Articles & Books

The Variant Saga: A happy ending?

Variant is like a union, only it tells you what it currently contains, and it will barf if you try to get something out of it that is does not currently contain. It's the type safe sibling of the union:

variant<double, string> v = 42.;
double d = get<double>(v);

I had proposed a variant many moons ago (N4218). After many discussions it seemed that the committee cannot agree on what the ideal C++ variant would look like. I will resolve this cliffhanger -- but before doing that let me introduce you to some of the key discussion points.

An ideal variant will always contain one of its alternative types. But look at this code snippet:

variant<string, MyClass> v = "ABC";
v = MyClass();

The second line will destroy the old value contained in the variant and construct the new value of a different type. Now suppose that the MyClass construction threw: what will the variant contain? What happens when you call get<1>(v)? What happens when the variant gets destroyed?

We either provide the strong exception guarantee (the variant would still contain the string) -- but this requires double buffering, as for instance boost::variant does. Or we could restrict the alternative types to only those that are nothrow_move_constructible. Or we make this a new state -- "invalid, because the variant has been derailed due to an exception thrown during type changing assignment". Or we say "you shouldn't write code that behaves like this; if you do you're on your own", i.e. undefined behavior. The committee was discussing what to do, and so was The Internet. There are other design decisions -- default construction, visitation etc -- but they are all insignificant compared to how to deal with the throwing, type-changing assignment.

I have tried to present the options and their pros and cons in P0086. In short: it's incredibly difficult and fragile to predict whether a type is_nothrow_move_constructible. And double buffering -- required for a strong exception guarantee -- kills the quest for an efficient variant. But efficiency is one of the main motivations for using a discriminated union.

After the second Library Evolution Working Group (LEWG) review in Lenexa, we got P0088R0: a design that was making this invalid state extremely rare. But if it happened, access to the value would result in undefined behavior. This caused a vivid reaction from the committee members. And from The Internet. Hundreds of emails on the committee email lists. Many many smart and convincing blog posts.

In the end, different parts of the committee strongly supported different designs -- and vetoing other designs. Massive disagreement. So when we came to our C++ Standards Meeting in Kona, it was immediately clear that we needed to expose this to the full committee (and not just LEWG). The expectation was that we would declare variant dead, and keep it locked away for the next five years. At least. (An I would have time to water my fishes again.)

So back to the cliffhanger. On the full committee, Monday evening stage in Kona were David Sankel and I. We presented (and represented) the different design options. While we were discussing with the committee members, live and uncut and on stage, David and I realized that we could make it happen. "The Kona Kompromise": similar to P0088R0, but instead of undefined behavior when extracting the value of such a zombie variant it would just throw!

The Kona Kompromise means that we don't pay any efficiency for the extremely rare case of a throwing move. The interface stays nice and clean. A variant of n alternatives is a "mostly" an n-state type. It offers the basic exception guarantee at no relevant performance loss. It is a safe vocabulary type for every-day use, also for novices. The vast majority of the committee was convinced by this idea. Almost everyone in the room was happy!

Do we have a std::variant now? Not yet. But we are a giant leap closer: variant is now under wording review with the Library Working Group (LWG); I will publish a new revision in the Kona post-mailing (P0088R1). This will get re-reviewed in Jacksonville, first week of March. Once LWG gives the green light, the full committee can vote variant into a Technical Specification (TS) as std::experimental::variant. Now that a large fraction of the committee has expressed its consent (happiness, even!), I expect that this will be in the TS called Library Fundamentals, v3. It might or might not make it into C++17 -- that depends mostly on how quickly I manage to bring P0088 into an acceptable state, and how quickly we will gain use experience with variant.

So there is one thing I'd really appreciate your help with: std::experimental::variant will show up in library implementations near you, likely in their first releases next year. It would be fantastic if you could try it out, and as importantly: give feedback, on the public forums or by contacting me directly ([email protected]). Your feedback will tell us whether the design decisions we took are the right ones, for instance regarding default construction, visitation, performance, and especially converting construction and assignment. As they say here: Mahalo!

Axel Naumann, CERN ([email protected])

 

6 topics on starting and running a User Group

I posted an update on my founding C++ User Groups article from 2 years ago:

6 topics on starting and running a User Group

by Jens Weller

From the article:

Almost two years ago I blogged about founding C++ User Groups, since then I have learned a lot more on the topic, and I want to share that experience with you in this blog post. While my focus here at Meeting C++ is C++, this post is more on the topic of a User Group, so its also useful to you, if you want to start a user group on something else. Yet, I might strive away into C++ lands in this post...

Becoming a Rule of Zero Hero--Glennan Carnie

Everything is in the title:

Becoming a Rule of Zero Hero

by Glennan Carnie

From the article:

Previously, we’ve looked at The Rule of Zero which, in essence, says: avoid doing your own resource management; use a pre-defined resource-managing type instead.

