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    {{Template:I18n/Language Navigation Bar|Development/Tutorials/Common Programming Mistakes}}


    {{TutorialBrowser|
    series=Getting Started|
    name=Errori di Programmazione Comuni|
    reading=[[Policies/API_to_Avoid|APIs to avoid]]
    }}
    <!-- == Abstract == -->
    == Prefazione ==
    <!-- This tutorial aims to combine the experience of KDE developers regarding Qt and KDE frameworks dos and don'ts. Besides actual mistakes, it also covers things which are not necessarily "bugs" but which make the code either slower or less readable. -->
    Questo tutorial mira a combinare le esperienze degli sviluppatori KDE su cosa fare e cosa non fare in merito alle librerie Qt e KDE. Oltre agli errori, vengono coperte anche cose che non sono necessariamente "bachi" ma che rendono il codice più lento e di difficile lettura.
    <!-- == General C++ == -->
    == C++ in generale ==
    <!-- This section guides you through some of the more dusty corners of C++ which either tend to be misused or which people often simply get wrong. -->
    Questa sezione ti guida attraverso alcuni degli angoli più remoti del C++ che tendono ad essere usati male o dei quali la gente si sbaglia.
    <!-- === Anonymous namespaces vs statics === -->
    === Namespace anonimi contro static ===
    <!-- If you have a method in a class that does not access any members and therefore does not need an object to operate, make it static. If additionally it is a private helper function that is not needed outside of the file, make it a file-static function. That hides the symbol completely. -->
    Se hai un metodo in una classe che non accede ad alcun membro e quindi non ha bisogno di un oggetto per funzionare, rendilo statico. Se in più è una funzione di supporto privata che non viene utilizzata all'esterno del file, rendila file-static. In questo modo essa viene nascosta completamente.
    <!-- Symbols defined in a C++ anonymous namespace do not have internal linkage. Anonymous namespaces only give a unique name for that translation unit and that is it; they do not change the linkage of the symbol at all. Linkage is not changed on those because the second phase of two-phase name lookup ignores functions with internal linkages. Also, entities with internal linkage cannot be used as template arguments. -->
    Le entità definite in un namespace anonimo in C++ non hanno linkage interni. I namespace anonimi offrono soltanto un nome unico per quella translation unit e basta; non cambiano in nessun modo il linkage dell'identificatore. Il linkage non viene cambiato perché la seconda delle due fasi di ricerca dei nomi ignora le funzioni con linkage interno. Per di più, le entità con linkage interno non possono essere usate come argomento di un template.
    <!-- So for now instead of using anonymous namespaces use static if you do not want a symbol to be exported. -->
    A questo punto invece di usare namespace anonimi usa la parola chiave static se non vuoi che un simbolo venga esportato.
    <!-- === NULL pointer issues === -->
    === Problemi di puntatore Nullo ===
    <!-- First and foremost: it is fine to delete a null pointer. So constructs like this that check for null before deleting are simply redundant: -->
    Prima di tutto: va bene eliminare un puntatore nullo. Quindi costruttori come il seguente che controllano che il valore sia nullo prima di eliminarlo sono semplicemente ridondanti:
    <code cppqt>
    if ( ptr ) {
      delete ptr;
    }
    </code>
    <!-- Note however, that '''a null check ''is'' required when you delete an array''' - that's because a relatively recent compiler on Solaris does not handle it properly otherwise. -->
    Da notare comunque, che '''un controllo per valore nullo ''è'' richiesto quando cancelli un array''' - questo perché altrimenti un compilatore relativamente recente per Solaris non lo gestisce opportunamente.
