Heise Developer: Mythos Proxy-Style

For you German speakers out there, my fifth Heise Developer article, “Mythos Proxy-Style: Wie eine verspätete Klasse neues Licht auf ein bekanntes Entwurfsmuster wirft” just went live on Heise’s site.

It examines the implementation of QProxyStyle in Qt 4.6 and, from that analysis, derives an important caveat regarding the applicability of a certain well-known design pattern.

This is the German translation of an unpublished English-language original, which I will publish here after a grace period.

From the article:

In der Qt-Gemeinde existiert ein Mythos: “QProxyStyle”. Auf der einen Seite stehen Entwickler wie David Faure, seines Zeichens KDE-Urgestein und geschätzter Kollege des Autors, die behaupten, ein QProxyStyle sei unmöglich. Auf der anderen Seite stehen Entwickler von Qt-Erweiterungen, die einen QProxyStyle entwickelt haben wollen. Eine Spurensuche im Märchenwald deckt interessante Komplikationen bei der Anwendung eines bekannten Entwurfsmusters auf.

Heise iX: QtQuick article, KDAB whitepaper; Qt Containers update

Together with my fellow KDABians Stephen Kelly and Volker Krause, I wrote a whitepaper on QtQuick usage in the KOMO project. From the whitepaper:

Developing a serious and large application from scratch, or from an existing desktop application, presents a different set of challenges compared to your normal smart-phone ‘app’. Portability to different platforms, and long-term maintainability in the face of ever-changing mobile target platforms suddenly come into play. This paper attempts to summarize the experience we gathered using the new declarative GUI framework, QtQuick, in this project.

Unfortunately, recent events (read: Tizen) have already invalidated much of the content in the supplemental box on the future of Qt, but that’s the bane of everyone writing about Qt’s future these days…

Get the English version (with slightly more details) at http://www.kdab.com/whitepapers, or read the German version (with better editing) in the current print issue of German computer magazine iX: http://www.heise.de/ix/inhalt/2011/11/114/.

In other news: I’ve updated “Understand the Qt Containers” with a discussion of the differences between STL and Qt containers regarding size types, associative container insertions, and error handling. I also mention QT_STRICT_ITERATORS now.

Heise Developer: Gelernt aus Fehlern

For you German speakers out there, Heise Developer just published a re-issue of my C++0x overview article from the 2009 Programmieren Heute, called “Gelernt aus Fehlern“.

Compared to the print version, we’ve updated the content in some places, such as spending less time on the removal of concepts and more about the adoption of the new standard, as well as turning the Top 10 into a Top 11 to celebrate the new standard 🙂

From the article:

Die spannendste Frage wird sein, wie schnell sich C++11 gegen seinen Vorgänger C++98 wird durchsetzen können. Es war in den vergangenen zwei Dekaden regelmäßig notwendig, C++-Neuerungen konservativ einzusetzen, da Compiler den Standard nur zögerlich umgesetzt haben. […] Hier muss sich die C++-Gemeinde umgewöhnen. Neuerungen wie Lambdas und auto machen C++11 zu einer deutlich produktiveren Sprache, als C++98 es war. Es gilt nun, diese Produktivitätssteigerung im Alltag auch zu nutzen, um gegen Java und .NET zu bestehen. Ein weiteres Jahrzehnt der Zersplitterung und Inkompatibilität könnte die momentan exzellente Ausgangsposition von C++ schnell wieder zunichte machen.

Hope you like it.

C++98 Support Costs Extra!

I should note, right up front, that this post reflects my personal opinion and not that of my employer. In particular, KDAB currently does not require extra payment for C++98 support, even though, as this post will make abundantly clear, I personally wouldn’t mind if it did :).

So, the new C++ standard is finalised and voted on unanimously. Does that mean that we’ll have another decade of C++ incompatibilities ahead of us?

My answer to that is: That depends on us.

Will we adopt C++11 faster than C++98? Our compilers do. Will we make use of the new productivity gains (auto, range-for, lambdas, …) or will we continue to castrate our code to work with 15-year-old compilers? The answers of the majority of C++ programmers will determine how the next decade goes for all of us.

In this post, I’ll argue (strongly) for dropping C++98 support asap, and using C++11 to the fullest possible extent in all our projects. This includes dropping support for compilers and platforms that don’t (yet) support C++11 to the extent required by a particular project instead of #ifdefing our way around it, as we (as an industry) have done for the past 30 years. Will these compilers and platforms be left out in the cold? Not really: they will have older versions of libraries and programs to support them. But yes, newer versions will require an up-to-date C++ compiler.

Is this realistic? I think it is.

Essentially, we have this situation already now. Many projects require fairly recent versions of libraries, precisely because 1) there’s not much point in supporting older versions, and 2) supporting many versions of one library with the same code-base is fairly expensive. I’ve tried. Gpgme++ theoretically supports gpgme’s that can be considered stone-aged. Has anyone ever tested this? Hmmm… How many Mac applications still support OS X 10.2? 10.3? 10.4? Half of the web doesn’t work anymore on KDE 3.5’s Konqueror.

Then why should we be so conservative when it comes to C++, the very core of what we work with? It’s not even customer-visible. Why do we accept an STL shipping in 2011 that is basically unchanged from the mid-1990s? Why do we #ifdef our way around MSVC6 problems in 2011? Why do we care about the quirks of GCC 3.3, or heaven forbid, 2.95?

My new-year’s resolution (ok, it’s a bit early, but hey): stop it!

