## trip report - CppCon 2016

25 september 2016

CppCon 2016 ended yesterday - I had the pleasure of attending and presenting at this amazing conference again this year.

I'm really grateful to Jon Kalb, Bryce Lelbach, the conference staff, the speakers, my company and everyone else involved for making this possible.

In the same vein as my C++Now 2016 trip report, I wanted to share my thoughts regarding the talks I liked the most and regarding my sessions.

### Favorite talks & lessons learned

#### Using Types Effectively - Ben Deane

(Slides available here.)

This is the first talk I saw after Bjarne's keynote, and I really found it valuable and entertaining. In short, the goal of the presentation was showing the audience how powerful C++'s type system is and how the upcoming C++17 std::variant and std::optional are game changers.

Ben was great at keeping the audience engaged by asking questions regarding the cardinality of sum and product types, starting with basic types (such as void, bool, and int) and moving onto function types and templated variants.

I personally love this presentation style and I wish more speakers would try it out.

The most important lessons I learned from the talk are:

• Use the type system to your advantage, in order to make "illegal states" irrepresentable:

I see code like the one in this example provided by Ben very often:

enum class ConnectionState
{
DISCONNECTED,
CONNECTING,
CONNECTED,
CONNECTION_INTERRUPTED
};

struct Connection
{
ConnectionState m_connectionState;

ConnectionId m_id;
std::chrono::system_clock::time_point m_connectedTime;
std::chrono::milliseconds m_lastPingTime;
Timer m_reconnectTimer;
};

Only some of the fields inside Connection are used for specific ConnectionState values. If m_connectionState equals ConnectionState::DISCONNECTED, why should we be able to access and modify m_id?

We can use the type system and std::variant to prevent representing illegal states, and also to improve the memory layout of our Connection class:

struct Connection
{

struct Disconnected
{
};

struct Connecting
{
};

struct Connected
{
ConnectionId m_id;
std::chrono::system_clock::time_point m_connectedTime;
std::optional<std::chrono::milliseconds> m_lastPingTime;
};

struct ConnectionInterrupted
{
std::chrono::system_clock::time_point m_disconnectedTime;
Timer m_reconnectTimer;
};

std::variant<Disconnected, Connecting, Connected, ConnectionInterrupted>
m_connection;
};

In the above example, using multiple struct types to represent the various possible states and their associated fields allows us to "compress" the memory layout of Connection and to avoid accessing/mutating inactive fields.

• Use "phantom types" to constrain behavior where possible:

Considering the following code snippet taken from Ben's talk:

std::string GetFormData();
std::string SanitizeFormData(const std::string&);
void ExecuteQuery(const std::string&);

The intended chain of calls for the user of this hypotetical library is:

1. Get the form data.

auto formData = GetFormData();
2. Sanitize the form data.

auto sanitizedFormData = SanitizeFormData(formData);
3. Execute the query.

ExecuteQuery(sanitizedFormData);

Can you spot the problem here? There is nothing preventing the user from calling ExecuteQuery with unsanitized form data.

C++'s type system comes to the rescue here - we can enforce our desired chain of operations by having different string types that represent the state of the form data. It is sufficient to "wrap and mark" std::string in order to achieve that. This technique is commonly known as "phantom types", and it's similar in concept to "strong typedefs".

By defining two empty "tag" structs, we can inform the type system of our intent of sanitizing the form data:

struct sanitized
{
};

struct unsanitized
{
};

Then we can "mark" std::string with one of the types above to create a separate type. In order to prevent implicit conversions (which would nullify the purpose of phantom types), an explicit constructor is required:

template <typename T>
struct FormData
{
explicit FormData(const string& input) : m_input(input) {}
std::string m_input;
};

Our marked types can then provide a type-safe interface to the user which won't allow ExecuteQuery to be called with an unsanitized string:

FormData<unsanitized> GetFormData();
FormData<sanitized> SanitizeFormData(const FormData<unsanitized>&);
void ExecuteQuery(const FormData<sanitized>&);

This is an extremely simple and powerful technique that you should always try to apply in your APIs.

Ben gave another talk which I didn't attend in person: "std::accumulate: Exploring an Algorithmic Empire" - I'm really looking forward to see the video when it's uploaded on CppCon's official YouTube channel.

