error: "'QObject::QObject(const QObject&)' is private" and "use of deleted function 'QObject::QObject(const QObject&)'"

[Lucas_1603]

Hi all,
I have a Song class like this:

#include <QObject>
class Song : public QObject
{
    Q_OBJECT
public:
    Song(const QString &title, const QString &singer, const QString &source, const QString &albumArt);
    explicit Song(QObject *parent = nullptr);
    Q_INVOKABLE QString title() const;
    QString singer() const;
    QString source() const;
    QString album_art() const;

private:
    QString m_title;
    QString m_singer;
    QString m_source;
    QString m_albumArt;
};

This returned some errors below:

'QObject::QObject(const QObject&)' is private within this context
use of deleted function 'QObject::QObject(const QObject&)'

Why I got these errors and how can I deal with it? Thanks for your help

Regard

[sharath]

hi @lucas_1603,

can we see your .cpp also?

Have you added your definition of explicit Song(QObject *parent = nullptr);
definition in .cpp?

[Lucas_1603]

@sharath
Yes, here it is:

#include <song.h>
Song::Song(QObject *parent) : QObject (parent)
{
    this->title() = "Hello World";
    this->singer() = "Dac Luc";
    this->source() = "lll";
    this->album_art() = "qrc:/Image/Hongkong1.png";
}
Song::Song(const QString &title, const QString &singer, const QString &source,  const QString &albumArt)
{
    m_title = title;
    m_singer = singer;
    m_source = source;
    m_albumArt = albumArt;
}

Q_INVOKABLE QString Song::title() const
{
    return m_title;
}

QString Song::singer() const
{
    return m_singer;
}

QString Song::source() const
{
    return m_source;
}

QString Song::album_art() const
{
    return m_albumArt;
}

[JonB]

@lucas_1603 said in error: "'QObject::QObject(const QObject&)' is private" and "use of deleted function 'QObject::QObject(const QObject&)'":

this->title() = "Hello World";
this->singer() = "Dac Luc";
this->source() = "lll";
this->album_art() = "qrc:/Image/Hongkong1.png";

Nope! Those are getter functions, hence the error message! You need to write setters too, as Qt does for all properties like these, to be called like:

    this->setTitle("Hello World");

[Lucas_1603]

@jonb
After added those setter functions and now my .cpp became like:

#include <song.h>
Song::Song(QObject *parent) : QObject (parent)
{
    this->setTitle("Hello world");
    this->setSinger("Dac Luc");
    this->setSource("lll");
    this->setAlbumArt("qrc:/Image/Hongkong1.png");
}
Song::Song(const QString &title, const QString &singer, const QString &source,  const QString &albumArt)
{
    m_title = title;
    m_singer = singer;
    m_source = source;
    m_albumArt = albumArt;
}

Q_INVOKABLE QString Song::title() const
{
    return m_title;
}

QString Song::singer() const
{
    return m_singer;
}

QString Song::source() const
{
    return m_source;
}

QString Song::album_art() const
{
    return m_albumArt;
}

But the errors are still the same

[jsulm]

@lucas_1603 Where exactly do you get the error?
The problem: QObject instances are NOT copyable.

[Lucas_1603]

@jsulm I think I'm getting trouble with Song class but actually I'm not sure about that because my project has other classes too. But I just see the errors in Song class now

[JonB]

@lucas_1603
I'm not an expert C++-er, but:

• Did you re-run qmake after adding the Q_OBJECT macro to the file?

• Song::Song(const QString &title, const QString &singer, const QString &source, const QString &albumArt)
  Does this constructor not need to invoke : QObject (parent) or your : Song(parent)? (I'm not sure about this one, but I'm thinking you need something like this?)

[Lucas_1603]

@jonb Thanks a lot, I'll try to find it out

[jsulm]

@lucas_1603 Your Song class is QObject based and thus not copyable. You're apparently trying to make a copy of it.
Double click at the first error line and see what is there.

[Lucas_1603]

@jsulm I don't know what you mean by said that I'm trying to make a copy of it. I actually didn't assign any ones to others!

[SGaist]

Hi,

Are you maybe using the copy constructor somewhere ?

[Lucas_1603]

@sgaist Nope, I don't use it in my project and I'm quite sure about that ^^.
I really want to know what does the error use of deleted function 'Song::Song(const Song&)' mean.

[SGaist]

It means that somewhere in your code you are using the copy constructor which is disabled since QObject objects cannot be copied.

[JKSH]

@lucas_1603 said in error: "'QObject::QObject(const QObject&)' is private" and "use of deleted function 'QObject::QObject(const QObject&)'":

Nope, I don't use it in my project and I'm quite sure about that ^^.