    <!-- When you delete a pointer, make sure you also set it to 0 so that future attempts to delete that object will not fail in a double delete. So the complete and proper idiom is: -->
    Quando elimini un puntatore, assicurati anche di settarlo a 0 in modo che futuri tentativi di cancellazione non falliscano in una doppia eliminazione. Per cui il modo completo e corretto di procedere è:
    <code cppqt>
    delete ptr;
    ptr = 0;
    </code>
    <!-- You may notice that null pointers are marked variously in one of three ways: 0, 0L and NULL. In C, NULL is defined as a null void pointer. However, in C++, this is not possible due to stricter type checking. Therefore, modern C++ implementations define it to a "magic" null pointer constant which can be assigned to any pointer. Older C++ implementations, OTOH, simply defined it to 0L or 0, which provides no additional type safety - one could assign it to an integer variable, which is obviously wrong. -->
    Potresti notare che i puntatori nulli sono variamente indicati in uno di questi tre modi: 0, 0L e NULL. In C, NULL è definito come un puntatore nullo di tipo void. Ma in C++ ciò non è possibile a causa di un controllo di tipo più stretto. Perciò, moderne implementazioni del C++ lo rendono come un "magico" puntatore nullo costante il quale può essere assegnato a qualunque altro puntatore. D'altra parte le implementazioni più vecchie di C++ semplicemente lo associano a 0 o 0L, il quale non tiene conto di alcuna sicurezza di tipo - si potrebbe assegnarlo ad una variabile intera, ovviamente sbagliando.
    <!-- In pointer context, the integer constant zero means "null pointer" - irrespective of the actual binary representation of a null pointer. This means that the choice between 0, 0L and NULL is a question of personal style and getting used to something rather than a technical one - as far as the code in KDE's SVN goes you will see 0 used more commonly than NULL. -->
    Nel contesto dei puntatori, la costante intera zero significa "puntatore nullo" - irrispettoso della rappresentazione binaria di un puntatore nullo. Ciò significa che la scelta tra 0, 0L e NULL è una questione di stile personale e abitudine piuttosto che tecnica - fintantoché nel codice SVN di KDE vedrai 0 usato più comunemente di NULL.
    <!-- Note, however, that if you want to pass a null pointer constant to a function in a variable argument list, you *must* explicitly cast it to a pointer - the compiler assumes integer context by default, which might or might not match the binary representation of a pointer. Again, it does not matter whether you cast 0, 0L or NULL, but the shorter representation is generally preferred. -->
    Notare, comunque, che se vuoi passare un puntatore nullo costante ad una funzione nella lista delle variabili degli argomenti, *devi* esplicitamente farne il cast in un puntatore - il compilatore assume di default il contesto degli interi, il quale può o non può coincidere con la rappresentazione binaria di un puntatore. Di nuovo, non ha importanza il fatto che fai il cast a 0, 0L o NULL, ma la rappresentazione più corta è generalmente preferita.
    <!-- === Member variables === -->
    === Variabili membro ===
    <!-- You will encounter four major styles of marking class member variables in KDE:
    * '''m_variable''' lowercase m, underscore and the name of the variable starting with a lowercase letter. This is the most common style and one prefered for code in kdelibs.
    * '''mVariable''' lowercase m and the name of variable starting with a uppercase letter
    * '''variable_''' variable name starting with a lowercase letter and then an underscore
    * '''_variable''' underscore and the name of variable starting with a lowercase letter. This style is actually usually frowned upon as this notation is also used in some code for function parameters instead. -->
    Incontrerai quattro maggiori stili per segnare le variabili membre delle classi in KDE:
    * '''m_variabile''' m minuscola, underscore ed il nome della variabile che comincia con una lettera minuscola. Questo è lo stile più comune ed uno dei preferiti nel codice delle kdelibs.
    * '''mVariabile''' m minuscola ed il nome della variabile che comincia con una lettera maiuscola.
    * '''variabile_''' il nome della variabile comincia con la lettera minuscola ed alla fine un underscore.
    * '''_variabile''' un underscore e poi il nome della variabile con la lettera iniziale minuscola. Questa notazione di solito è sconsigliata siccome è anche utilizzata in qualche codice per i parametri delle funzioni.