#### C++17 in Breadth - Alisdair Meredith

An extremely informative two-part presentation regarding all the changes present in the upcoming C++17 standard. While Bjarne's keynote made me feel disappointed that many major features (e.g. concepts, modules, UFCS) were either delayed or canceled, Alisdair's talk made me realize that C++17 is going to be a massive improvement over C++14 and that there will be many useful language and features for everyone.

No major programming paradigm shifts will be introduced by the new standard, but many interesting features that could severely make complex code much easier to write and reason about will be introduced. I'm particularly excited for:

• Template argument deduction for class templates, which will finally allow us to get rid of make_xxx(...) functions. Something cool I realized is that implementing a "scope guard" will now be trivial:

template <typename TF>
struct scope_guard : TF
{
template <typename TFFwd>
scope_guard(TFFwd&& f) : TF{std::forward<TFFwd>(f)} { }

~scope_guard()
{
static_cast<TF&>(*this)();
}
};

int main()
{
scope_guard x{[]{ std::cout << "bye!\n"; }};
}
• constexpr lambdas: oh boy. This feature opens up a whole new world for lambda-based metaprogramming. I'm a huge fan of Boost.Hana and I think that this allow it become more powerful with simpler interfaces - I can't wait to see what Louis Dionne is going to implement with them.

• fold expressions and if constexpr(...): these two new features hit close to home, as they will allow users to implement patterns I've talked about in conferences in much cleaner and safer ways:

• Fold expressions over the "comma operator" will allow a trivial implementation of for_each_argument, a code snippet that I covered both at CppCon 2015, C++Now 2016 and CppCon 2016.

• if constexpr(...) is a sane implementation of static_if, which can be implemented (with a slightly less enticing syntax) in C++14. I presented a way of doing that at C++Now 2016 and CppCon 2016.

#### Hybrid Data Structures - Chandler Carruth

An enlightening overview of some high-performance data structures used in LLVM's codebase. I really liked the idea of a "small vector" class that uses SBO ("small buffer optimization") to prevent unnecessary allocations. This is sadly impossible for std::vector and I think that code making use of many small vector instantiations could greatly benefit from SBO.

Chandler also presented some very interesting ways of using "tagged pointers", in order to implement lightweight variants as small as a regular pointer.

Questions regarding the possibility of implementing these data structures using allocators were also asked during the talk, which allowed Chandler to explain why sometimes it is easier to build these high-performance structures from scratch and lead to interesting discussion.

#### Constant Fun - Dietmar Kuhl

(Slides available here.)

An in-depth analysis of "constant expressions" and constexpr in both C++11 and C++14.

If you have never used constexpr in the past and want to implement useful compile-time functions and structures (e.g. mapping strings to types or factory functions), this is a fantastic introduction to the constexpr specifier.

If you have used constexpr and are still not completely confident on when constexpr function arguments and return values are constant expressions (like I was before the talk), definitely watch the video when it's available: Dietmar does an excellent job at clearly explaining how and when functions, types and variables can be evaluated as constant expression in constexpr contexts and how to write your code to make sure it is as flexible as possible.

#### From Zero to Iterators - Patrick Niedzielski

(Slides available here.)

Patrick really loves iterators. If you don't already feel the same way, I'm confident you'll start loving them after watching this very well-presented and informative talk.

Patrick implements an hierarchy of iterator concepts (using "concepts lite") and various generic functions that show how powerful this abstraction is. The audience is guided from a very simple and naive pointer-based copy function to the definition and implementation of a CacheAwareIterator concept that can be used to automatically parallelize algorithms (potentially avoiding "false sharing" with C++17 features).

If that doesn't sound exciting to you, I recommend watching the first minutes of this talk when it becomes available - I guarantee you'll be hooked.

Patrick's heavy "concepts lite" usage also shows their benefits and drawbacks clearly. If you haven't yet made up your mind on this feature that could be part of C++20, this talk is also a great place to start developing an opinion.

### My sessions

#### Implementing static control flow in C++14 - Vittorio Romeo

(Slides and code available here.)

Abstract:

There has always been great interest in imperative compile-time control flow: as an example, consider all the existing static_if proposals and the recently accepted constexpr_if construct for C++17.

What if you were told that it is actually possible to implement imperative control flow in C++14?

