Serialization Example

[ofmrew]

I am trying to serialize my class but I cannot find an example. I tried the code below, but I get that it does not define a type.

#include <QObject>
#include <QDataStream>
#include <QRectF>
#include <QLineF>
#include <QDebug>

class MyClass : public QObject
{
    Q_OBJECT
public:
    explicit MyClass(QObject *parent = nullptr);

    QDataStream &MyClass;::operator <<(QDataStream &out, const MyClass &mc){
        out << mc.br << mc.ln;
        return out;

    }
    QDataStream &MyClass;::operator >>(QDataStream &in, const MyClass &mc){
        in >> mc.br >> mc.ln;
        return in;
    }
    QRectF br;
    QLineF ln;

signals:

public slots:
};

[SGaist]

Hi,

Declare the stream operators in the same file but outside of your class and implement them in the implementation file from your class.

By the way, you have a semicolon between MyClass and the two colon.

[ofmrew]

@SGaist As for the semicolon, that came from an example "http://www.bogotobogo.com/Qt/Qt5_QFile_Serialization_Class.php".

class MyClass : public QObject
{
    Q_OBJECT
public:
    explicit MyClass(QObject *parent = nullptr);


    QRectF br;
    QLineF ln;

signals:

public slots:
};
QDataStream &MyClass;::operator <<(QDataStream &out, const MyClass &mc){
    out << mc.br << mc.ln;
    return out;

}
QDataStream &MyClass;::operator >>(QDataStream &in, const MyClass &mc){
    in >> mc.br >> mc.ln;
    return in;
}

That did not work.

[ofmrew]

@ofmrew Forgot to remove the semicolons. With them removed I get:

/home/xxx/SerializationExperiments/myclass.h:24: error: ‘QDataStream& MyClass::operator<<(QDataStream&, const MyClass&)’ must take exactly one argument
 QDataStream &MyClass::operator <<(QDataStream &out, const MyClass &mc){
                                                                      ^

[SGaist]

That's a typo, remove it.

By the way, the operator>> takes a reference, not a const reference. Otherwise you won't be able to update the content of the object you pass.

[ofmrew]

@SGaist

QDataStream &MyClass::operator <<(QDataStream &out, MyClass &mc){
    out << mc.br << mc.ln;
    return out;

}
QDataStream &MyClass::operator >>(QDataStream &in, MyClass &mc){
    in >> mc.br >> mc.ln;
    return in;
}

/home/xxx/SerializationExperiments/myclass.h:24: error: ‘QDataStream& MyClass::operator<<(QDataStream&, MyClass&)’ must take exactly one argument
 QDataStream &MyClass::operator <<(QDataStream &out, MyClass &mc){
                                                                ^

It dtill complains about more than one argument. How do I fix that?

[SGaist]

Remove MyClass;:: from the declaration.

It's shown in the Reading And Writing Other Qt Classes part of QDataStream's documentation.

[ofmrew]

@SGaist Did the "http://www.bogotobogo.com/Qt/Qt5_QFile_Serialization_Class.php" ever work? That is the only example I could find to use as a model.

QDataStream &operator <<(QDataStream &out, MyClass &mc);

Compiles, but do I need to put the implementation in the cpp file? If so, what is the format?

[SGaist]

Copy the methods as they are in the implementation file and just leave the declaration in the header.

[ofmrew]

@SGaist That compiled. Not to testing. Thanks.

[ofmrew]

@ofmrew The status after the write to datastream is 0, but the >> does not appear to work. I added MyClass to a vector and tried to serialize it, but it failed to return the vector from the stream. A breakpoint, in MyClass, on << is activated, but one on >> is not.

class MyCanvas : public QWidget
{
    Q_OBJECT
public:
    explicit MyCanvas(QWidget *parent = nullptr);

    void create();
    void store();
    void load();

    QVector<MyClass> cell;
    QDataStream stream;


MyCanvas::MyCanvas(QWidget *parent) : QWidget(parent)
{

}

void MyCanvas::create()
{
//    QVector<MyClass>cell;

    MyClass line1(QRectF(50.0, 50.0, 200.0, 200.0), QLineF(50.0, 50.0, 250.0, 250.0));
    cell.append(line1);
    MyClass line2(QRectF(250.0, 50.0, 200.0, 200.0), QLineF(250.0, 250.0, 500.0, 50.0));
    cell.append(line2);
}

void MyCanvas::store()
{
//    stream = new QDataStream();
    stream<<cell;
    qDebug() << "status" << stream.status();
//    qDebug() << stream;
}

void MyCanvas::load()
{
//    cell2 = new QVector<MyClass>();
    cell.clear();
    stream>>cell;
    qDebug() << cell.size();
//    qDebug() << cell;
}
class MyClass
{
public:
    explicit MyClass();
    MyClass(QRectF r, QLineF l);
    void addBoundingRect(QRectF);
    void addLine(QLineF);


    QRectF br;
    QLineF ln;

signals:

public slots:
};
QDataStream &operator <<(QDataStream &out, const MyClass &mc);
QDataStream &operator >>(QDataStream &in, MyClass &mc);

MyClass::MyClass() {}
MyClass::MyClass(QRectF r, QLineF l) : br{r}, ln{l}
{

}

void MyClass::addBoundingRect(QRectF r)
{
    br = r;
}
QDataStream &operator <<(QDataStream &out, const MyClass &mc){
    out << mc.br << mc.ln;
    return out;

}
QDataStream &operator >>(QDataStream &in, MyClass &mc){
    in >> mc.br >> mc.ln;
    return in;
}

```Is it failing to create MyClass?

[ofmrew]

@ofmrew Ignore the last post: Like an idiot I missed the fact that QDataStream is a file. Thanks for the help.

[ofmrew]

@ofmrew I tested the serialization both out and in. Now I am trying to serialize QMap<int, QVector<PixmapTile>> levelMap;. The out seemed to work, but on in it is having a problem with QVector<PixmapTile>.

/home/xxx/Qt/5.10.0/gcc_64/include/QtCore/qdatastream.h:242: error: no match for ‘operator>>’ (operand types are ‘QDataStream’ and ‘QVector<PixmapTile>::value_type {aka PixmapTile}’)
         s >> t;
 QDataStream &operator<<(QDataStream &s, const PixmapTile &pixmt);
QDataStream &operator<<(QDataStream &s, PixmapTile &pixmt);
QDataStream &operator<<(QDataStream &s, QVector<PixmapTile> &vofpt);

Am I asking too much?

[SGaist]

IIRC, you don't need to implement the QVector version because it should be automatically handled as you implemented the operators for your custom data type.

There might be a typo you only have operator<< declared in your last code sample.

[ofmrew]

@SGaist Good eyes.

QDataStream &operator>>(QDataStream &s, PixmapTile &pixmt);
//QDataStream &operator<<(QDataStream &s, QVector<PixmapTile> &vofpt);

Got it to compile. Copy and paste can bit you. Thanks.

[ofmrew]

@ofmrew I checked it with the debugger and it looks good. I am impressed.

[VRonin]

@ofmrew said in Serialization Example:

QDataStream &operator<<(QDataStream &s, QVector<PixmapTile> &vofpt);

As mentioned by @SGaist you don't need to implement the operator for QVector<PixmapTile>. Qt already has template operators for QMap and QVector you just need to implement QDataStream &operator>>(QDataStream &s, PixmapTile &pixmt); and QDataStream &operator<<(QDataStream &s, const PixmapTile &pixmt);. the rest is for free