    <!-- As it often happens there is not one correct way of doing it, so remember to always follow the syntax used by the application/library to which you are committing. -->
    Come accade spesso non c'è un modo corretto per farlo, perciò ricorda sempre di rispettare la sintassi utilizzata dall'applicazione/libreria alla quale stai facendo commit.
    <!-- === Static variables === -->
    === Variabili statiche ===
    <!-- Try to limit the number of static variables used in your code, especially when committing to a library. Construction and initialization of large number of static variables really hurts the startup times. -->
    Cerca di limitare il numero di variabili statiche nel tuo codice, specialmente quando fate il commit per una libreria. Costruzione ed inizializzazione di un grande numero di variabili statiche fa veramente male ai tempi di avvio.
    <!-- Do not use class-static variables, especially not in libraries and loadable modules though it is even discouraged in applications. Static objects lead to lots of problems such as hard to debug crashes due to undefined order of construction/destruction. -->
    Non usare variabili class-static, in particolare non nelle librerie e nei moduli sebbene sia anche scoraggiato nelle applicazioni. Oggetti statici portano ad un sacco di problemi tra cui difficoltà di debug dei crash dovuto ad un ordine indefinito di costruttore/distruttore.
    <!-- Instead, use a static pointer, together with <tt>K_GLOBAL_STATIC</tt> which is defined in <tt>kglobal.h</tt> and is used like this: -->
    Invece, usa un puntatore statico insieme a <tt>K_GLOBAL_STATIC</tt> definito in <tt>kglobal.h</tt> ed usato in questo modo:
    <!-- <code cppqt>
    class A { ... };
    K_GLOBAL_STATIC(A, globalA)
    void doSomething()
    {
        A *a = globalA;
        ...
    }
    void doSomethingElse()
    {
        if (globalA.isDestroyed()) {
            return;
        }
        A *a = globalA;
        ...
    }
    void installPostRoutine()
    {
        qAddPostRoutine(globalA.destroy);
    }
    </code> -->
    <code cppqt>
    class A { ... };
    K_GLOBAL_STATIC(A, globaleA)
    void faQualcosa()
    {
        A *a = globaleA;
        ...
    }
    void faQualcosAltro()
    {
        if (globaleA.isDestroyed()) {
            return;
        }
        A *a = globaleA;
        ...
    }
    void installaPostRoutine()
    {
        qAddPostRoutine(globaleA.destroy);
    }
    </code>
    <!-- See the [http://www.englishbreakfastnetwork.org/apidocs/apidox-kde-4.0/kdelibs-apidocs/kdecore/html/kglobal_8h.html#75ca0c60b03dc5e4f9427263bf4043c7 API documentation] for <tt>K_GLOBAL_STATIC</tt> for more information. -->
    Vedi la [http://www.englishbreakfastnetwork.org/apidocs/apidox-kde-4.0/kdelibs-apidocs/kdecore/html/kglobal_8h.html#75ca0c60b03dc5e4f9427263bf4043c7 documentazione delle API] per più informazioni su <tt>K_GLOBAL_STATIC</tt>.
    <!-- === Constant data === -->
    === Dati costanti ===
    <!-- If you need some constant data of simple data types in several places, you do good by defining it once at a central place, to avoid a mistype in one of the instances. If the data changes there is also only one place you need to edit. -->
    Se hai bisogno di qualche dato costante per semplici tipi di dato in molti punti, fai bene a definirli una volta sola in un posto centrale, onde evitare errori di digitazione in una delle istanze. Se i dati cambiano hai bisogno di editare solo in un punto.
    <!-- Even if there is only one instance you do good by defining it elsewhere, to avoid so-called "magic numbers" in the code which are unexplained (cmp. 42). Usually this is done at the top of a file to avoid searching for it. -->
    Anche se usati una sola volta è meglio definirli da un'altra parte, per evitare inspiegabili "numeri magici" nel codice (cmp. 42). Di solito ciò viene fatto in cima al file per non doverli ricercare.