In this tutorial, the implementation and design of a compile-time static_if branching construct and of a compile-time static_for iteration construct will be shown and analyzed. These constructs will then be compared to traditional solutions and upcoming C++17 features, examining advantages and drawbacks.

I've previously talked about "static control flow" in a lightning talk at Meeting C++ 2015 and at C++Now 2015. I realized that this topic is of more general interest and that it would be a great fit for CppCon's program - I'm really glad my submission got accepted and that I had a chance to show developers how C++14 makes it reasonably easy to implement imperative-like compile-time control flow.

In short, this talk shows how to implement a static_if C++14 construct similar to C++17's if constexpr:

template <typename T>
auto consume(T&& x)
{
static_if(bool_v<is_solid<T>>)
.then([](auto&& y)
{
y.eat();
std::cout << "eating solid\n";
})
.else_if(bool_v<is_liquid<T>>)
.then([](auto&& y)
{
y.drink();
std::cout << "drinking liquid\n";
})
.else_([](auto&&)
{
std::cout << "cannot consume\n";
})(FWD(x));
}

It also covers two techniques that can be used to implement compile-time iteration:

• A for_args function, similar to the previously covered for_each_argument, which uses the ellipsis ... operator to avoid recursion in order to prevent significantly increasing compilation times.

• A more advanced and feature-rich static_for function, which allows users to keep track of the iteration index and use imperative-like continue and break instructions. This is implemented recursively and has a bigger impact on compilation times but is way more flexible than for_args.

In constrast to the previous versions, which were 100% code, I added some animated slides that clearly show the idea of the implementation behind static_if, giving the audience a visual representation of how branches are "collapsed" and how execution is "deferred" in order to prevent unmatched branches to generate compiler errors.

Additionally, I covered the history of static control flow proposals in the C++ standards, and the rules (plus some usage examples) of the upcoming if constexpr, which makes static_if` obsolete.

I'm really happy about the talk: I finished right in time with a few extra minutes for questions, the audience looked engaged and was interested in knowing more about the patterns. I received only positive feedback so far and I'm looking forward to more of that and to constructive criticism.

#### C++ in Colleges - (panel)

Abstract:

Join a panel of college professors, recent and current undergraduates, and professionals with teaching and outreach experience as we discuss how to engage students in C++. Why is it important to teach C++ early, and is it an appropriate choice for a first programming language? Are colleges teaching C++ less than they used to? Are colleges teaching C++ well? How can the C++ community better engage students in open source and in industry?

I was very lucky to be part of this panel moderated by Jackie Kay as a recent college graudate, surrounded by professors and students with many interesting stories and opinions to share (Bjarne joined the panel as well!).

I think that the way C++ is being taught is a huge issue that needs to be effectively addressed by the community in order to prevent spreading misinformation about the language and to bring professors up-to-date with the newest standards and best practices. In way too many classes, mine included, C++ is introduced as an old and outdated language, still alive only for legacy reasons.

In other classes, C++ is taught unidiomatically, excessively using dynamical memory allocations and Java-like idioms.

I really really would love to see all college professors in the world get up-to-date with the newest standards and with idiomatic C++ code: it's a shame that many students avoid the language or even despise it due to improper teaching and misconceptions. Making this a reality is a big problem that I do not know how to solve - interesting suggestions and possible solutions were discussed during the panel, but the only thing that I'm sure will help is doing your small part: tell your coworkers and classmates about modern C++, spread the word on how powerful and safe idiomatic C++ is, and make an effort to clear misconceptions away.

One of this year's keynotes by Dan Saks, Keynote: extern "C": Talking to C Programmers About C++, is an invaluable resource on how to speak to people who truly believe incorrect information about the language, and I think that the lessons learned from that keynote can be effectively applied in the context of college education as well.

I really enjoyed being part of the discussion on this panel, and also being able to share a very interesting story heard at ++it Florence Meetup 2015: Marco Foco taught C++14 using Raspberry Pi2 to a class containing students with no previous C++ experience starting from modern code and idioms, achieving very positive results. This is, in my opinion, a very remarkable experience that shows how teaching the complexity of the language (which exists due to compatibility reasons) can be deferred to a later step in the teaching process.

Another resource that you might find interesting is Kate Gregory's CppCon 2015 "Stop Teaching C" talk.