into a single design seem to have been abandoned for now; the respective proposal authors maintain that the two kinds of coroutines are useful for different purposes, and could reasonably co-exist (no pun intended) in the language.

    Ranges TS

    The Ranges TS was sent out for its PDTS ballot two meetings ago, but due to the focus on C++17, the committee didn’t get through all of the resulting comments at the last meeting. That work was finished at this meeting, and this revised TS also successfully passed a publication vote.

    Networking TS

    Like the Ranges TS, the Networking TS was also sent out for its PDTS ballot two meetings ago, and resolving the ballot comments was completed at this meeting, leading to another successful publication vote.

    Modules TS

    Modules had come close to being sent out for its PDTS ballot at the previous meeting, but didn’t quite make it due to some procedural mis-communication (detailed in my previous report if you’re interested).

    Modules is kind of in an interesting state. There are two relatively mature implementations (in MSVC and Clang), whose development either preceded or was concurrent with the development of the specification. Given this state of affairs, I’ve seen the following dynamic play out several times over the past few meetings:

    • a prospective user, or someone working on a new implementation (such as the one in GCC), comes to the committee seeking clarification about what happens in a particular scenario (like this one)
    • the two existing implementers consult their respective implementations, and give different answers
    • the implementers trace the difference in outcome back to a difference in the conceptual model of Modules that they have in their mind
    • the difference in the conceptual model, once identified, is discussed and reconciled by the committee, typically in the Evolution Working Group (EWG)
    • the implementers work with the Core Working Group (CWG) to ensure the specification wording reflects the new shared understanding

    Of course, this is a desirable outcome – identifying and reconciling differences like this, and arriving at a specification that’s precise enough that someone can write a new implementation based purely on the spec, is precisely what we want out of a standards process. However, I can’t help but wonder if there isn’t a more efficient way to identify these differences – for example, by the two implementers actually studying each other’s implementations (I realize that’s complicated by the fact that one is proprietary…), or at least discussing their respective implementation strategies in depth.

    That said, Modules did make good progress at this meeting. EWG looked at several proposal changes to the spec (I summarize the technical discussion below), CWG worked diligently to polish the spec wording further, and in the end, we achieved consensus to – finally – send out Modules for its PDTS ballot!

    Parallelism TS v2

    The Parallelism TS v2 (working draft here) picked up a new feature, vector and wavefront policies. Other proposals targeting it, like vector types and algorithms, are continuing to work their way through the library groups.

    Concurrency TS v2

    SG 1, the Study Group that deals with concurrency and parallelism, reviewed several proposals targeting the Concurrency TS v2 (which still does not yet have a working draft) at this meeting, including a variant of joining_thread with cooperative cancellation, lock-free programming techniques for reclamation, and stackful coroutines (which I’ve mentioned above in connection with the Coroutines TS).

    Executors are still likely slated for a separate TS. The unified proposal presented at the last meeting has been simplified as requested, to narrow its scope to something manageable for initial standardization.

    Merging Technical Specifications Into C++20

    We have some technical specifications that are already published and haven’t been merged into C++17, and are thus candidates for merging into C++20. I already mentioned that Concepts was merged with some modifications (details below).

    Parts of the Concurrency TS are slated to be merged into C++20: latches, with barriers to hopefully follow after some design issues are ironed out, and an improved version of atomic shared pointers. future.then() is going to require some more iteration before final standardization.

    The Transactional Memory TS currently has only one implementation; the Study Group that worked on it hopes for some more implementation and usage experience prior to standardization.

    The Library Fundamentals TS v2 seems to be in good shape to be merged into C++20, though I’m not sure of the exact status / concrete plans.

    In addition to the TSes that are already published, many people are eager to see the TSes that were just published (Coroutines, Ranges, and Networking), as well as Modules, make it into C++20 too. I think it’s too early to try and predict whether they will make it. From a procedural point of view, there is enough time for all of these to complete their publication process and be merged in the C++20 timeframe. However, it will really depend on how much implementation and use experience these features get between now and the C++20 feature-complete date (sometime in 2019), and what the feedback from that experience is.

    Future Technical Specifications

    Finally, I’ll give some mention to some planned future Technical Specifications that don’t have an official project or working draft yet:

    Reflection

    A proposal for static introspection (sometimes called “reflexpr” after the keyword it uses; see its summary, design, and specification for details) continues to head towards a Reflection TS. It has been approved by SG 7 (the Reflection and Metaprogramming Study Group) and the Evolution Working Group at previous meetings. This week, it was successfully reviewed by the Library Evolution Working Group, allowing it to move on to the Core and Library groups going forward.

