Qt ListView doesn't show C++ model content

Question

I'm creating a QList<> in C++ using a QML ListView to display it. The application runs without errors but the ListView stubbornly remains empty.

The QML will show a rectangle for the presence of each list item. I checked the UI code by creating a list in QML. It displays correctly for the QML created list.

Here's my QML:

import Processes 1.0
...
ListView {
    id: qInterfaceList
    height: parent.height;
    width: parent.width;
    model: myModel
    orientation: ListView.Vertical
    delegate:
        Rectangle {
            height: 30;
            width: 120;
            border.color: "red"
            border.width: 3
        }

The C++ code that creates and registers the list object:

// Register C++ classes as a QML type named Processes (version 1.0)
qmlRegisterType<Process>("Processes", 1, 0, "Process");

QQmlApplicationEngine engine;

// read the configuration file
Config conf;
if ( conf.read() )
{
    QQmlContext* ctxt = engine.rootContext();
    if ( ctxt )
    {
        qDebug()
            << "--- conf.Interfaces: "
            << conf.Interfaces.length()
            ;
        ConfigInterface c;
        QVariant v = QVariant::fromValue( conf.Interfaces );
        qDebug()
            << "--- ConfigInterface: "
            << v
            << "--- typeName: "
            << v.typeName()
            ;
        ctxt->setContextProperty("myModel", QVariant::fromValue( conf.Interfaces ));
    }
}

engine.load(QUrl(QStringLiteral("qrc:/main.qml")));
if (engine.rootObjects().isEmpty())
    return -1;
return app.exec();

To debug I output information about list from C++ and QML: In C++ the count of list items is correct. In C++ the conversion to QVariant is working. In QML it sees the defined list.

The debugging output:

Debugging starts
--- conf.Interfaces:  65
--- ConfigInterface:  QVariant(QList<ConfigInterface*>, ) --- typeName:  QList<ConfigInterface*>
qml: myModel: QVariant(QList<ConfigInterface*>)
Debugging has finished

Any ideas what's wrong or how to debug this?

Thanks

EDIT: Here's the class being used as a list item

Class declaration:

class ConfigInterface : public QObject
{
    Q_OBJECT

    Q_PROPERTY(QString sql READ getTag WRITE setTag NOTIFY tagChanged)
    Q_PROPERTY(QString description READ getDescription WRITE setDescription NOTIFY descriptionChanged)
public:
    /*explicit*/ ConfigInterface();
    /*explicit*/ ConfigInterface(QObject *parent);
    ~ConfigInterface();

    // Copy constructor needed because these are copied when added to a QList
    ConfigInterface(const ConfigInterface &p2) {_tag = p2._tag; _description = p2._description; }

    QString getDescription() const;
    void setDescription(QString&);

    QString getTag() const;
    void setTag(QString&);

signals:
    void tagChanged(QString);
    void descriptionChanged(QString);

public:
    QString _tag;
    QString _description;
    QString QueryTemplate;
    QString ConnectString;
    QString MinimumId;
};

Q_DECLARE_METATYPE(ConfigInterface*)

C++ code:

ConfigInterface::ConfigInterface()
    : QObject( nullptr )
{
}

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

ConfigInterface::~ConfigInterface()
{
}

QString ConfigInterface::getTag() const
{
    return _tag;
}
void ConfigInterface::setTag(QString& str)
{
    _tag = str;
    emit tagChanged(_tag);
}

Edited. The class code is shown at the bottom of the post. Thanks for helping :) — Jay
– Jay, Commented Jan 20, 2018 at 20:03

eyllanesc · Accepted Answer · 2018-01-21 14:15:46Z

1

The main problem is caused because it has a list of ConfigInterface *, according to the examples provided in the documentation should be a list of QObject *:

class Config{
    [...]
public:
    QList<QObject *> Interfaces;
    [...]
};

In addition to this you should get the following warning:

/..../configinterface.h:17: warning: base class ‘class QObject’ should be explicitly initialized in the copy constructor [-Wextra]
     ConfigInterface(const ConfigInterface &p2) {_tag = p2._tag; _description = p2._description; }
     ^~~~~~~~~~~~~~~

This is caused because QObject and its derived classes must not have a copy constructor or assignment operator, For more information read the following:

http://doc.qt.io/qt-5/qobject.html#no-copy-constructor-or-assignment-operator

Another improvement is that both constructors can be united in only one, in the end their class could have the following structure:

configinterface.h

#ifndef CONFIGINTERFACE_H
#define CONFIGINTERFACE_H

#include <QObject>

class ConfigInterface : public QObject
{
    Q_OBJECT

    Q_PROPERTY(QString sql READ getTag WRITE setTag NOTIFY tagChanged)
    Q_PROPERTY(QString description READ getDescription WRITE setDescription NOTIFY descriptionChanged)
public:
    ConfigInterface(QObject *parent=Q_NULLPTR);
    ~ConfigInterface();

    QString getTag() const;
    void setTag(const QString &tag);
    QString getDescription() const;
    void setDescription(const QString &description);

signals:
    void tagChanged(QString);
    void descriptionChanged(QString);

private:
    QString _tag;
    QString _description;
    QString QueryTemplate;
    QString ConnectString;
    QString MinimumId;
};

#endif // CONFIGINTERFACE_H

configinterface.cpp

#include "configinterface.h"

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

ConfigInterface::~ConfigInterface()
{
}

QString ConfigInterface::getDescription() const
{
    return _description;
}

void ConfigInterface::setDescription(const QString &description)
{
    if(_description == description)
        return;
    emit descriptionChanged(description);
    _description = description;
}

QString ConfigInterface::getTag() const
{
    return _tag;
}

void ConfigInterface::setTag(const QString &tag)
{
    if(tag == _tag)
        return;
    emit tagChanged(tag);
    _tag = tag;
}

edited Jan 21, 2018 at 14:15

answered Jan 20, 2018 at 22:59

eyllanesc

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4 Comments

Jay Over a year ago

My class is derived from QObject. The Qt examples show a QList of derived objects being used as a QML model. The docs for Q_DECLARE_METATYPE say "Any class or struct that has a public default constructor, a public copy constructor, and a public destructor can be registered". That conflicts with your statement I shouldn't have a copy constructor. I'm dubious but I'll certainly try your code.

eyllanesc Over a year ago

@Jay If the class inherits from QObject it is not necessary to use Q_DECLARE_METATYPE, also check this question stackoverflow.com/questions/47803531/…

eyllanesc Over a year ago

@Jay The complete example of my test can be found in github.com/eyllanesc/stackoverflow/tree/master/48349672

Jay Over a year ago

Woot! Thank you for all the effort. I'd give you two upvotes if I could

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