Using QDataStream to serialize custom class causes C2679 error

Question

I'm writing an application in which I need serialization to store some data in files. For serialization I want to use the QDataStream class.

I can't compile my code due to this compiler error:

Error 1 error C2679: binary '<<' : no operator found which takes a right-hand operand of type 'const CListItem' (or there is no acceptable conversion) c:\qt\4.8.6\src\corelib\io\qdatastream.h 265

See the relevant code below. Anyone knows what's going on here?

There is a similar question which didn't help me. When I follow the steps described there, I get this (similar) error:

Error 1 error C2678: binary '<<' : no operator found which takes a left-hand operand of type 'QDataStream' (or there is no acceptable conversion) c:\qt\4.8.6\src\corelib\io\qdatastream.h 265

This is my problem:

What I want to serialize is a class called CMPProject.

CMPProject holds

CListModel* m_pData;
QDateTime m_dateTimeCreated;

This is basically what should be serialized.

CMPProject has operators to stream its contents into a QDataStream.

MPProject.h:

#ifndef _MPPROJECT_
#define _MPPROJECT_

#include <QtCore/QString>
#include <QtCore/QFile>
#include <QtCore/QDateTime>

#include "ListModel.h"

class CMPProject
{
public:
    // ...

    friend QDataStream& operator <<(QDataStream& stream, const CMPProject& project);
    friend QDataStream& operator >>(QDataStream& stream, CMPProject& project);

private:
    static const quint32 m_streamHeader = 0x1329453;
    QFile* m_pFile;
    CListModel* m_pData;
    QDateTime m_dateTimeCreated;
};

#endif // _MPPROJECT_

The data stream operators in MPProject.cpp:

QDataStream& operator <<(QDataStream& stream, const CMPProject& project)
{
    return stream << project.m_dateTimeCreated << *(project.m_pData);
}

QDataStream& operator >>(QDataStream& stream, CMPProject& project)
{
    return stream >> project.m_dateTimeCreated >> *(project.m_pData);
}

m_pData is of type CListModel. CListModel contains the actual data stored as a QList<CListItem>.

To serialize the CListModel I added the according operators in ListModel.h:

#ifndef _LISTMODEL_
#define _LISTMODEL_

#include <QtCore/QAbstractListModel>
#include <QtCore/QList>
#include <QtCore/QStringList>

#include "ListItem.h"

typedef QMap<unsigned int, QString> TValueMap;

class CListModel : public QAbstractListModel
{
public:
    // ...

    template<typename T>
    friend void operator <<(QVariant& data, const QList<T>& target);

    template<typename T>
    friend void operator >>(const QVariant& data, QList<T>& target);

    friend QDataStream& operator <<(QDataStream& stream, const CListModel& listModel);
    friend QDataStream& operator >>(QDataStream& stream, CListModel& listModel);

private:    

    QList<CListItem> m_items;
};

#endif // !_LISTMODEL_

and ListModel.cpp:

template<typename T>
void operator <<(QVariant& data, const QList<T>& target)
{
    QVariantList list;
    list.reserve(target.count());
    for (int i = 0; i < target.count(); i++) {
        QVariant item;
        item << target[i];
        list.append(item);
    }
    data = list;
}

template<typename T>
void operator >>(const QVariant& data, QList<T>& target)
{
    QVariantList list = data.toList();
    target.reserve(list.count());
    for (int i = 0; i < list.count(); i++) {
        T item;
        list[i] >> item;
        target.append(item);
    }
}

QDataStream& operator <<(QDataStream& stream, const CListModel& listModel)
{
    // ERROR C2679 does not occur when I change this line to "return stream;"
    return stream << listModel.m_items;
}

QDataStream& operator >>(QDataStream& stream, CListModel& listModel)
{
    return stream >> listModel.m_items;
}

In order to serialize the contents of listModel.m_items (objects of type CListItem) I implemented the according operators in ListItem.h:

#ifndef _LISTITEM_
#define _LISTITEM_

#include <QtCore/QString>
#include <QtCore/QStringList>
#include <QtCore/QDateTime>
#include <QtCore/QVariantMap>

class CListItem
{
public:
    // ...

    friend void operator <<(QVariant& data, const CListItem& target);
    friend void operator >>(const QVariant& data, CListItem& target);
private:
    QString m_name;
    QString m_domain;
    QString m_login;
    QString m_password;
    QDateTime m_LastModified;
};

#endif // !_LISTITEM_

and ListItem.cpp:

template<typename T>
void operator <<(QVariant& data, const T& target)
{
    data = QVariant::fromValue<T>(target);
}

template<typename T>
void operator >>(const QVariant& data, T& target)
{
    target = data.value<T>();
}

void operator <<(QVariant& data, const CListItem& target)
{
    QVariantMap map;
    map["name"] << target.m_name;
    map["domain"] << target.m_domain;
    map["login"] << target.m_login;
    map["password"] << target.m_password;
    map["dateModified"] << target.m_LastModified;
    data << map;
}

void operator >>(const QVariant& data, CListItem& target)
{
    QVariantMap map;
    data >> map;
    map["name"] >> target.m_name;
    map["domain"] >> target.m_domain;
    map["login"] >> target.m_login;
    map["password"] >> target.m_password;
    map["dateModified"] >> target.m_LastModified;
}

Have you been able to determine which line of your code is causing this? — LogicStuff, Nov 22 '15 at 17:24
@LogicStuff I edited my question to highlight the line. It's in _ListModel.cpp_. — Exa, Nov 22 '15 at 17:36

score 2 · Accepted Answer · answered Nov 22 '15 at 20:17

I fixed it.

The problem was that I didn't have operators to put my QList into a QDataStream. But I defined wrappers to store my list in QVariant, so (of course) I just should have used those.

QDataStream& operator <<(QDataStream& stream, const CListModel& listModel)
{
    QVariant var;
    var << listModel.m_items;
    return stream << var;
}

QDataStream& operator >>(QDataStream& stream, CListModel& listModel)
{
    QVariant var;
    stream >> var;

    var >> listModel.m_items;

    return stream;
}

score 1 · Answer 2 · edited May 23 '17 at 12:04

It looks like here is no operator<< overloaded for arguments: QDataStream & and QList<CListItem> const&. In the reference here and here, there aren't any documented.

The second error addresses this, but there also should be QList<CListItem> argument mentioned. Was that the full error message?

Basically, you should be using this:

QDataStream& operator <<(QDataStream& stream, const CListModel& listModel)
{
    for(auto const& item : listModel.m_items)
        stream << item; // write elements one by one

    return stream;
}

This template might serve you for multiple types (or something akin to Pretty-print C++ STL containers):

template <typename Stream, typename Containter>
Stream& operator <<(Stream& stream, const Containter& container)
{
    for(auto const& item : container)
        stream << item;

    return stream;
}

Your code would be valid without making change to operator<< you defined. Hope that helps.

Using QDataStream to serialize custom class causes C2679 error

2 Answers2