[Solved]qml can not recognize the c++ parameters with &



  • pass two arguments into the c++ api from qml site
    when I add & on the api, the app can not recognize the signature
    of the function.

    .hpp
    @
    #ifndef TESTPARAM_HPP
    #define TESTPARAM_HPP

    #include <QList>
    #include <QObject>
    #include <QUrl>

    class testParam : public QObject
    {
    Q_OBJECT
    public:
    explicit testParam(QObject *parent = 0);

    signals:

    public slots:
    void test(QList<QUrl> &first, QList<QUrl> &second)
    {

    }
    

    };

    #endif // TESTPARAM_HPP

    @

    .cpp
    @
    #include "testParam.hpp"

    testParam::testParam(QObject *parent) :
    QObject(parent)
    {
    }

    @

    main.qml
    @
    import QtQuick 2.1
    import QtQuick.Dialogs 1.0

    import Test 1.0

    Rectangle {
    width: 100
    height: 62

    TestParam{
        id: testParam
    }
    
    FileDialog{
        id: fileDialog
        //selectFolder: param.selectFolder
        selectMultiple: true
        nameFilters: [ "Image files (*.bmp *.jpg *.JPEG *.png *.ppm *.tiff *.xbm *.xpm)" ]
        onAccepted: {
            testParam.test(fileDialog.fileUrls, fileDialog.fileUrls);
        }
    }
    
    MouseArea{
        anchors.fill: parent
    
        onClicked: {
            fileDialog.open();
        }
    }
    

    }
    @

    error message

    Error: Unknown method parameter type: QList<QUrl>&

    They are fine if
    1 : strip the &
    2 : reduce the parameters of "test" to a single parameter, that way you can preserve the &

    Anyway to pass the parameters as reference into the "test" with two parameters?


  • Moderators

    Data will be copied anyway, as JavaScript lists and QList are different. It does seem a bit strange, though, especially the part about using only one parameter.

    Try usign QVariant, maybe? And then getting the data using QVariant::value<>().



  • [quote author="sierdzio" date="1372142579"]
    Try usign QVariant, maybe? And then getting the data using QVariant::value<>().[/quote]

    QVariant can prevent the copy of the data?
    Could you show me an example?


  • Moderators

    It might, although it probably won't. I was rather thinking about allowing you to use 2 parameters.



  • Do we have a way to pass heavy data between qml and c++ without copy?


  • Moderators

    By using QObject pointers.

    For me, const QString references and QLists of pointers also do work. I don't know the internals of the engine well enough to answer your question with confidence.



  • [quote author="sierdzio" date="1372144978"]By using QObject pointers.

    For me, const QString references and QLists of pointers also do work. I don't know the internals of the engine well enough to answer your question with confidence.[/quote]

    Could you show me some example?


  • Moderators



  • Thanks for your help.

    Since a lot of heavy jobs would be done on c++ sites, I hope in the future we could
    pass the parameters between c++ and qml by reference.



  • Can't find a reasonable way to transform between string and QObject
    so I try const&

    @
    /**

    • @brief find out the different string of new_data compare with the old data
    • @param old_data : the old data
    • @param new_data : the new data
    • @return the combination of the old data and the different string of new_data compare with the old data
    • @example old_data = {1, 2, 3, 4}, new_data = {1, 3, 4, 5}, results = {1, 2, 3, 4, 5}
    • @caution : qml site could not recognize the parameters with &(even it can, it still copy the parameters rather than take the reference)
    •        pass the parameters by const& seems work, but I am not sure it would not copy the data by this way
      

    */
    QList<QString> fileProcess::unique_strs(QList<QString> const &old_data, QList<QString> const &new_data)
    {
    //awkward solution and maybe meaningless, hope that in the future this could be solved
    return set_difference_lazy(const_cast<QList<QString>&>(new_data), const_cast<QList<QString>&>(old_data));
    }
    @



  • After some experiment, the last post of my solution is useless, the data of
    qml and c++ are always copy


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