    Meanwhile, SG 7 is continuing to look at more forward-looking reflection and metaprogramming topics, such as a longer-term vision for metaprogramming, and a proposal for metaclasses (I talk more about these below).

    Graphics

    The Graphics TS, which proposes to standardize a set of 2D graphics primitives inspired by cairo, continues to be under active review by the Library Evolution Working Group; the latest draft spec can be found here. The proposal is close to being forwarded to the Library Working Group, but isn’t quite there yet.

    While I wasn’t present for its discussion in LEWG, I’m told that one of the changes that have been requested is to give the library a stateless interface. This matches the feedback I’ve heard from Mozilla engineers knowledgeable about graphics (and which I’ve tried to relay, albeit unsuccessfully, at a previous meeting).

    Evolution Working Group

    I’ll now write in a bit more detail about the technical discussions that took place in the Evolution Working Group, the subgroup that I sat in for the duration of the week.

    All proposals discussed in EWG at this meeting were targeting C++20 (except for Modules, where we discussed changes targeting the Modules TS). I’ve categorized them into the usual “accepted”, “further work encouraged”, and “rejected” categories:

    Accepted proposals:

    • Default member initializers for bitfields. Previously, bit-fields couldn’t have default member initializers; now they can, with the “natural” syntax, int x : 5 = 42; (brace initialization is also allowed). A disambiguation rule was added to deal with parsing ambiguities (since e.g. an = could conceivably be part of the bitfield width expression).
    • Tweaking the rules for constructor template argument deduction. At the last meeting, EWG decided that for wrapper types like tuple, copying should be preferable to wrapping; that is, something like tuple t{tuple{1, 2}}; should deduce the type of t as tuple<int, int> rather than tuple<tuple<int, int>>. However, it had been unclear whether this guidance applied to types like vector that had std::initializer_list constructors. EWG clarified that copying should indeed be preferred to wrapping for those types, too. (The paper also proposed several other tweaks to the rules, which did not gain consensus to be approved just yet; the author will come back with a revised paper for those.)
    • Resolving a language defect related to defaulted copy constructors. This was actually a proposal that I co-authored, and it was prompted by me running into this language defect in Mozilla code (it prevented the storing of an nsAutoPtr inside a std::tuple). It’s also, to date, my first proposal to be approved by EWG!
    • A simpler solution to the problem that allowing the template keyword in unqualified-ids aimed to solve. While reviewing that proposal, the Core Working Group found that the relevant lookup rules could be tweaked so as to avoid having to use the template keyword at all. The proposed rules technically change the meaning of certain existing code patterns, but only ones that are very obscure and unlikely to occur in the wild. EWG was, naturally, delighted with this simplification.
    • An attribute to mark unreachable code. This proposal aims to standardize existing practice where a point in the code that the author expects cannot be reached is marked with __builtin_unreachable() or __assume(false). The initial proposal was to make the standardized version an [[unreachable]] attribute, but based on EWG’s feedback, this was revised to be a std::unreachable() library function instead. The semantics is that if such a call is reached during execution, the behaviour is undefined. (EWG discussed at length whether this facility should be tied to the Contracts proposal. The outcome was that it should not be; since “undefined behaviour” encompasses everything, we can later change the specified behaviour to be something like “call the contract violation handler” without that being a breaking change.) The proposal was sent to LEWG, which will design the library interface more precisely, and consider the possibility of passing in a compile-time string argument for diagnostic purposes.
    • Down with typename! This paper argued that in some contexts where typename is currently required to disambiguate a name nested in a dependent scope as being a type, the compiler can actually disambiguate based on the context, and proposed removing the requirement of writing typename in such contexts. The proposal passed with flying colours. (It was, however, pointed out that the proposal prevents certain hypothetical future extensions. For example, one of the contexts in question is y in using x = y;: that can currently only be a type. However, suppose we later want to add expression aliases to C++; this proposal rules out re-using the using x = y; syntax for them.)
    • Removing throw(). Dynamic exception specifications have been deprecated since C++11 (superseded by noexcept), and removed altogether in C++17, with the exception of throw() as an alias for noexcept(true). This paper proposed removing that last vestige, too, and EWG approved it. (The paper also proposed removing some other things that were deprecated in C++17, which were rejected; I mention those in the list of rejected proposals below.)
    • Ranged-based for statement with initializer. This introduces a new form of range-for: for (T var = init; U elem : <range-expression>); here, var is a variable that lives for the duration of the loop, and can be referenced by <range-expression> (whereas elem is the usual loop variable that takes on a new value on every iteration). This is useful for both scope hygiene (it avoids polluting the enclosing scope with var) and resolving a category of lifetime issues with range-for. EWG expressed concerns about parseability (parsers will now need to perform more lookahead to determine which form of loop they are parsing) and readability (the “semicolon colon” punctuation in a loop header of this form can look deceptively like the “semicolon semicolon” punctuation in a traditional for loop), but passed the proposal anyways.
    • Some changes to the Modules TS (other proposed changes were deferred to Modules v2) – I talk about these below
    • Changes to Concepts – see below