The copy constructor is invoked when you try to assign one Song object to another, for example:

Song s1("Happy Birthday to You", "Me", "/path/to/song", "/path/to/art");

Song s2(s1);  // copy-constructor invoked here
Song s3 = s1; // operator=() and copy-constructor invoked here

I really want to know what does the error use of deleted function 'Song::Song(const Song&)' mean.

It means your code contains something like s2 or s3 above.

• Song::Song(const Song&) is the copy-constructor
• A "deleted function" is a function which the creator has removed/deleted from the class -- QObjects are not allowed to have copy-constructors. See https://thispointer.com/c11-c14-delete-keyword-and-deleted-functions-with-use-cases-examples/ for more info

[Lucas_1603]

@jksh Yep, I got it. I realized what all you guys said.

So I change the approach to my project now, assume that I have a PlaylistModel class:

#include <QAbstractListModel>
class PlaylistModel : public QAbstractListModel
{
    Q_OBJECT

public:
    enum Roles {
        TitleRole = Qt::UserRole + 1,
        SingerRole,
        SourceRole,
        AlbumArtRole
    };

    explicit PlaylistModel(QObject *parent = nullptr);

    int rowCount(const QModelIndex &parent = QModelIndex()) const override;

    QVariant data(const QModelIndex &index, int role = Qt::DisplayRole) const override;
    void addSong(Song &song);

protected:
    QHash<int, QByteArray> roleNames() const override;

private:
    QList<Song> m_data;
};

Now I want to access m_data (is the private attribute) from QML file, how can I do that?

Thanks a lot for your help.

[JonB]

@lucas_1603
You can only access private variables in the class which defines them. So expose a getter/setter if you really need to. But since QAbstractListModel already has data() & setData() methods you would normally go via those.

[JKSH]

@lucas_1603 said in error: "'QObject::QObject(const QObject&)' is private" and "use of deleted function 'QObject::QObject(const QObject&)'":

QList<Song> m_data

Does that compile?

Store pointers-to-QObjects inside containers, rather than QObjects themselves.

[Lucas_1603]

@jksh Yes, it compiled.
@JonB As you said, I can access m_data via data() and setData() method. In data(), I have to assign QModelIndex &index and int role to it .
How I can use data() method with enum TitleRole?

[IntruderExcluder]

You should override roleNames method in your model:

QHash<int, QByteArray> PlaylistModel::roleNames() const
{
    static QHash<int, QByteArray>* ret = nullptr;
    if (ret)
        return *ret;

    ret = new QHash<int, QByteArray>();
    (*ret)[TitleRole] = "title";
    ...

    return *ret;
}

Then in QML you can use them as model property: model.title.

[Lucas_1603]

@intruderexcluder yes, I overrided it. When I use PlaylistModel as a model in QML, I actually just want to access m_data variable, not all of PlaylistModel attributes.
Any suggestions to do that? Thanks a lot

[IntruderExcluder]

You are probably doing something wrong, but you can at any time simply add slot or invokable method to access any data.

[JKSH]

@lucas_1603 said in error: "'QObject::QObject(const QObject&)' is private" and "use of deleted function 'QObject::QObject(const QObject&)'":

Yes, it compiled
...
I actually just want to access m_data variable, not all of PlaylistModel attributes.

Is Song still a subclass of QObject?

[Lucas_1603]

@jksh No, it's not now. I think I'm doing right way in this time ^^
The challenge now is accessing to m_data to get title, singer, source, album_art.

[JKSH]

@lucas_1603 said in error: "'QObject::QObject(const QObject&)' is private" and "use of deleted function 'QObject::QObject(const QObject&)'":

I think I'm doing right way in this time ^^

There's often more than 1 way to achieve something; some are just easier or more efficient than others :)

The challenge now is accessing to m_data to get title, singer, source, album_art.

I see that you've implemented a Model. The idea behind a model is that you shouldn't need to access the internal data structure directly. Instead, you use the model's standard interface (rows, columns, and maybe roles) to access individual fields.

In your example, you can treat each Row as an individual song. You can also treat each Column as a song field (e.g. Col 1 = Title, Col 2 = Singer, etc.). Older examples use Roles instead of Columns

In QML, you normally don't call data() and setData() directly; you let the View and Delegate call them for you.

Spend some time getting to know the model-view framework first:

• https://doc.qt.io/qt-5/qtquick-modelviewsdata-cppmodels.html
• https://doc.qt.io/qt-5/qtquick-models-objectlistmodel-example.html
  • https://code.qt.io/cgit/qt/qtdeclarative.git/tree/examples/quick/models/objectlistmodel?h=5.13

This is quite a lengthy topic, so it's not something that we can teach you over a few forum posts. You need to spend time going through examples and modifying them to see how they behave. trying things out.

[Lucas_1603]

@jksh Thank you so much
All you guys helped me a lot and I really appreciate it.
I'll spend more time to learn more about model-view framework to see how really they work.