Read array of custom metatype from QSettings

Question

I have a problem with reading custom metatype data from QSetting. I have a class:

class MusicOwner
{
public:
    MusicOwner() :
        songs_count(0),
        id(0)
    {}

    explicit MusicOwner(const Song &owner_radio);
    Song toOwnerRadio() const;

    static QList<MusicOwner> parseMusicOwnerList(const QVariant &request_result);

private:
    friend QDataStream &operator <<(QDataStream &stream, const VkService::MusicOwner &val);
    friend QDataStream &operator >>(QDataStream &stream, VkService::MusicOwner &val);
    friend QDebug operator<< (QDebug d, const MusicOwner &owner);

    int songs_count;
    int id;
    QString name;
    QString screen_name;
    QUrl photo;
};

Q_DECLARE_METATYPE(VkService::MusicOwner)

with overloaded:

QDataStream &operator <<(QDataStream &stream, const VkService::MusicOwner &val)
{
    stream << val.id;
    stream << val.name;
    stream << val.songs_count;
    stream << val.screen_name;
    return stream;
}

QDataStream &operator >>(QDataStream &stream, VkService::MusicOwner &val)
{
    stream >> val.id;
    stream >> val.name;
    stream >> val.songs_count;
    stream >> val.screen_name;
    return stream;
}

QDebug operator <<(QDebug d, const VkService::MusicOwner &owner)
{
    d << "MusicOwner("
      << owner.id << ","
      << owner.name << ","
      << owner.songs_count << ","
      << owner.screen_name << ")";
    return d;
}

and somewhere at program's start I call:

qRegisterMetaTypeStreamOperators<MusicOwner>("MusicOwner");

To read and write I use two functions: Saving works fine, all data that I saving appearing in settings file.

void VkService::SaveBookmarks()
{
    TRACE;
    QSettings s;
    s.beginGroup(kSettingGroup);

    s.beginWriteArray("bookmarks");
    int index = 0;
    for (int i = 0; i < root_item_->rowCount(); ++i){
        auto item = root_item_->child(i);
        if (item->data(InternetModel::Role_Type).toInt() == Type_Bookmark){
            Song song = item->data(InternetModel::Role_SongMetadata).value<Song>();
            s.setArrayIndex(index);
            MusicOwner owner(song);
            qLog(Info) << "Save" << index << ":" << owner;
            s.setValue("owner", QVariant::fromValue(owner));
            ++index;
        }
    }
    s.endArray();
}

Problems with this function, It's load count correctly, but it's loading empty default constructed items.

void VkService::LoadBookmarks()
{
    QSettings s;
    s.beginGroup(kSettingGroup);

    int max = s.beginReadArray("bookmarks");
    for (int i = 0; i < max; ++i){
        s.setArrayIndex(i);
        MusicOwner owner = s.value("owner").value<MusicOwner>();
        qLog(Info) << "Load" << i << ":" << owner;
        AppendBookmarkFromRadio(root_item_, owner.toOwnerRadio());
    }
    s.endArray();
}

I rewrote it to new project for testing, but it works FINE. I spend two hour to find out, why this variant reading data correctly but first variant not. Owner in first variant converting correctly. Even data in settings file in both variants the same.

#include <QCoreApplication>
#include <QDataStream>
#include <QDebug>
#include <QUrl>
#include <QSettings>
#include <QVariant>

class MusicOwner
{
public:
    MusicOwner() :
        songs_count(0),
        id(0)
    {}

private:
    friend QDataStream &operator <<(QDataStream &stream, const MusicOwner &val);
    friend QDataStream &operator >>(QDataStream &stream, MusicOwner &val);
    friend QDebug operator<< (QDebug d, const MusicOwner &owner);

public:
    int songs_count;
    int id;
    QString name;
    QString screen_name;
    QUrl photo;
};


QDataStream &operator <<(QDataStream &stream, const MusicOwner &val)
{
    stream << val.id;
    stream << val.name;
    stream << val.songs_count;
    stream << val.screen_name;
    return stream;
}

QDataStream &operator >>(QDataStream &stream, MusicOwner &val)
{
    stream >> val.id;
    stream >> val.name;
    stream >> val.songs_count;
    stream >> val.screen_name;
    return stream;
}

QDebug operator <<(QDebug d, const MusicOwner &owner)
{
    d << "MusicOwner("
      << owner.id << ","
      << owner.name << ","
      << owner.songs_count << ","
      << owner.screen_name << ")";
    return d;
}