    Proposals for which further work is encouraged:

    • Non-throwing container operations. This paper acknowledges the reality that many C++ projects are unable to or choose not to use exceptions, and proposes that standard library container types which currently rely on exceptions to report memory allocation failure, provide an alternative API that doesn’t use exceptions. Several concrete alternatives are mentioned. EWG sent this proposal to the Library Evolution Working Group to design a concrete alternative API, with the understanding that the resulting proposal will come back to EWG for review as well.
    • Efficient sized deletion for variable-sized classes. This proposal builds on the sized deletion feature added to C++14 to enable this optimization for “variable-sized classes” (that is, classes that take control of their own allocation and allocate a variable amount of extra space before or after the object itself to store extra information). EWG found the use cases motivating, but encouraged the development of a syntax that less prone to accidental misuse, as well as additional consultation with implementers to ensure that ABI is not broken.
    • Return type deduction and SFINAE. This paper proposes a special syntax for single-expression lambdas, which would also come with the semantic change that the return expression be subject to SFINAE (this is a desirable property that often leads authors to repeat the return expression, wrapped in decltype, in an explicit return type declaration (and then to devise macros to avoid the repetition)). EWG liked the goal but had parsing-related concerns about the syntax; the author was encouraged to continue exploring the syntax space to find something that’s both parseable and readable. Continued exploration of terser lambdas, whether as part of the same proposal or a separate proposal, was also encouraged. It’s worth noting that there was another proposal in the mailing (which wasn’t discussed since the author wasn’t present) that had significant overlap with this proposal; EWG observed that it might make sense to collaborate on a revised proposal in this space.
    • Default-constructible stateless lambdas. Lambdas are currently not default-constructible, but for stateless lambdas (that is, lambdas that do not capture any variables) there is no justification for this restriction, so this paper proposed removing it. EWG agreed, but suggested that they should also be made assignable. (An example of a situation where one currently runs into these restrictions is transform iterators: such iterators often aggregate the transformation function, and algorithms often default-construct or assign iterators.)
    • Product type access. Recall that structured bindings work with both tuple-like types, and structures with public members. The former expose get<>() functions to access the tuple elements by index; for the latter, structured bindings achieve such index-based access by “language magic”. This paper proposed exposing such language magic via a new syntax or library interface, so that things other than structured bindings (for example, code that wants to iterate over the public members of a structure) can take advantage of it. EWG agreed this was desirable, expressed a preference for a library interface, and sent the proposal onward to LEWG to design said interface. (Compilers will likely end up exposing intrinsics to allow library implementers to implement such an interface. I personally don’t see the advantage of doing things this way over just introducing standard language syntax, but I’m happy to get the functionality one way or the other.)
    • Changing the attack vector of constexpr_vector. At the previous meeting, implementers reported that supporting full-blown dynamic memory allocation in a constexpr context was not feasible to implement efficiently, and suggested a more limited facility, such as a special constexpr_vector container. This proposal argues that such a container would be too limiting, and suggests supporting a constexpr allocator (which can then be used with regular containers) instead. Discussion at this meeting suggested that (a) on the one hand, a constexpr allocator is no less general (and thus no easier to support) than new itself; but (b) on the other hand, more recent implementer experiments suggest that supporting new itself, with some limitations, might be feasible after all. Continued exploration of this topic was warmly encouraged.
    • Implicit evaluation of auto variables. This is a resurrection of an earlier proposal to allow a class to opt into having a conversion function of some sort called when an instance of it is assigned to an auto-typed variable. The canonical use case is an intermediate type is an expression template system, for which it’s generally desirable to trigger evaluation when initializing an auto-typed variable. EWG wasn’t fond of widening the gap between the deduction rules for auto and the deduction rules for template parameters (which is what auto is modelled on), and suggested approaching the problem form a different angle; one idea that was thrown around was the possibility of extending the notion of deduction guides (currently used for class template argument deduction) to apply to situations like this.
    • Allowing class template specializations in unrelated namespaces. The motivation here is to avoid having to reopen the namespace in which a class template was defined, to provide a specialization of that template. EWG liked the idea, but suggested that it might be prudent to still restrict such specializations to occur within associated namespaces of the specialization (e.g. the namespaces of the specialization’s template arguments) – kind of like how Rust doesn’t allow you to implement a trait unless you’re either the author of the trait, or the author of the type you’re implementing the trait for.
    • Precise semantics for contract assertions. This paper explores the design space of contract assertions, enumerating the various (sometimes contradictory) objectives we may want to achieve by using them, and proposes a set of primitive operations that facilitate implementing assertions in ways that meet useful subsets of these objectives. EWG expressed an interest in standardizing some of the proposed primitives, specifically a mechanism to deliberately introduce unspecified (nondeterministic) behaviour into a program, and a “prevent continuation handler” that an assertion check can invoke if an assertion fails and execution should not continue as a result. (A third primitive, for deliberately invoking undefined behaviour, is already handled by the independently proposed std::unreachable() function that EWG approved at this meeting.)

