Writing property that contains QList (problem converting to QVariant)

Lotte

Hello,

The following question applies to Qt 6. I'm trying to load properties from an xml file into a class using the QMetaProperty::write() function. To simplify the example I left out the xml part and just hardcoded some values that I would like to write to the properties in this class. I'm using QMetaProperty::write() instead of the set functions because it allows me to easily loop over all properties in a class. This works perfectly, except for two types: a list of a list of points (QList<QList<QPointF>>) and a list of rectangles (QList<QRectF>). Somehow Qt is unable to convert these types into a QVariant, even though I used Q_DECLARE_METATYPE(QList<QList<QPointF>>) and Q_DECLARE_METATYPE(QList<QRectF>). How can I solve this problem? Minimal example below.

listexample.h

#ifndef LISTEXAMPLE_H
#define LISTEXAMPLE_H

#include <QMetaProperty>
#include <QRect>

Q_DECLARE_METATYPE(QList<QList<QPointF>>);
Q_DECLARE_METATYPE(QList<QRectF>);

class ListExample : public QObject
{
    Q_OBJECT
    Q_PROPERTY(QList<QList<QPointF>> series MEMBER m_series READ getSeries WRITE setSeries NOTIFY seriesChanged)
    Q_PROPERTY(QList<QRectF> axisLims MEMBER m_axisLims READ getAxisLims WRITE setAxisLims NOTIFY axisLimsChanged)

public:
    explicit ListExample();

    QList<QList<QPointF>> m_series;
    QList<QRectF> m_axisLims;

    Q_INVOKABLE QList<QList<QPointF>> getSeries() const;
    Q_INVOKABLE void setSeries(const QList<QList<QPointF>> series);
    Q_INVOKABLE QList<QRectF> getAxisLims() const;
    Q_INVOKABLE void setAxisLims(const QList<QRectF> axisLims);

    Q_INVOKABLE void deserializeObject();

signals:
    void seriesChanged(QList<QList<QPointF>> m_series);
    void axisLimsChanged(QList<QRectF> m_axisLims);
};

#endif // LISTEXAMPLE_H

listexample.cpp

#include "listexample.h"

ListExample::ListExample() : QObject() { }

QList<QList<QPointF>> ListExample::getSeries() const
{
    return m_series;
}

void ListExample::setSeries(const QList<QList<QPointF>> series)
{
    if (series == m_series) return;
    m_series = series;
    emit seriesChanged(m_series);
}

QList<QRectF> ListExample::getAxisLims() const
{
    return m_axisLims;
}

void ListExample::setAxisLims(const QList<QRectF> axisLims)
{
    if (axisLims == m_axisLims) return;
    m_axisLims = axisLims;
    emit axisLimsChanged(m_axisLims);
}

void ListExample::deserializeObject()
{
    const QMetaObject* metaObject = this->metaObject();
    int numProperties = metaObject->propertyCount();
    for (int propertyNr=0; propertyNr<numProperties; propertyNr++)
    {
        QMetaProperty property = metaObject->property(propertyNr);
        QString propertyType = QString(property.typeName()).remove(" ");
        QVariant propertyValue; // Normally loaded from xml, but hardcoded for example
        if (propertyType == "QList<QList<QPointF>>")
        {
            QList<QList<QPointF>> series({{QPointF(1,2), QPointF(3,4)}, {QPointF(5,6), QPointF(7,8)}});
            propertyValue = QVariant(series); // DOES NOT COMPILE!
            propertyValue = series; // DOES NOT COMPILE!
        }
        else if (propertyType == "QList<QRectF>")
        {
            QList<QRectF> axisLims({QRectF(1,2,3,4), QRectF(5,6,7,8)});
            propertyValue = QVariant(axisLims); // DOES NOT COMPILE!
            propertyValue = axisLims; // DOES NOT COMPILE!
        }
        if (!property.write(this, propertyValue))
        {
            QString propertyName(property.name());
            QString className(metaObject->className());
            qDebug() << "Unable to write property " + propertyName + " to class " + className;
        }
    }
}

