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.

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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!

Translated: Pimp My Pimpl (part 2)

I’m happy to report that I finally sat down to translate the second part of my Heise Developer article on the Pimpl Idiom to English. Please find it here.

Translated: Pimp My Pimpl (part 1)

I’m happy to report that I finally sat down to translate the first part of my Heise Developer article on the Pimpl Idiom to English. Please find it here.

Heise iX: iX Spezial “Programmieren Heute” available as eBook

I just noticed that the iX Spezial “Programmieren Heute”, to which yours truly contributed a state-of-the-union article on C/C++, is “now” (well, since April) available as eBook. Needless to say (the title indicates it already), the whole thing is in German.

From the article:

Auch wenn C++ nicht gerade als beliebt unter Entwicklern gelten darf, wird die Sprache sie noch eine Weile begleiten – vor allem, weil der Bedarf an hardwarenahen Sprachen wie C++ oder seiner Alternative D sich in nächster Zeit erhöhen dürfte.

I’m still not through all the other articles in my tr(Belegexemplar)—it’s that interesting 🙂

Go get it at Dpunkt.de.

Disclaimer: I don’t get any proceeds from this, only the author’s fee, which was fixed.

Heise Developer: Dem Elfenbeinturm entreißen

A little belatedly, I guess, but here we go:

For you German speakers out there, my first Heise Developer article, “Dem Elfenbeinturm entreißen: Über den praxisrelevanten Einsatz der Template-Metaprogrammierung“, “just” went live on Heise’s site.

It shows how to use TMP to convert from QVariant to boost::variant<>. Even if you don’t speak German, you might be interested in the sample source code: ftp://ftp.heise.de/pub/ix/developer/elfenbein.zip.

From the article:

Als Ansatz für eine Konvertierungsfunktion lässt sich Folgendes schreiben.

template <typename T_Variant>
T_Variant qvariant2variant( const QVariant & qv ) {
  for( T : types(T_Variant) )
    if ( qv.type() == T )
      return T_Variant( qvariant_cast<T>( qv ) );
  throw bad_cast( "..." );
};

Es fehlen noch zwei Dinge, um aus dem Pseudocode “richtigen” Code zu erstellen: Erstens ein Weg, um einen Typ T auf den dazugehörigen QVariant-internen Diskriminator abzubilden. Das leistet das Funktions-Template qMetaTypeId() […].

Zweitens ein Weg, um “für jeden Typ T, den T_Variant enthalten kann”, abzubilden. Das lässt sich mit TMP lösen (Stichwort: Liste von Typen), was die Aufgabe für den Rest des Artikels sein wird.