    <!-- Define the constant data using the language constructs of C++, not the preprocessor instructions, like you may be used to from plain C. This way the compiler can help you to find mistakes by doing type checking. -->
    Definisci i dati costanti usando i costrutti del C++, non le istruzioni del preprocessore, come potresti essere abituato a fare dal C. In questo modo il compilatore può aiutarti a trovare errori facendo il controllo di tipo.
    <!-- <code cppqt>
    // Correct!
    static const int AnswerToAllQuestions = 42;
    // Wrong!
    #define AnswerToAllQuestions 42
    </code> -->
    <code cppqt>
    // Corretto!
    static const int LaRispostaATutteLeDomande = 42;
    // Sbagliato!
    #define LaRispostaATutteLeDomande 42
    </code>
    <!-- If defining a constant array do not use a pointer as data type. Instead use the data type and append the array symbol with undefined length, <tt>[]</tt>, behind the name. Otherwise you also define a variable to some const data. That variable could mistakenly be assigned a new pointer to, without the compiler complaining about. And accessing the array would have one indirection, because first the value of the variable needs to be read. -->
    Se stai definendo un array costante non usare un puntatore come tipo di dato. Invece usa il suo tipo ed appendi il simbolo dell'array di indefinita lunghezza, <tt>[]</tt>, dopo il nome. Altrimenti definirai anche una variabile che punta a qualche dato costante. La variabile potrebbe per sbaglio essere assegnata ad un altro puntatore, senza che il compilatore se ne lamenti. E l'accesso all'array sarebbe indiretto, perché per primo deve essere letto il valore della variabile.
    <!-- <code cppqt>
    // Correct!
    static const char SomeString[] = "Example";
    // Wrong!
    static const char* SomeString = "Example";
    // Wrong!
    #define SomeString "Example"
    </code> -->
    <code cppqt>
    // Corretto!
    static const char UnaStringa[] = "Esempio";
    // Sbagliato!
    static const char* UnaStringa = "Esempio";
    // Sbagliato!
    #define UnaStringa "Esempio"
    </code>
    <!-- === Forward Declarations === -->
    === Dichiarazioni anticipate ===
    <!-- You will reduce compile times by forward declaring classes when possible instead of including their respective headers. For example: -->
    Ridurrai i tempi di compilazione dichiarando anticipatamente le classi quando possibile invece di includere i rispettivi headers. Per esempio:
    <!-- <code cppqt>
    #include <QWidget>    // slow
    #include <QStringList> // slow
    #include <QString>    // slow
    class SomeInterface
    {
    public:
        virtual void widgetAction( QWidget *widget ) =0;
        virtual void stringAction( const QString& str ) =0;
        virtual void stringListAction( const QStringList& strList ) =0;
    };
    </code> -->
    <code cppqt>
    #include <QWidget>    // lento
    #include <QStringList> // lento
    #include <QString>    // lento
    class QualcheInterfaccia
    {
    public:
        virtual void azioneWidget( QWidget *widget ) =0;
        virtual void azioneStringa( const QString& str ) =0;
        virtual void azioniListaStringhe( const QStringList& strLista ) =0;
    };
    </code>
    <!-- The above should instead be written like this: -->
    Dovrebbe invece essere scritto in questo modo:
    <!-- <code cppqt>
    class QWidget;    // fast
    class QStringList; // fast
    class QString;    // fast
    class SomeInterface
    {
    public:
        virtual void widgetAction( QWidget *widget ) =0;
        virtual void stringAction( const QString& str ) =0;
        virtual void stringListAction( const QStringList& strList ) =0;
    };
    </code> -->
    <code cppqt>
    class QWidget;    // veloce
    class QStringList; // veloce
    class QString;    // veloce
    class QualcheInterfaccia
    {
    public:
        virtual void azioneWidget( QWidget *widget ) =0;
        virtual void azioneStringa( const QString& str ) =0;
        virtual void azioniListaStringhe( const QStringList& strLista ) =0;
