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ACCU 2019 Slides and Trip Report

Monday, 22 April 2019

I attended ACCU 2019 a couple of weeks ago, where I was presenting my session Here's my number; call me, maybe. Callbacks in a multithreaded world.

The conference proper started on Wednesday, after a day of pre-conference workshops on the Tuesday, and continued until Saturday. I was only there Wednesday to Friday.


I didn't arrive until Wednesday lunchtime, so I missed the first keynote and morning sessions. I did, however get to see Ivan Čukić presenting his session on Ranges for distributed and asynchronous systems. This was an interesting talk that covered similar ground to things I've thought about before. It was good to see Ivan's take, and think about how it differed to mine. It was was also good to see how modern C++ techniques can produce simpler code than I had when I thought about this a few years ago. Ivan's approach is a clean design for pipelined tasks that allows implicit parallelism.

After the break I then went to Gail Ollis's presentation and workshop on Helping Developers to Help Each Other . Gail shared some of her research into how developers feel about various aspects of software development, from the behaviour of others to the code that they write. She then got us to try one of the exercises she talked about in small groups. By picking developer behaviours from the cards she provided to each group, and telling stories about how that behaviour has affected us, either positively or negatively, we can share our experiences, and learn from each other.


First up on Thursday was Herb Sutter's keynote: De-fragmenting C++: Making exceptions more affordable and usable . Herb was eloquent as always, talking about his idea for making exceptions in C++ lower cost, so that they can be used in all projects: a significant number of projects currently ban exceptions from at least some of their code. I think this is a worthwhile aim, and hope to see something like Herb's ideas get accepted for C++ in a future standard.

Next up was my session, Here's my number; call me, maybe. Callbacks in a multithreaded world. It was well attended, with interesting questions from the audience. My slides are available here, and the video is available on youtube. Several people came up to me later in the conference to say that they had enjoyed my talk, and that they thought it would be useful for them in their work, which pleased me no end: this is what I always hope to achieve from my presentations.

Thursday lunchtime was taken up with book signings. I was one of four authors of recently-published programming books set up in the conservatory area of the hotel to sell copies of our books, and sign books for people. I sold plenty, and signed more, which was great.

Kate Gregory's talk on What Do We Mean When We Say Nothing At All? was after lunch. She discussed the various places in C++ where we can choose to specify something (such as const, virtual, or explicit), but we don't have to. Can we interpret meaning from the lack of an annotation? If your codebase uses override everywhere, except in one place, is that an accidental omission, or is it a flag to say "this isn't actually an override of the base class function"? Is it a good or bad idea to omit the names of unused parameters? There was a lot to think about with this talk, but the key takeaway for me is Consistency is Key: if you are consistent in your use of optional annotations, then deviation from your usual pattern can convey meaning to the reader, whereas if you are inconsistent then the reader cannot infer anything.

The final session I attended on Thursday was the C++ Pub Quiz, which was hosted by Felix Petriconi. The presented code was intended to confuse, and elicit exclamations of "WTF!", and succeeded on both counts. However, it was fun as ever, helped by the free drinks, and the fact that my team "Ungarian Notation" were the eventual winners.


Friday was the last day of the conference for me (though there the conference had another full day on Saturday). It started with Paul Grenyer's keynote on the trials and tribulations of trying to form a "community" for developers in Norwich, with meet-ups and conferences. Paul managed to be entertaining, but having followed Paul's blog for a few years, there wasn't anything that was new to me.

Interactive C++ : Meet Jupyter / Cling - The data scientist's geeky younger sibling was the next session I attended, presented by Neil Horlock. This was an interesting session about cling, a C++ interpreter, complete with a REPL, and how this can be combined with Jupyter notebooks to create a wiki with embedded code that you can edit and run. Support for various libraries allows to write code to plot graphs and maps and things, and have the graphs appear right there in the web page immediately. This is an incredibly powerful tool, and I had discussions with people afterwards about how this could be used both as an educational tool, and for "live" documentation and customer-facing tests: "here is sample code, try it out right now" is an incredibly powerful thing to be able to say.

After lunch I went to see Andreas Weis talk about Taming Dynamic Memory - An Introduction to Custom Allocators. This was a good introduction to various simple allocators, along with how and why you might use them in your C++ code. With John Lakos in the front row, Andreas had to field many questions. I had hoped for more depth, but I thought the material was well-paced, and so there wouldn't have been time; that would have been quite a different presentation, and less of an "introduction".

The final session I attended was Elsewhere Memory by Niall Douglas. Niall talked about the C++ object model, and how that can cause difficulties for code that wants to serialize the binary representation of objects to disk, or over the network, or wants to directly share memory with another process. Niall is working on a standardization proposal which would allow creating objects "fully formed" from a binary representation, without running a constructor, and would allow terminating the lifetime of an object without running its destructor. This is a difficult area as it interacts with compilers' alias analysis and the normal deterministic lifetime rules. However, this is an area where people currently do have "working" code that violates the strict lifetime rules of the standard, so it would be good to have a way of making such code standards-conforming.

Between the Sessions

The sessions at a conference at ACCU are great, and I always enjoy attending them, and often learn things. However, you can often watch these on Youtube later. One of the best parts of physically attending a conference is the discussions had in person before and after the sessions. It is always great to chat to people in person who you primarily converse with via email, and it is exciting to meet new people.

The conference tries to encourage attendees to be open to new people joining discussions with the "Pacman rule" — don't form a closed circle when having a discussion, but leave a space for someone to join. This seemed to work well in practice.

I always have a great time at ACCU conferences, and this one was no different.

Posted by Anthony Williams
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