C++11 support in C++ compilers now is much stronger than C++98 support was in 1998 in the then-C++-compilers. I fully expect every C++ compiler to have caught up on final C++11 with its next major release, except for source-code invisible changes like the new memory model. I want Qt 5 to require a C++11 compiler. I want KDE 5 to require a C++11 compiler. I will lobby inside KDAB to do the same.

Why? Because I’m a programmer, and we’re lazy.

I want to use auto. If you make me type std::map<std::string,std::string>::const_iterator instead of auto, it’s more work, less fun, so you should need to pay extra.

I want to use lambdas. If you make me write

struct ByName : std::binary_function<Foo,Foo,bool> {
    result_type operator()( const Foo & lhs, const Foo & rhs ) const {
        return lhs.name() < rhs.name() ;
    }
};

instead of [](const Foo &lhs, const Foo &rhs) { return lhs.name() < rhs.name() ; }, it’s more work, less fun, so you should need to pay extra.

C++11 is a much more productive language than C++98. We as an industry can, however, only reap that productivity gain if we stop throwing C++ productivity out of the window by the bucket-load in the hopeless pursuit of compatibility with non-standard implementations. The worst offender, Microsoft, has seen the light. Their current compilers are very standards-compliant, and actively maintained. I hope that Apple won’t be the next blocker: By choosing Clang over GCC for—apparently—political reasons (GCC 4.2 was the last GCC with GPLv2), they have lost at least one year of C++11 support that GCC and VC are ahead of Clang. Let’s hope Clang catches up soon.

[[EDIT 2011-09-21: If you think that VC isn’t implementing C++11 fast enough, you can make yourselves heard here: http://visualstudio.uservoice.com, in particular here and here.]]

[[EDIT 2012-03-30: Microsoft has announced that the VC++ compiler will have a faster release cycle than Visual Studio. That makes me hope that we’ll see a “reasonably conforming” (Sutter) VC++ a long time before 2014. Also, Clang++ 3.1 has caught up a lot with GCC (http://clang.llvm.org/cxx_status.html#cxx11, 3.1 currently shows up as “SVN”); it seems tooling will indeed not be the problem, after all.]]

Early-Bird Registration open for “In-depth Multi-Threading with Qt” @ DevDays

I’ll be giving a two-day “In-depth multi-threading with Qt” training just after the DevDays in Munich, ie. Oct 27th-28th. The training language will be English. The training material is the one from the usual Qt training, but “seriously expanded” to fill the two-day schedule.

From the announcement:

This two-day training teaches how to use multithreading with the primitives provided by Qt. Participants will gain insight into multithreading problems in general, and how they pertain to Qt programs in particular.

Examples include how to offload work from the GUI thread, how to increase throughput to the maximum in your application, and optimal strategies for communication between threads.

The training covers topics such as cross-thread signal/slot connections, QThreadPool, QObjects and multithreading, QtConcurrent, QFuture, and Qt atomic operations.

Participants are expected to have a working knowledge of C++ and Qt. Prior experience with multithreaded programming is recommended, but not necessary. The course will start with a one hour multithreading refresher.

To book, visit http://www.kdab.com/schedule.

Until October 1st, you’ll save 200€ on the early-bird offer. Just use d94ec6fd in the Voucher(s) field.

Hope to see you there!

Understand the Qt Containers: The Data

This is just a short heads-up to those that have asked for quantitative measurements. I’ve begun updating “Understand the Qt Containers” with a section on data.

As of now, the memory and append performance is in. I’ll re-run the iteration test over the night once more (I’ve detected an anomaly that I’d like the verify first), and re-add them again, when done.

Effective Qt: Understand the Qt Containers


The third of my Effective Qt columns, “Understand the Qt Containers“, just went live on my site.

If you know me, you will know what awaits you there 🙂 I hope I have not disappointed you. That said, this is just a first part. I plan to eventually extend it to cover more containers (currently, just QVector and QList), and give more data and assembler code for the containers I’ve already described.

These are the guidelines this item covers:

  • Remember that QList has nothing to do with std::list, and QSet has nothing to do with std::set.
  • Be aware of the two Qt container APIs, Qt-ish and STL-compatible, and avoid mixing their use in the same project.
  • Familiarise yourself with the STL containers and the additional features they offer.
  • Prefer member-swap over assignment wherever possible to express move semantics. Use C++11 rvalue move semantics if the class and the compiler support them.
  • [[EDIT 2011-08-25: new]] Always use constBegin()/constEnd() (cbegin()/cend()) when assigning the result to a const_iterator.
  • Avoid modifying a container (remove/insert elements) while iterating. If you have to, use an STL algorithm, such as one of the std::remove() family.
  • Prefer the STL-compatible iterators over the Java-style ones.
  • If you do use the Java-style iterators, avoid using the mutating ones.
  • Prefer to use const references as the first argument of Q_FOREACH, unless the element type is customarily passed by value.
  • Familiarise yourself with BOOST_FOREACH, and the additional features it offers.
  • Declare your enums and QFlags as Q_PRIMITIVE_TYPE if there’s a chance they will be held in Qt containers.
  • Declare your value-types Q_MOVABLE_TYPE if there’s a chance they will be held in Qt containers.
  • Don’t change the classification of a type if you need to maintain binary compatibility.
  • Prefer vector (std or Q) over QList.
  • Avoid QList<T> where T is not declared as either Q_MOVABLE_TYPE or Q_PRIMITIVE_TYPE or where sizeof(T) != sizeof(void*) (remember to check both 32 and 64-bit platforms).
  • [[EDIT 2011-08-16: new]] Avoid using vectors of types for which Qt APIs customarily use QList, and for which QList is not inefficient.

Hope you enjoy!