QT: Error Overloading the "<<" operator in Qdatastream

Question

I have a class

class budget
{
    float transportation, grocery, food, stationery;
    QString key;
public:
//input output functions here.

};

I created a QHash> and << operators for my class.

QDataStream &operator <<(QDataStream &stream, budget &myclass)
{
    stream<<myclass.getFood();
    stream<<myclass.getGrocery();
    stream<<myclass.getKey();
    stream<<myclass.getStatn();
    stream<<myclass.getTransport();

    return stream;
}

QDataStream &operator >>(QDataStream &stream, budget &myclass)
{
    float f;
    QString s;
    stream>>f;
    myclass.addFood(f);
    stream>>f;
    myclass.addGrocery(f);
    stream>>s;
    myclass.addDate(s);
    stream>>f;
    myclass.addStatn(f);
    stream>>f;
    myclass.addTransport(f);

    return stream;
}

But even now i still get an error:

C:\Users\Karthik\QT\Mendrive-build-simulator-Simulator_Qt_for_MinGW_4_4__Qt_SDK__Debug........\QtSDK\Simulator\Qt\mingw\include\QtCore\qdatastream.h:381: error: no match for 'operator<<' in 'operator<<(((QDataStream&)((QDataStream*)out)), ((const QString&)((const QString*)it.QHash::const_iterator::key with Key = QString, T = budget))) << it.QHash::const_iterator::value with Key = QString, T = budget'

Why is this happening? Apparently the >> operator seems to be overloaded i get the error only for the << operator.

Thanks.

score 3 · Accepted Answer · edited May 23 '17 at 12:20

3

The error stems from the declaration of the function signatures. Change your declarations from:

QDataStream &operator <<(QDataStream &stream, budget &myclass);
QDataStream &operator >>(QDataStream &stream, budget &myclass);

to:

QDataStream &operator <<(QDataStream &stream, const budget &myclass);
QDataStream &operator >>(QDataStream &stream, budget &myclass);

The same problem has described here

Sorry I have missed one thing. Methods must be declares as friends also. So I have tried it out and here is working result;

class budget
{
  float transportation, grocery, food, stationery;
  QString key;
public:
  budget() {}

friend QDataStream &operator <<(QDataStream &stream, const budget &myclass) {

      stream<< myclass.food;
      stream<< myclass.grocery;
      stream<< myclass.key;
      stream<< myclass.stationery;
      stream<< myclass.transportation;

    return stream;

}
friend QDataStream &operator >>(QDataStream &stream, budget &myclass) {

    stream >> myclass.food;
    stream >> myclass.grocery;
    stream >> myclass.key;
    stream >> myclass.stationery;
    stream >> myclass.transportation;
    return stream;

}

update

To answer your question regarding friend functions let me point you to the answer already available here

edited May 23 '17 at 12:20

Community

1
1

answered Feb 13 '12 at 08:51

Neox

2,000
13
12

I tried the solution above in the beginning, but that lead to 5 errors of the form: `C:\Users\Karthik\QT\Mendrive-build-simulator-Simulator_Qt_for_MinGW_4_4__Qt_SDK__Debug\..\Mendrive\fileoverload.h:11: error: passing 'const budget' as 'this' argument of 'float budget::**getFood()**' discards qualifiers` Where **getFood()** is replaced by by other member functions. That's why i tried removing the `const` – Airmine Feb 13 '12 at 10:12
yea thats because the funcs have to return const values. – Neox Feb 13 '12 at 10:27
1

Yes! The declaring it as a friend function did it! Could you please explain though, how it made a difference? I still can't understand why it wasn't working as a non-friend function. Thanks a lot! – Airmine Feb 13 '12 at 10:58
You really should learn how to use 'const', look up a tutorial somewhere and read it. Maybe this http://www.parashift.com/c++-faq-lite/const-correctness.html . change your "float budget::getFood();" method to "float budget::getFood() const;" – 0xbaadf00d Feb 13 '12 at 11:04

QT: Error Overloading the "<<" operator in Qdatastream

1 Answers1