Q_DECLARE_METATYPE(MusicOwner)

const QString kSettingGroup = "Group";

void Save() {
    QSettings s;
    s.beginGroup(kSettingGroup);

    s.beginWriteArray("bookmarks");
    int index = 0;
    for (int i = 0; i < 100; ++i){
        if (random() % 5 == 0) {
            s.setArrayIndex(index);
            MusicOwner owner;
            owner.id = i;
            owner.name ="Hello world";
            owner.songs_count = i * 2;
            owner.screen_name = "hello_world";

            s.setValue("owner", QVariant::fromValue(owner));
            qDebug() << "Saved" << i << ":" << owner;
            ++index;
        }
    }
    s.endArray();
    qDebug() << "Saved" << index << "elements";
}

void Load() {
    QSettings s;
    s.beginGroup(kSettingGroup);

    int max = s.beginReadArray("bookmarks");
    qDebug() << "To load" << max << "elements";

    for (int i = 0; i < max; ++i){
        s.setArrayIndex(i);
        MusicOwner owner = s.value("owner").value<MusicOwner>();
        qDebug() << "\tLoaded" << i << ":" << owner;
    }
    s.endArray();
}

int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);
    qRegisterMetaTypeStreamOperators<MusicOwner>("MusicOwner");
    QSettings s;

    Load();
    Save();

    return a.exec();
}

Maybe you can find difference in work with QSettings in this variants?

score 1 · Answer 1 · answered Aug 26 '13 at 14:46

You are not careful at all with your namespaces in the example. You can't make a "minimal case" that doesn't use namespaces if you do, in fact, use them in your non-working code.

The following compileable example uses namespaces and works fine in both Qt 4.8 and 5.1. Note that those settings are not portable by default between 4.8 and 5.1, I don't know if that's a bug or a feature.

#include <QCoreApplication>
#include <QDataStream>
#include <QDebug>
#include <QSettings>
#include <QVariant>

namespace VkService {

class MusicOwnerA
{
    friend QDataStream &operator <<(QDataStream &stream, const MusicOwnerA &val);
    friend QDataStream &operator >>(QDataStream &stream, MusicOwnerA &val);
    friend QDebug operator<< (QDebug d, const MusicOwnerA &owner);
public:
    MusicOwnerA() : id(0) {}
    int id;
    QString name;
};

QDataStream &operator <<(QDataStream &stream, const VkService::MusicOwnerA &val)
{
    stream << val.id;
    stream << val.name;
    return stream;
}

QDataStream &operator >>(QDataStream &stream, VkService::MusicOwnerA &val)
{
    stream >> val.id;
    stream >> val.name;
    return stream;
}

QDebug operator <<(QDebug d, const VkService::MusicOwnerA &owner)
{
    d << "VkService::MusicOwnerA("
      << owner.id << ","
      << owner.name << ")";
    return d;
}

}

Q_DECLARE_METATYPE(VkService::MusicOwnerA)

void Save() {
    QSettings s;
    s.beginWriteArray("bookmarks");
    int index = 0;
    for (int i = 0; i < 100; ++i){
        if (random() % 5 == 0) {
            s.setArrayIndex(index);
            VkService::MusicOwnerA owner;
            owner.id = i;
            owner.name ="Hello world";
            s.setValue("owner", QVariant::fromValue(owner));
            qDebug() << "Saved" << i << ":" << owner;
            ++index;
        }
    }
    s.endArray();
    qDebug() << "Saved" << index << "elements";
}

void Load() {
    QSettings s;
    int max = s.beginReadArray("bookmarks");
    qDebug() << "To load" << max << "elements";
    for (int i = 0; i < max; ++i){
        s.setArrayIndex(i);
        VkService::MusicOwnerA owner = s.value("owner").value<VkService::MusicOwnerA>();
        qDebug() << "\tLoaded" << i << ":" << owner;
    }
    s.endArray();
}

int main(int argc, char **argv)
{
    QCoreApplication a(argc, argv);
    a.setOrganizationDomain("16549302.stackoverflow.com");
    qRegisterMetaTypeStreamOperators<VkService::MusicOwnerA>("VkService::MusicOwnerA");
    Load();
    Save();
}

Thank you for your answer, but I found solution. All metatypes are registering in external method as it turned out. When I add needed lines to it, it worked. For the future what should I do in this case on stackoverflow? I don't found button 'delete question'. — shed, Aug 28 '13 at 19:13
Well, the reasons to flag a question for deletion are rather limited. Yours doesn't really qualify. — Kuba hasn't forgotten Monica, Aug 28 '13 at 20:04
@shed When you've found the answer by yourself, you can create an answer by your own describing how you've solved it — Random Citizen, Nov 10 '13 at 09:18

Read array of custom metatype from QSettings

1 Answers1