QVector serialization / deserialization

RahibeMeryem

Hi,

I have a Qvector :

struct enrolment {
        QString person_name;
        QPixmap person_foto;
        QString category;
    };

QVector<enrolment> enrolment_db;

And populating the enrolment_db vector around 1000 item.

I want to save file and get back from file to QVector again.

I tried:

QFile personel_file("/data/personelVec.dat");
        personel_file.open(QIODevice::WriteOnly);
        QDataStream out_personelVec(&personel_file);   // we will serialize the data into the file
        out_personelVec.setVersion(QDataStream::Qt_5_10);
        out_personelVec << enrolment_Db;

for serialization but gives error.

 invalid operands to binary expression ('QDataStream' and 'const typename QVector<enrolment>::value_type' (aka 'const enrolment'))

What is the proper way to serialize / deserialize such QVector<enrolment> enrolment_db . ?

examole appreciated.

Best

SGaist

Hi,

You have implement the QDataStream operators for your enrolment struct. See here.

RahibeMeryem

struct enrolment {
        QString person_name;
        QPixmap person_foto;
        QString category;
    };
QFile file ("/data/personel.ser");
            QDataStream & operator<< (QDataStream& stream, const enrolment& enrolment);
            
            QDataStream out(&file);
            out.setVersion(QDataStream::Qt_5_10);
            out << enrolment_Db;

is this the way ?

SGaist

Since I don't know where you put what in your source tree I can't say yes for sure.

RahibeMeryem

I searched the internet and couldn't find a similar working example .

if somebody kindly make an example it could be useful for new beginners.

struct enrolment {
        QString person_name;
        QPixmap person_foto;
        QString category;
    };

    QVector <enrolment> enrolment_Db;

//add 1000 item to the enrolment_Db

serialize  enrolment_Db to file
deserilaize from file.

I am lack of QDatastream deep knowledge. appreciated for example.

mrjj

Hi
Lucky its not super complex.

in .h file
struct enrolment {
  QString person_name;
  QPixmap person_foto;
  QString category;
};

QDataStream& operator <<(QDataStream& out, const enrolment Entry);
QDataStream& operator >>(QDataStream& in, enrolment Entry);

and in .cpp

// save
QDataStream& operator <<(QDataStream& out, const enrolment Entry) {
   out << Entry.person_name;
  out << Entry.person_foto;
  out << Entry.category;
  return out;
}

// load. MUST BE SAME ORDER as when saved
QDataStream& operator >>(QDataStream& in, enrolment &Entry) { 
  in >> Entry.person_name;
  in >> Entry.person_foto;
  in >> Entry.category;
  return in;
}

RahibeMeryem

it gave:

error: overloaded 'operator<<' must be a binary operator (has 3 parameters)
    QDataStream& operator <<(QDataStream& out, const enrolment Entry);

for

QDataStream& operator <<(QDataStream& out, const enrolment Entry);
QDataStream& operator >>(QDataStream& in, enrolment &Entry);

mrjj

@RahibeMeryem
Can you show the actual code ?
My compiler had zero issues.

That errors come if you put them inside class. like with friend.
did you add just like in sample ?
under class?

RahibeMeryem

I put the private section of the header below :

in widget.h file
struct enrolment {
  QString person_name;
  QPixmap person_foto;
  QString category;
};

QDataStream& operator <<(QDataStream& out, const enrolment Entry);
QDataStream& operator >>(QDataStream& in, enrolment &Entry);

in the widget.cpp :

void Widget::on_select_folder_enrolment_clicked()
{
 // save
QDataStream& operator <<(QDataStream& out, const enrolment Entry) {
   out << Entry.person_name;
  out << Entry.person_foto;
  out << Entry.category;
  return out;
}

// load. MUST BE SAME ORDER as when saved
QDataStream& operator >>(QDataStream& in, enrolment &Entry) { 
  in >> Entry.person_name;
  in >> Entry.person_foto;
  in >> Entry.category;
  return in;
}
....
....

mrjj

@RahibeMeryem

ohhh o.O
Ok. do not put them inside the functions.
have them outside as a function on its own.
it allows you to do

QFile personel_file("/data/personelVec.dat");
        personel_file.open(QIODevice::WriteOnly);
        QDataStream out_personelVec(&personel_file);   // we will serialize the data into the file
        out_personelVec.setVersion(QDataStream::Qt_5_10);
        out_personelVec << enrolment_Db;

as you tried before as we now gave it a << to be used
out_personelVec << enrolment_Db;

so its


QDataStream& operator <<(QDataStream& out, const enrolment Entry) {
... not showing code. but dont delete !
}

QDataStream& operator >>(QDataStream& in, enrolment &Entry) { 
...
}


void Widget::on_select_folder_enrolment_clicked()
{
QFile personel_file("/data/personelVec.dat");
        personel_file.open(QIODevice::WriteOnly);
        QDataStream out_personelVec(&personel_file);   // we will serialize the data into the file
        out_personelVec.setVersion(QDataStream::Qt_5_10);
        out_personelVec << enrolment_Db;
}

RahibeMeryem

:))

Thanks a lot .. I am math and deep learning expert.. most of the my time spending non main subject related struggling like this.