    Rejected proposals:

    • Attributes for structured bindings. This proposal would have allowed applying attributes to individual bindings, such as auto [a, b [[maybe_unused]], c] = f();. EWG found this change insufficiently motivated; some people also thought it was inappropriate to give individual bindings attributes when we can’t even give them types.
    • Making pointers-to-members callable. This would have allowed p(s) to be valid and equivalent to s.*p when p is a pointer to a member of a type S, and s is an object of that type. It had been proposed before, and was rejected for largely the same reason: some people argued that it was a half-baked unified call syntax proposal. (I personally thought this was a very sensible proposal – not at all like unified call syntax, which was controversial for changing name lookup rules, which this proposal didn’t do.)
    • Explicit structs. The proposal here was to allow marking a struct as explicit, which meant that all its fields had to be initialized, either by a default member initializer, by a constructor intializer, or explicitly by the caller (not relying on the fallback to default initialization) during aggregate initialization. EWG didn’t find this well enough motivated, observing that either your structure has an invariant, in which case it’s likely to be more involved than “not the default values”, or it doesn’t, in which case the default values should be fine. (Uninitialized values, which can arise for primitive types, are another matter, and can be addressed by different means, such as via the [[uninitialized]] attribute proposal.)
    • Changing the way feature-test macros are standardized. Feature test macros (like __cpp_constexpr, intended to be defined by an implementation if it supports constexpr) are currently standardized in the form of a standing document published by the committee, which is not quite a standard (for example, it does not undergo balloting by national bodies). As they have become rather popular, Microsoft proposed that they be standardized more formally; they claimed that it’s something they’d like to support, but can’t unless it’s a formal standard, because they’re trying to distance themselves from their previous habit of supporting non-standard extensions. (I didn’t quite follow the logic behind this, but I guess large companies sometimes operate in strange ways.) However, there was no consensus to change how feature test macros are standardized; some on the committee dislike them, in part because of their potential for fragmentation, and because they don’t completely remove the need for compiler version checks and such (due to bugs etc.)
    • Removing other language features deprecated in C++17. In addition to throw() (whose removal passed, as mentioned above), two other removals were proposed.
      • Out-of-line declarations of static constexpr data members. By making static constexpr data members implicitly inline, C++17 made it so that the in-line declaration which provides the value is also a definition, making an out-of-line declaration superfluous. Accordingly, the ability to write such an out-of-line declaration at all was deprecated, and was now proposed for removal in C++20.
      • Implicit generation of a copy constructor or copy assignment operator in a class with a user-defined copy assignment operator, copy constructor, or destructor. This has long been known to be a potential footgun (since generally, if you need to user-define one of these functions, you probably need to user-define all three), and C++11 already broke with this pattern by having a user-defined move operation disable implicit generation of the copy operations. The committee has long been eyeing the possibility extending this treatment to user-defined copy operations, and the paper suggested that perhaps C++20 would be the time to do so. However, the reality is that there still is a lot of code out there that relies on this implicit generation, and much of it isn’t actually buggy (though much of it is).

