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.