    };
    </code>
    <!-- === Iterators === -->
    === Iteratori ===
    <!-- ==== Prefer const iterators and cache end() ==== -->
    <! --Prefer to use <tt>const_iterators</tt> over normal iterators when possible. Containers, which are being implicitly shared often detach when a call to a non-const <tt>begin()</tt> or <tt>end()</tt> methods is made ({{qt|QList}} is an example of such a container). When using a const_iterator also watch out that you are really calling the const version of <tt>begin()</tt> and <tt>end()</tt>. Unless your container is actually const itself this probably will not be the case, possibly causing an unnecessary detach of your container. So basically whenever you use const_iterator initialize them using <tt>constBegin()</tt>/<tt>constEnd()</tt> instead, to be on the safe side. -->
    ==== Preferire iteratori costanti e conservare end() ====
    Preferire l'uso dei <tt>const_iterators</tt> rispetto ai normali iteratori quando possibile. I containers, implicitamente condivisi, spesso
    Cache the return of the <tt>end()</tt> (or <tt>constEnd()</tt>) method call before doing iteration over large containers. For example:
    <code cppqt>
    QValueList<SomeClass> container;
    //code which inserts a large number of elements to the container
    QValueListConstIterator end( container.constEnd() );
    for ( QValueListConstIterator itr( container.constBegin() );
        itr != end; ++itr ) {
    }
    </code>
    This avoids the unnecessary creation of the temporary <tt>end()</tt> (or <tt>constEnd()</tt>) return object on each loop iteration, largely speeding it up.
    Prefer to use pre-increment over post-increment operators on iterators as this avoids creating an unnecessary temporary object in the process.
    ====Take care when erasing elements inside a loop====
    When you want to erase some elements from the list, you maybe would use code similar to this:
    <code cppqt>
    QMap<int, Job *>::iterator it = m_activeTimers.begin();
    QMap<int, Job *>::iterator itEnd = m_activeTimers.end();
    for( ; it!=itEnd ; ++it )
    {
        if(it.value() == job)
        {
            //A timer for this job has been found. Let's stop it.
            killTimer(it.key());
            m_activeTimers.erase(it);
        }
    }
    </code>
    This code will potentially crash because it is a dangling iterator after the call to erase().
    You have to rewrite the code this way:
    <code cppqt>
    QMap<int, Job *>::iterator it = m_activeTimers.begin();
    while (it != m_activeTimers.end())
    {
        QMap<int, Job *>::iterator prev = it;
        ++it;
        if(prev.value() == job)
        {
            //A timer for this job has been found. Let's stop it.
            killTimer(prev.key());
            m_activeTimers.erase(prev);
        }
    }
    </code>
    This problem is also discussed in the [http://doc.trolltech.com/4.3/qmap-iterator.html#details Qt documentation for QMap::iterator] but applies to '''all''' Qt iterators
    === memory leaks ===
    A very "popular" programming mistake is to do a <tt>new</tt> without a <tt>delete</tt> like in this program:
    '''mem_gourmet.cpp'''
    <code cppqt>
    class t
    {
      public:
        t() {}
    };
    void pollute()
    {
      t* polluter = new t();
    }
    int main()
    {
      while (true) pollute();
    }
    </code>
    You see, ''pollute()'' instanciates a new object ''polluter'' of the class ''t''. Then, the variable ''polluter'' is lost because it is local, but the content (the object) stays on the heap. I could use this program to render my computer unusable within 10 seconds.
    To solve this, there are the following approaches:
    * keep the variable on the stack instead of the heap:
    <code cppqt>
      t* polluter = new t();
    </code>
    would become
    <code cppqt>
      t polluter();
    </code>
    * delete polluter using the complementing function to new:
    <code cppqt>
      delete polluter;
    </code>
    A tool to detect memory leaks like this is [[Development/Tools/Valgrind|Valgrind]].