CMakeLists.txt

cmake_minimum_required(VERSION 3.0 FATAL_ERROR)

project(listexample)

set(CMAKE_INCLUDE_CURRENT_DIR ON)

set(CMAKE_AUTOUIC ON)
set(CMAKE_AUTOMOC ON)
set(CMAKE_AUTORCC ON)

set(CMAKE_CXX_STANDARD 11)
set(CMAKE_CXX_STANDARD_REQUIRED ON)

find_package(Qt6 REQUIRED COMPONENTS Core)

add_executable(listexample ${GUI_TYPE} listexample.h listexample.cpp)

target_compile_definitions(listexample PRIVATE $<$<OR:$<CONFIG:Debug>,$<CONFIG:RelWithDebInfo>>:QT_QML_DEBUG>)

target_link_libraries(listexample PRIVATE Qt${QT_VERSION_MAJOR}::Core)

ivan.solovev

@Lotte said in Writing property that contains QList (problem converting to QVariant):

propertyValue = QVariant(series); // DOES NOT COMPILE!

Hi.
This would not compile because QVariant does not have a constructor overload that takes a QList<QList<QPoint>> or QList<QRectF> as a parameter.

I believe you need to do like that:

propertyValue = QVariant::fromValue(series);

Also, as mentioned above, consider passing parameters to the setter methods by constant reference, i.e.:

Q_INVOKABLE void setAxisLims(const QList<QRectF> &axisLims); // note the '&'

SGaist

Hi and welcome to devnet,

Did you register them as well ?

Lotte

Adding the following two lines to the ListExample constructor does not seem to make a difference. Code still does not compile. Any suggestions are very welcome...

qRegisterMetaType<QList<QList<QPointF>>>("QList<QList<QPointF>>");
qRegisterMetaType<QList<QRectF>>("QList<QList<QPointF>>");

JonB

@Lotte
Hi there. This may be totally unrelated to your current problem, so you may choose to ignore it for now. I also don't know anything about QML. But your definitions like:

void ListExample::setSeries(const QList<QList<QPointF>> series)
{
    if (series == m_series) return;

Because you have not used any &s for "reference" in QList<QList<QPointF>> series that means it will copy the QList<QList<QPointF>> passed in. And so even if you pass in m_series as actual parameter I believe if (series == m_series) will never be true. (You could check this to verify if I am correct.) Efficiency-wise you might want to rethink your parameter type here and in setAxisLims() etc.

Lotte

@JonB Thanks for the reply. I tested your suggestion by adding the following 2 functions to the ListExample class.

void ListExample::compareSeries(const QList<QList<QPointF>> series1, const QList<QList<QPointF>> series2)
{
    if (series1 == series2) qDebug() << "series are the same";
    else qDebug() << "series are different";
}

void ListExample::runTest()
{
    QList<QList<QPointF>> seriesA({{QPointF(1,2), QPointF(3,4)}, {QPointF(5,6), QPointF(7,8)}});
    QList<QList<QPointF>> seriesB({{QPointF(1,2), QPointF(3,4)}, {QPointF(5,6), QPointF(7,8)}});
    QList<QList<QPointF>> seriesC({{QPointF(8,7), QPointF(6,5)}, {QPointF(4,3), QPointF(2,1)}});
    compareSeries(seriesA, seriesB);
    compareSeries(seriesA, seriesC);
    compareSeries(seriesB, seriesC);
}

The output for runTest() is:

series are the same
series are different
series are different

So the check in if (series == m_series) seems to be working correctly. However, the code still does not compile when uncommenting the problematic lines

propertyValue = QVariant(series); // DOES NOT COMPILE!
propertyValue = series; // DOES NOT COMPILE!

and

propertyValue = QVariant(axisLims); // DOES NOT COMPILE!
propertyValue = axisLims; // DOES NOT COMPILE!

Any more ideas? I'm completely stuck so anything is welcome!

ivan.solovev

@Lotte said in Writing property that contains QList (problem converting to QVariant):

propertyValue = QVariant(series); // DOES NOT COMPILE!

Hi.
This would not compile because QVariant does not have a constructor overload that takes a QList<QList<QPoint>> or QList<QRectF> as a parameter.

I believe you need to do like that:

propertyValue = QVariant::fromValue(series);

Also, as mentioned above, consider passing parameters to the setter methods by constant reference, i.e.:

Q_INVOKABLE void setAxisLims(const QList<QRectF> &axisLims); // note the '&'

JonB

@Lotte said in Writing property that contains QList (problem converting to QVariant):

So the check in if (series == m_series) seems to be working correctly.

Just on that one. I forgot that QList has Qt's only-copy-on-write semantics, hence why it works.
But agree this is not related to your issue anyway.

Lotte

Thank you @ivan-solovev for the answer! It works!