      Neither removal gained consensus. In each case, undeprecating them was also proposed, but that was rejected too, suggesting that the hope that these features can be removed in a future standard remains alive.

    • Capturing *this with initializer. C++17 added the ability to have a lambda capture the entire *this object by value. However, it’s still not possible to capture it by move (which may be reasonable if e.g. constructing the lambda is the last thing you do with the current object). To rectify this, this paper proposed allowing the capture of *this with the init-capture syntax. Unfortunately, this effectively allows rebinding this to refer to a completely unrelated object inside the lambda, which EWG believed would be far too confusing, and there didn’t appear to be a way to restrict the feature to only work for the intended use case of moving the current object.
    • bit_sizeof and bit_offsetof. These are similar to sizeof and offsetof, but count the number of bits, and work on bitfield members. EWG preferred to hold off on these until they are implementable with a library interface on top of reflection.
    • Parametric functions. This oddly-named proposal is really a fresh approach to named arguments. In contrast with the named arguments proposal that I co-authored a few years back, which proposed to allow using named arguments with existing functions and existing parameter names (and garnered significant opposition over concerns that it would make parameter names part of a function’s interface when the function hadn’t been written with that in mind), this paper proposed introducing a new kind of function, which can have named arguments, declared with a new syntax, and for which the parameter names are effectively part of the function’s type. While this approach does address the concerns with my proposal, EWG felt the new syntax and new language machinery it would require was disproportionate to the value of the feature. In spite of the idea’s repeated rejection, no one was under any illusion that this would be the last named arguments proposal to come in front of EWG.

    There were a handful of proposals that were not discussed due to their authors not being present. They included the other terse lambdas proposal and its offshoot idea of making forwarding less verbose, and a proposal for an [[uninitialized]] attribute.

    Concepts

    A major focus of this meeting was to achieve consensus to get Concepts into C++20. To this end, EWG spent half a day plus an evening session discussing several papers on the topic.

    Two of the proposals – a unified concept definition syntax and semantic constraint matching – were write-ups of design directions that had already been discussed and approved in Kona; their discussion at this meeting was more of a rubber-stamp. (The second paper contained a provision to require that re-declarations of a constrained template use the same syntax (e.g. you can’t have one using requires-clauses and the other using a shorthand form); this provision had some objections, just as it did in Kona, but was passed anyways.)

    EWG next looked at a small proposal to address certain syntax ambiguities; the affected scenarios involve constrained function templates with a requires-clause, where it can be ambiguous where the require-clause after the template parameter list ends, and where the function declaration itself begins. The proposed solution was to restrict the grammar for the expression allowed in a top-level requires-clause so as to remove the ambiguity; expressions that don’t fit in the restricted grammar can still be used if they are parenthesized (as in requires (expr)). This allows common forms of constraints (like trait<T>::value or trait_v<T>) to be used without parentheses, while allowing any expression with parentheses. This was also approved.

    That brings us to the controversial part of the discussion: abbreviated function templates (henceforth, “AFTs”), also called “terse templates”. To recap, AFTs are function templates declared without a template parameter list, where the parameter types use concept names (or auto), which the compiler turns into invented template parameters. A canonical example is void sort(Sortable& s);, which is a shorthand for template <Sortable S> void sort(S& s); (which is itself a shorthand for template <typename S> requires Sortable<S> void sort(S& s);).

    AFTs have been controversial since their introduction, due to their ability to make template code look like non-template code. Many have argued that this is a bad idea, beacuse template code is fundamentally different from non-template code (e.g. consider different name lookup rules, the need for syntactic disambiguators like typename, and the ability to define a function out of line). Others have argued that making generic programming (programming with templates) look more like regular programming is a good thing.

    (A related feature that shared some of the controversy around AFTs was concept introductions, which were yet another shorthand, making Mergeable{In1, In2, Out} void merge(In1, In1, In2, Out); short for template <typename In1, typename In2, typename Out> requires Mergeable<In1, In2, Out> void merge(In1, In1, In2, Out);. Concept introductions don’t make a template look like a non-template the way AFTs do, but were still controversial as many felt they were an odd syntax and offered yet another way of defining constrained function templates with relatively little gain in brevity.)