    === dynamic_cast ===
    You can only dynamic_cast to type T from type T2 provided
    that:
    * T is defined in a library you link to (you'd get a linker error if this isn't the case, since it won't find
    the vtable or RTTI info)
    * T is "well-anchored" in that library. By "well-anchored" I mean that the vtable is not a COMMON symbol subject to merging at run-time by the dynamic linker. In other words, the first virtual member in the class definition must exist and not be inlined: it must be in a .cpp file.
    * T and T2 are exported
    For instance, we've seen some hard-to-track problems in non-KDE C++ code we're linking with (I think NMM) because of that. It happened that:
    * libphonon loads the NMM plugin
    * NMM plugin links to NMM
    * NMM loads its own plugins
    * NMM's own plugins link to NMM
    Some classes in the NMM library did not have well-anchored vtables, so dynamic_casting failed inside the Phonon NMM plugin for objects created in the NMM's own plugins.
    == Program Design ==
    In this section we will go over some common problems related to the design of Qt/KDE applications.
    === Delayed Initialization ===
    Although the design of modern C++ applications can be very complex, one recurring problem, which is generally easy to fix, is not using the technique of [http://www.kdedevelopers.org/node/view/509 delayed initialization].
    First, let us look at the standard way of initializing a KDE application:
    <code cppqt>
    int main( int argc, char **argv )
    {
        ....
        KApplication a;
        KCmdLineArgs *args = KCmdLineArgs::parsedArgs();
        MainWindow *window = new MainWindow( args );
        a.setMainWidget( window );
        window->show();
        return a.exec();
    }
    </code>
       
    Notice that <tt>window</tt> is created before the <tt>a.exec()</tt> call that starts the event loop. This implies that we want to avoid doing anything non-trivial in the top-level constructor, since it runs before we can even show the window.
    The solution is simple: we need to delay the construction of anything besides the GUI until after the event loop has started. Here is how the example class MainWindow's constructor could look to achieve this:
    <code cppqt>
    MainWindow::MainWindow()
    {
        initGUI();
        QTimer::singleShot( 0, this, SLOT(initObject()) );
    }
    void MainWindow::initGUI()
    {
        /* Construct your widgets here.  Note that the widgets you
        * construct here shouldn't require complex initialization
        * either, or you've defeated the purpose.
        * All you want to do is create your GUI objects and
        * QObject::connect
        * the appropriate signals to their slots.
        */
    }
    void MainWindow::initObject()
    {
        /* This slot will be called as soon as the event loop starts.
        * Put everything else that needs to be done, including
        * restoring values, reading files, session restoring, etc here.
        * It will still take time, but at least your window will be
        * on the screen, making your app look active.
        */
    }
    </code>
     
    Using this technique may not buy you any overall time, but it makes your app ''seem'' quicker to the user who is starting it. This increased perceived responsiveness is reassuring for the user as they get quick feedback that the action of launching the app has succeeded.
    When (and only when) the start up can not be made reasonably fast enough, consider using a {{class|KSplashScreen}}.
    == Data Structures ==
    In this section we will go over some of our most common pet-peeves which affect data structures very commonly seen in Qt/KDE applications.
    === Passing non-POD types ===
    Non-POD ("plain old data") types should be passed by const reference if at all possible. This includes anything other than the basic types such as <tt>char</tt> and <tt>int</tt>.
    Take, for instance, {{qt|QString}}. They should always be passed into methods as <tt>const {{qt|QString}}&</tt>. Even though {{qt|QString}} is implicitly shared it is still more efficient (and safer) to pass const references as opposed to objects by value.
    So the canonical signature of a method taking QString arguments is:
    <code cppqt>
    void myMethod( const QString & foo, const QString & bar );
    </code>
    === QObject ===
    If you ever need to delete a QObject derived class from within one of its own methods, do not ever delete it this way:
    <code cppqt>
      delete this;
    </code>
    This will sooner or later cause a crash because a method on that object might be invoked from the Qt event loop via slots/signals after you deleted it.
    Instead always use <tt>{{QtMethod|QObject|deleteLater}}</tt> which tries to do the same thing as <tt>delete this</tt> but in a safer way.