Qt is Wonderfull but needs deep knowledge and operational expertise which is require time and facing problems.

Best

mrjj

@RahibeMeryem
well operator overloading ( << and >> are operators ) is somewhat
intermediate level.

Just for the fullness of this post.

You could have done
out_personelVec << enrolment_Db. person_name << enrolment_Db. person_foto ;
since it know how to stream QString and pixmap already.

However, the operator overloading is smart if you later do
struct SomeOther {
enrolment en;
};

if you then overload for "SomeOther"
then
SomeOther test;
out << test;
will just work as it calls << for enrolment
so it allows for deep nesting of Types, each with the ability to save it self.

SGaist

One small thing, it should be:

QDataStream& operator >>(QDataStream& in, enrolment& Entry);

Otherwise there's no way to get the data back in the caller function.

mrjj

@SGaist
Good catch. Thank you. Updated sample.
and remember to change both in .h and in .cpp. (RahibeMeryem)

RahibeMeryem

Serialization is ok ;

deserialization gives below error:

/Users/alpullu/Qt/5.11.0/clang_64/lib/QtCore.framework/Headers/qdatastream.h:243: error: use of overloaded operator '>>' is ambiguous (with operand types 'QDataStream' and 'typename QVector<enrolment_>::value_type' (aka 'enrolment_'))
        s >> t;
        ~ ^  ~

// load. MUST BE SAME ORDER as when saved
QDataStream& operator >>(QDataStream& in, enrolment_& Entry) {
  in >> Entry.person_name;
  in >> Entry.person_foto;
  in >> Entry.category;
  in >> Entry.face_indexed_status;
  return in;
}

mrjj

@RahibeMeryem

Please always show code as you being unused to Qt / c++ might
give situation where you did something in code we would never think of.

RahibeMeryem

I exactly copied your deserialization example :

// load. MUST BE SAME ORDER as when saved
QDataStream& operator >>(QDataStream& in, enrolment_& Entry) {
  in >> Entry.person_name;
  in >> Entry.person_foto;
  in >> Entry.category;
  in >> Entry.face_indexed_status;
  return in;
}

honestly didnt understand why its not working . :(

mrjj

@RahibeMeryem
hi
but does the error point to this ?
I assumed it was where you used it.
as in where you try to load in the file again.
Can you show that code ?

RahibeMeryem

the error is in this line : in_personelVec >> enrolment_Db;
gives:
/Users/xx/Qt/5.11.0/clang_64/lib/QtCore.framework/Headers/qdatastream.h:243: error: use of overloaded operator '>>' is ambiguous (with operand types 'QDataStream' and 'typename QVector<enrolment_>::value_type' (aka 'enrolment_'))
s >> t;
~ ^ ~

void Widget::load_enrollment_if_exist()
{
    bool fileExists = QFileInfo::exists(enrollment_path) && QFileInfo(enrollment_path).isFile();
    if (fileExists){
        QFile personel_file(enrollment_path);
                personel_file.open(QIODevice::ReadOnly);
                QDataStream in_personelVec(&personel_file);   // we will deserialize the data into the file
                in_personelVec.setVersion(QDataStream::Qt_5_10);
                in_personelVec >>  enrolment_Db;  ///////////THE ERROR is HERE
                qDebug() << " enrolment yüklendi: , personel sayısı: " << enrolment_Db.size()  ;
    }

}

// save
QDataStream& operator <<(QDataStream& out, const enrolment_ Entry) {
  out << Entry.person_name;
  out << Entry.person_foto;
  out << Entry.category;
  out << Entry.face_indexed_status;
  return out;
}

// load. MUST BE SAME ORDER as when saved
QDataStream& operator >>(QDataStream& in, enrolment_& Entry) {
  in >> Entry.person_name;
  in >> Entry.person_foto;
  in >> Entry.category;
  in >> Entry.face_indexed_status;
  return in;
}

RahibeMeryem

Do we need to write QVector<enrolment_> size ? because it has 1000 elements ?