    The controversy around AFTs and concept introductions was one of the reasons the proposed merger of the Concepts TS into C++17 failed to gain consensus. Eager not to repeat this turn of events for C++20, AFTs and concept introductions were proposed for removal from Concepts, at least for the time being, with the hope that this would allow the merger of Concepts into C++20 to gain consensus. After a long and at times heated discussion, EWG approved this removal, and approved the merger of Concepts, as modified by this removal (and by the other proposals mentioned above), into C++20. As mentioned above, this merger was subsequently passed by the full committee at the end of the week, resulting in Concepts now being in the C++20 working draft!

    It’s important to note that the removal of AFTs was not a rejection of having a terse syntax for defining constrained function templates in general. There is general agreement that such a terse syntax is desirable; people just want such a syntax to come with some kind of syntactic marker that makes it clear that a function template (as opposed to a non-template function) is being declared. I fully expect that proposals for an alternative terse syntax that comes with such a syntactic marker will forthcome (in fact, I’ve already been asked for feedback on one such draft proposal), and may even be approved in the C++20 timeframe; after all, we’re still relatively early in the C++20 cycle.

    There was one snag about the removal of AFTs that happened at this meeting. In the Concepts wording, AFTs are specified using a piece of underlying language machinery called constrained type specifiers. Besides AFTs, this machinery powers some other features of Concepts, such as the ability to write ConceptName var = expr;, or even vector<auto> var = expr;. While these other features weren’t nearly as controversial as AFTs were, from a specification point of view, removing AFTs while keeping these in would have required a significant refactor of the wording that would have been difficult to accomplish by the end of the week. Since the committee wanted to “strike the iron while it’s hot” (meaning, get Concepts into C++20 while there is consensus for doing so), it was decided that for the time being, constrained type specifiers would be removed altogether. As a result, in the current C++20 working draft, things like vector<auto> var = expr; are ill-formed. However, it’s widely expected that this feature will make it back into C++20 Concepts at future meetings.

    Lastly, I’ll note that there were two proposals (one I co-authored, and a second one that didn’t make the pre-meeting mailing) concerning the semantics of constrained type specifiers. The removal of constrained type specifiers made these proposals moot, at least for the time being, so they were not discussed at this meeting. However, as people propose re-adding some of the uses of constrained type specifiers, and/or terse templates in some form, these papers will become relevant again, and I expect they will be discussed at that time.

    Modules

    Another major goal of the meeting was to send out the Modules TS for its PDTS ballot. I gave an overview of the current state of Modules above. Here, I’ll mention the Modules-related proposals that came before EWG this week:

    • Distinguishing the declaration of a module interface unit from the declaration of a module implementation unit. The current syntax is module M; for both. In Kona, a desire was expressed for interface units to have a separate syntax, and accordingly, one was proposed: export module M;. (The re-use of the export keyword here is due to the committee’s reluctance to introduce new keywords, even context-sensitive ones. module interface M; would probably have worked with interface being a keyword in this context only.) This was approved for the Modules TS.
    • Module partitions (first part of the paper only). These are module units that form part of a module interface, rather than being a complete module interface; the proposed syntax in this paper is module M [[partition]];. This proposal failed to gain consensus, not over syntax concerns, but over semantic concerns: unlike the previous module partitions proposal (which was not presented at this meeting, ostensibly for lack of time), this proposal did not provide for a way for partitions to declare dependencies on each other; rather, each partition was allowed to depend on entities declared in all other partitions, but only on their forward-declarations, which many felt was too limiting. (The corresponding implementation model was to do a quick “heuristic parse” of all partitions to gather such forward-declarations, and then do full processing of the partitions in parallel; this itself resulted in some raised eyebrows, as past experience doing “heuristic parsing” of C++ hasn’t been very promising.) Due to the controversy surrounding this topic, and not wanting to hold up the Modules TS, EWG decided to defer module partitions to Modules v2.
    • Exporting using-declarations. This wasn’t so much a proposal, as a request for clarification of the semantics. The affected scenario was discussed, and the requested clarification given; no changes to the Modules TS were deemed necessary.
    • Name clashes between private (non-exported) entities declared in different modules. Such a name clash is ill-formed (an ODR violation) according to the current spec; several people found that surprising, since one of the supposed advantages of Modules is to shield non-exported entities like private helpers from the outside world. This matter was discussed briefly, but a resolution was postponed to after the PDTS ballot (note: that’s not the same as being postponed to Modules v2; changes can be made to the Modules TS between the PDTS ballot and final publication).
    • A paper describing some requirements that a Modules proposal would need to have to be useful in evolving a particular large codebase (Bloomberg’s). Discussion revealed that the current spec meets some but not all of these requirements; the gaps mainly concern the ability to take a legacy (non-modular) code component, and non-intrusively (“additively”) provide a modular “view” of that component. No changes were proposed at this time, but some changes to fill these gaps are likely to appear as comments on the PDTS ballot.
    • Identifying module source code. Currently, the module M; or export module M; declaration that introduces a module unit is not required to be the first declaration in the file. Preceding declarations are interpreted as being part of the global module (and this is often used to e.g. include legacy headers). The author of this proposal would nonetheless like something that’s required to be the first declaration in the file, that announces “this file is a module unit”, and proposed module ; as being such a marker. EWG was favourable to the idea, but postponed discussion of a concrete syntax until after the PDTS.