    === Empty QStrings ===
    It is common to want to see if a {{qt|QString}} is empty. Here are three ways of doing it, the first two of which are correct:
    <code cppqt>
    // Correct
    if ( mystring.isEmpty() ) {
    }
    // Correct
    if ( mystring == QString() ) {
    }
    // Wrong! ""
    if ( mystring == "" ) {
    }
    </code>
    While there is a distinction between "null" {{qt|QString}}s and empty ones, this is a purely historical artifact and new code is discouraged from making use of it.
    === QString and reading files ===
    If you are reading in a file, it is faster to convert it from the local encoding to Unicode ({{qt|QString}}) in one go, rather than line by line. This means that methods like <tt>{{qt|QIODevice}}::readAll()</tt> are often a good solution, followed by a single {{qt|QString}} instantiation.
    For larger files, consider reading a block of lines and then performing the conversion. That way you get the opportunity to update your GUI. This can be accomplished by reentering the event loop normally, along with using a timer to read in the blocks in the background, or by creating a local event loop.
    While one can also use <tt>qApp->processEvents()</tt>, it is discouraged as it easily leads to subtle yet often fatal problems.
    === Reading QString from a KProcess ===
    {{class|KProcess}} emits the signals <tt>readyReadStandard{Output|Error}</tt> as data comes in.
    A common mistake is reading all available data in the connected slot and converting it to {{qt|QString}} right away: the data comes in arbitrarily segmented chunks, so multi-byte characters might be cut into pieces and thus invalidated. Several approaches to this problem exist:
    <ul>
    <li>Do you really need to process the data as it comes in? If not, just use <tt>readAllStandard{Output|Error}</tt> after the process has exited. Unlike in KDE3, KProcess is now able to accumulate the data for you.</li>
    <li>Wrap the process into a {{qt|QTextStream}} and read line-wise. This should work starting with Qt 4.4.</li>
    <li>Accumulate data chunks in the slots and process them each time a newline arrives or after some timeout passes. [http://websvn.kde.org/trunk/KDE/kdevplatform/util/processlinemaker.cpp?view=markup Example code]</li>
    </ul>
    === QString and QByteArray ===
    While {{qt|QString}} is the tool of choice for many string handling situations, there is one where it is particularly inefficient. If you are pushing about and working on data in {{qt|QByteArray}}s, take care not to pass it through methods which take {{qt|QString}} parameters; then make QByteArrays from them again.
    For example:
    <code cppqt>
    QByteArray myData;
    QString myNewData = mangleData( myData );
    QString mangleData( const QString& data ) {
        QByteArray str = data.toLatin1();
        // mangle
        return QString(str);
    }
    </code>
       
    The expensive thing happening here is the conversion to {{qt|QString}}, which does a conversion to Unicode internally. This is unnecessary because, the first thing the method does is convert it back using <tt>toLatin1()</tt>. So if you are sure that the Unicode conversion is not needed, try to avoid inadvertently using QString along the way.
    The above example should instead be written as:
    <code cppqt>
    QByteArray myData;
    QByteArray myNewData = mangleData( myData );
    QByteArray mangleData( const QByteArray& data )
    </code>
    === QDomElement ===
    When parsing XML documents, one often needs to iterate over all the elements. You may be tempted to use the following code for that:
    <code cppqt>
    for ( QDomElement e = baseElement.firstChild().toElement();
          !e.isNull();
          e = e.nextSibling().toElement() ) {
          ...
    }
    </code>
     
    That is not correct though: the above loop will stop prematurely when it encounters a {{qt|QDomNode}} that is something other than an element such as a comment.
    The correct loop looks like:
    <code cppqt>
    for ( QDomNode n = baseElement.firstChild(); !n.isNull();
          n = n.nextSibling() ) {
        QDomElement e = n.toElement();
        if ( e.isNull() ) {
            continue;
        }
        ...
    }
    </code>

    Latest revision as of 13:10, 27 November 2008