Copy QStringList pointer to QStringList



  • Hi,

    how can I make a copy of a QStringList pointer that is not a new pointer? I am learning by doing and even if I've been reading about pointers I am still very unsure of the concept.

    My goal is to dynamically update a QCompleter:

    void CodeEditor::scanForVariables(){
        QStringList customCompList = new QStringList(completerList); // <- this is not working because completerList is a pointer. If it is not a pointer other parths of my code will not work
    
        QRegularExpression variableExpression = QRegularExpression("\\$[a-zA-Z_\x7f-\xff][a-zA-Z0-9_\x7f-\xff]*");
        QRegularExpressionMatchIterator matches = variableExpression.globalMatch(this->toPlainText());
        while (matches.hasNext()) {
            QRegularExpressionMatch match = matches.next();
            customCompList << match.captured();
        }
    
        QRegularExpression functionExpression = QRegularExpression("(?<=function )[a-zA-Z_\x7f-\xff][a-zA-Z0-9_\x7f-\xff]*(?=\\s*\\()");
        matches = functionExpression.globalMatch(this->toPlainText());
        while (matches.hasNext()) {
            QRegularExpressionMatch match = matches.next();
            customCompList << match.captured();
        }
    
        completerModel->setStringList(customCompList);
    }
    

    My problem is that setStringList does not allow pointers.

    Thanks for any hints!


  • Lifetime Qt Champion

    Hi,

    The first question is: why are you using pointers to QStringList ?

    There's no need for that in your use case.



  • Hi,

    the completerList is used in different codeEditors. If it is not a pointer it will get copied over and over, right?

    Best



  • Hi,
    first of, @SGaist is correct, no need to use pointers here.

    But to answer your question, to copy your QStringlist you'll have to do either do a "deep copy" or copy each string and append it to your new list.

    I would recommend the later one.

    QStringList customCompList;
    for( QString s : completerList)
           customCompList.append(s);
    


  • Okay, this was quite a mess and a lot of try and error. Now I am not using pointers but the program crashes on the last line. "Segmentation error". The customCompList looks okay. The old and new values are there. Can I not do this:

    QStringList customCompList = completerList;
    

    And then append to it via <<? Will this mess up the list in the memory?


  • Lifetime Qt Champion

    If you pass your QStringList around using const references then no there won't be copies done and if you need really need to pass a pointer from time to time for a good reason, then just pass the address of your QStringList to that function.



  • Hi,

    I am new to the debugging tool. Here is what I get when I try to qDebug the list:

        for(QString s: customCompList)
            qDebug() << s;
    
    		__for_begin	@0x28c1b8	QList<QString>::iterator
    		__for_end	@0x28c1b4	QList<QString>::iterator
    			i	@0x28c1b4	QList<QString>::Node
    				v	0x123eb08 <vtable for QTextDocument+8>	void*
    		__for_range	<2818 Elemente>	QStringList &
    			[0]	"$GLOBALS"	QString
    			[...]
    			[2817]	"zlib_get_coding_type"	QString
    		customCompList	<2819 Elemente>	QStringList
    
    

    Why is the for range with 2818 elements one element short? The missing element is the one I added in the function.


  • Lifetime Qt Champion

    Where exactly are you calling that ?



  • I replaced the setStringList with the for loop. The output is displayed in the Qt Debug window (the right one with all the variables).



  • I got it. I needed to add

    completerModel = new QStringListModel();
    

    Prior setting the string list.


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