    With these discussions having taken place, the committee was successful in getting Modules sent out for its PDTS ballot. This is very exciting – it’s a major milestone for Modules! At the same time, I think it’s clear from the nature of the some of the proposals being submitted on the topic (including the feedback from implementation and deployment experience at Google, some of which is yet to be fully discussed by EWG) that this is a feature where there’s still a fair amount of room for implementation convergence and user feedback to gain confidence that the feature as specified will be useful and achieve its intended objectives for a broad spectrum of C++ users. The PDTS ballot formally begins the process of collecting that feedback, which is great! I am very curious about the kinds of comments it will garner.

    If you’re interested in Modules, I encourage you to give the prototype implementations in Clang and MSVC a try, play around with them, and share your thoughts and experiences. (Formal PDTS comments can be submitted via your national standards body, but you can also provide informal feedback on the committee’s public mailing lists.)

    Other Working Groups

    The Library Working Group had a busy week, completing its wording review of the Ranges TS, Networking TS, and library components of the Coroutines TS, and allowing all three of these to be published at the end of the week. They are also in the process of reviewing papers targeting C++20 (including span, which provides an often-requested “array view” facility), Parallelism TS v2, and Library Fundamentals TS v3.

    The Library Evolution Working Group was, as usual, working through its large backlog of proposed new library features. As much as I’d love to follow this group in as much detail as I follow EWG, I can’t be in two places at once, so I can’t give a complete listing of the proposals discussed and their outcomes, but I’ll mention a few highlights:

    SG 7 (Reflection and Metaprogramming)

    SG 7 met for an evening session and discussed three topics.

    The first was an extension to the existing static reflection proposal (which is headed towards publication as the initial Reflection TS) to allow reflection of functions. Most of the discussion concerned a particular detail: whether you should be allowed to reflect over the names of a function’s parameters. It was decided that you should, but that in the case of a function with multiple declarations, the implementation is free to choose any one of them as the source of the reflected parameter names.

    The second topic was what we want metaprogramming to look like in the longer term. There was a paper exploring the design space that identified three major paradigms: type-based metaprogramming (examples: Boost.MPL, the current static reflection proposal), heterogenerous value-based (example: Boost.Hana), and homogeneous value-based (this would be based on constexpr metaprogramming, and would require some language extensions). Another paper then argued that the third one, homogeneous value-based metaprogramming, is the best choice, both from a compile-time performance perspective (the other two involve a lot of template instantiations which are compile-time expensive), and because it makes metaprogramming look more like regular programming, making it more accessible. SG 7 agreed that this is the long-term direction we should aim for. Note that this implies that, while the Reflection TS will likely be published in its current form (with its type-based interface), prior to merger into the C++ IS it would likely be revised to have a homogenerous value-based interface.

    The third topic was a proposal for a more advanced reflection/metaprogramming feature, metaclasses. Metaclasses combine reflection facilities with proposed code injection facilities to allow class definitions to undergo arbitrary user-defined compile-time transformations. A metaclass can be thought of as a “kind” or category of class; a class definition can be annotated (exact syntax TBD) to declare the class as belonging to that metaclass, and such a class definition will undergo the transformations specified in the metaclass definition. Examples of metaclasses might be “interfaces”, where the transformation includes making every method pure virtual, and “value types”, where the transformation includes generating memberwise comparison operators; widely used metaclasses could eventually become part of the standard library. Obviously, this is a very powerful feature, and the proposal is at a very early stage; many aspects, including the proposed code injection primitives (which are likely to be pursued as a separate proposal), need further development. Early feedback was generally positive, with some concerns raised about the feature allowing codebases to grow their own “dialects” of C++.

