从QSettings中读取自定义元类型数组

Question

我有一个从QSetting读取自定义元类型数据的问题。我有一个类:

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)

与重载:

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;
}

在程序开始的某处我调用:

qRegisterMetaTypeStreamOperators<MusicOwner>("MusicOwner");

要读和写，我使用两个函数:保存工作正常，我保存的所有数据都出现在设置文件中。

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();
}

这个函数的问题，加载计数正确，但是加载的是空的默认构造项

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();
}

我将它重写到新项目中进行测试，但它工作良好。我花了两个小时来找出，为什么这个变体正确读取数据而第一个变体不正确。第一个变量的所有者正确转换。即使两个变体的设置文件中的数据相同。

#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();
}

也许你可以在这些变体中找到与QSettings工作的差异?

Answer 1

在这个示例中，您对名称空间一点也不小心。如果您在非工作代码中使用了名称空间，那么您就不能做出不使用名称空间的"最小情况"。

下面的可编译示例使用名称空间，并且在Qt 4.8和5.1中都工作良好。请注意，这些设置在4.8和5.1之间默认是不可移植的，我不知道这是一个bug还是一个特性。

#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();
}