This is an excellent guideline and can significantly improve the quality of your application code. However, there are some circumstances where you might not get exactly what you were expecting. It’s not that the code will fail; it just might not be as efficient as you thought.

Luckily, the solution is easy to implement and has the additional side-effect of making your code even more explicit.

C++ User Group Meetings in November

The montly overview on upcoming C++ User Group Meetings:

23 C++ User Group meetings in November

by Jens Weller

The list:

    4.11 C++ UG Saint Louis - Unit Test++, Scott Meyers "gotchas", group exercise
    4.11 C++ UG Washington, DC - Q & A / Info Sharing
    4.11 C++ UG Austin - OpenMP
    9.11 C++ UG Edinburgh - C++ Edinburgh
    9.11 C++ UG Zentralschweiz - Coding Dojo
    11.11 C++ UG Utah - Regular Monthly Meeting
    11.11 C++ UG San Francisco/ Bay area - Presentation and Q&A
    12.11 C++ UG NRW/Aachen - Variadic Templates und TMP ohne Rekursion
    12.11 C++ UG Dresden - User perspektive on Catch
    16.11 C++ UG Austin - North Austin Monthly C/C++ Pub Social
    17.11 C++ UG Berlin - November Meetup
    17.11 C++ UG Hamburg - OpenCL
    18.11 C++ UG San Francisco/ Bay area - Workshop and Discussion Group
    18.11 C++ UG Warsaw - LLVM a jakość i bezpieczeństwo oprogramowania
    18.11 C++ UG Washington, DC - Q & A / Info Sharing
    18.11 C++ UG Bristol - Thomas Guest
    18.11 C++ UG Düsseldorf - Lightning Talks
    18.11 C++ UG North West/Seattle - CppCon Keynote - Bjarne Stroustrup
    18.11 C++ UG Sacramento - Sacramento Area C++, First Meetup
    25.11 C++ UG San Francisco/ Bay area - Workshop and Discussion Group
    26.11 C++ UG Rhein-Neckar - C++11/C++14 loops of fun
    26.11 C++ UG Bremen - C++ User Group
    26.11 C++ UG Munich - Lightning Talks

New updates of CppHints.com, service of recommendations on C++ programming from PVS-Studio team

New updates of C++Hints, service of recommendations on C++ programming

Changes in CppHints.com

by PVS-Studio Team

We got a lot of positive feedback from our readers and continue developing the C++Hints project. We have also answered two requests, that we saw quite often in the letters:

By this moment we have published the following articles:

An introduction to C++'s SFINAE concept -- Jean Guegant

Jean Guegant describes in a very detailed way different implementation stategies of compile-time introspection of a class member for C++98, C++11 and C++14.

An introduction to C++'s SFINAE concept: compile-time introspection of a class member

by Jean Guegant

From the article:

As a C++ enthusiast, I usually follow the annual C++ conference cppconf or at least try to keep myself up-to-date with the major events that happen there. One way to catch up, if you can't afford a plane ticket or the ticket, is to follow the youtube channel dedicated to this conference. This year, I was impressed by Louis Dionne talk entitled "C++ Metaprogramming: A Paradigm Shift". One feature called is_valid that can be found in Louis's Boost.Hana library particulary caught my attention. This genious is_valid function heavily rely on an even more "magic" C++ programming technique coined with the term SFINAE discovered at the end of the previous century. If this acronym doesn't speak to you, don't be scared, we are going to dive straight in the subject.

Random Acts of Optimization --Tony Albrecht

Discussion regarding systematic approach to go about optimization of logic.

Random Acts of Optimization

by Tony Albrecht

From the article:

The three stages mentioned here, while seemingly obvious, are all too often overlooked when programmers seek to optimize. Just to reiterate:

    1. Identification: profile the application and identify the worst performing parts.
    2. Comprehension: understand what the code is trying to achieve and why it is slow.
    3. Iteration: change the code based on step 2 and then re-profile. Repeat until fast enough.

The solution above is not the fastest possible version, but it is a step in the right direction—the safest path to performance gains is via iterative improvements.

Using Variadic Templates cleanly--Florian Weber

Variadics are even more easy to use than we tought:

Using Variadic Templates cleanly

by Florian Weber

From the article:

When one comes across examples for variadic templates, almost always recursion is used to achieve almost everything, for example like this:

// We are lucky: the author correctly used zero
// arguments instead of one as the base-case,
// thereby avoiding code-duplication:
inline void print_to_stream(std::ostream&) {}

template<typename Head, typename...Tail>
void print_to_stream(std::ostream& stream, const Head& h, const Tail&... t) {
  stream << h;
  print_to_stream(stream, t...);
}

In this article we will see what better alternatives for this rather clumsy hack exist and see how we can write a better version with less code...