    The Velocity of C++ Evolution

    C++ standardization has accelerated since the release of C++11, with the adoption of a three-year standardization cycle, the use of Technical Specifications to gather early feedback on major new features, and an increase in overall participation and the breadth of domain areas and user communities represented in the committee.

    All the same, sometimes it feels like the C++ language is still slow to evolve, and a big part of that is the significant constraint of needing to remain backwards-compatible, as much as possible, with older versions of the language. (Minor breakages have occurred, of course, like C++11 user-defined literals changing the meaning of "literal"MACRO. But by and large, the committee has held backwards-compatibility as one of its cornerstone principles.)

    A paper, discussed at an evening session this week, explores the question of whether, in today’s age of modern tools, the committee still needs to observe this constraint as strictly as it has in the past. The paper observes that it’s already the case that upgrading to a newer compiler version typically entails making some changes / fixes to a codebase. Moreover, in cases where a language change does break or change the meaning of code, compilers have gotten pretty good at warning users about it so they can fix their code accordingly (e.g. consider clang’s -Wc++11-compat warnings). The paper argues that, perhaps, the tooling landscape has matured to the point where we should feel free to make larger breaking changes, as long as they’re of the sort that compilers can detect and warn about statically, and rely on (or even require) compilers to warn users about affected code, allowing them to make safe upgrades (tooling could potentially help with the upgrades, too, in the form of automated refactorings). This would involve more work for compiler implementers, and more work for maintainers of code when upgrading compilers, but the reward of allowing the language to shed some of its cumbersome legacy features may be worth it.

    The committee found this idea intriguing. No change in policy was made at this time, but further exploration of the topic was very much encouraged.

    If the committee does end up going down this path, one particularly interesting implication would be about the future of the standard library. The Concepts-enabled algorithms in the Ranges TS are not fully backwards-compatible with the algorithms in the current standard library. As a result, when the topic of how to merge the Ranges TS into the C++ standard came up, the best idea people had was to start an “STLv2”, a new version of the standard library that makes a clean break from the current version, while being standardized alongside it. However, in a world where we are not bound to strict backwards-compatibility, that may not be necessary – we may just be able to merge the Ranges TS into the current standard library, and live with the resulting (relatively small) amount of breakage to existing code.

    Conclusion

    With C++17 effectively done, the committee had a productive meeting working on C++20 and advancing Technical Specifications like Modules and Coroutines.

    The merger of Concepts into C++20 was a definite highlight of this meeting – this feature has been long in the making, having almost made C++11, and its final arrival is worth celebrating. Sending out Modules for its PDTS ballot was a close second, as it allows us to start collecting formal feedback on this very important C++ feature. And there are many other goodies in the pipeline: Ranges, Networking, Coroutines, contracts, reflection, graphics, and many more.

    The next meeting of the Committee will be in Albuquerque, New Mexico, the week of November 6th, 2017. Stay tuned for my report!

    Other Trip Reports

    Others have written reports about this meeting as well. Some that I’ve come across include Herb Sutter’s and Guy Davidson’s. Michael Wong also has a report written just before the meeting, that covers concurrency-related topics in more depth than my reports. I encourage you to check them out!

    Posted in c++, programming | Tagged , , | 16 Comments

    Featured Song: Misery Is a Butterfly

    It’s been a while since I wrote a Featured Song post. Today, I’m going to share a song from a band I’ve discovered in recent months, Blonde Redhead.

    A small band with just three members (none of whom, incidentally, is blonde or a redhead 🙂 ), their music has a unique sound and a contemplative atmosphere that I’ve taken to.

    I particularly enjoy lead singer Kazu Makino‘s thin, clear high voice which gives the band’s sound an almost ethereal twist. She also has really good stage presence (check out some live recordings of the band).

    The song I’m featuring today – my favourite of the Blonde Redhead songs I’ve heard so far – is the title track of their 2004 album Misery Is a Butterfly:



    Featured lyrics:

    Remember when we found misery
    We watched her, watched her spread her wings
    And slowly fly around our room
    And she asked for your gentle mind

    Full lyrics and discussion can be found here.

    I’d also like to share a beautiful instrumental (piano and cello) cover of this song that I’ve come across:



    Hope you’ve enjoyed them!

    Posted in featured song, music | Leave a comment