QTList Custom class Serlization

[saravanadel]

Hi,

I have a problem in qt list serialization.

I am getting error no match for operator '>>' in s '>>' t (from qdatastream.h). I also tried declaring meta but not use.



class CustomClass
{
QString name;
QDataStream &operator<<(QDataStream &out, const CustomClass& myClass)
{
out<<myClass.name;
return out;
}
QDataStream &operator>>(QDataStream& in,CustomClass& myClass)
{
in>>myClass.name;
return in;
}
}
Q_DECLARE_METATYPE(CustomClass*);

class CustomClass2
{
QList<CustomClass> list;
QDataStream &operator<<(QDataStream &out, const CustomClass2& myClass)
{
out<<myClass.list;
return out;
}
QDataStream &operator>>(QDataStream& in,CustomClass2& myClass)
{
in>>myClass.list;
return in;
}
};

Without the meta tag declaration 'Q_DECLARE_METATYPE(CustomClass*);'. it says no match for operator '>>' and '<<' myClass.list

Please help.


Development in,
Qt SDK for Nokia

[wysota]

Define the operators as standalone functions and not methods of your class.

[ChrisW67]

Without the meta tag declaration 'Q_DECLARE_METATYPE(CustomClass*);'. it says no match for operator '>>' and '<<' myClass.list

With the macro in place you will receive an error because CustomClass declaration is missing a terminating semi-colon.

After doing as wysota suggests you may also need to make the free operator functions friends of the class.

[saravanadel]

Define the operators as standalone functions and not methods of your class.

Still the same.

no match for 'operator >>' in 'in >> myClass.list'

I missed the semi-colon in the post.

Added after 41 minutes:


Define the operators as standalone functions and not methods of your class.

It worked when I declared them inline in h.

Secondly I cannot serialize the objects to the datastream.

QList<CustomClass*> list;
QDataStream out(&file);
out <<list; //works fine

QList<CustomClass*> ob;
QDataStream in(&file);
in >>ob; //no match for operator' >> in s >> t'

I more thing, I am inheriting CustomClass from QObject.

[ChrisW67]

Congratulations. Would you like us to guess what you have actually done? Clearly it wasn't what wysota and I suggested because I know that works.

Edit... since you added to your prior post:

In your header you should have a prototype for the operator>> and operator<< free functions.
In your cpp you should have the implementation. Just like you do for the classes.

You are probably streaming a series of pointer values to a store, not the content of the pointed-to objects. This does not bode well for recovering the objects. What do your actual streaming operators look like?

[saravanadel]

What do your actual streaming operators look like?

inline QDataStream& operator<<(QDataStream &out, const ContactModel& myClass)
inline QDataStream& operator>>(QDataStream& in,ContactModel& myClass)

[ChrisW67]

Those operators have nothing to do with your last example.

This does not do what you think it does:


QList<CustomClass*> list;
QDataStream out(&file);
out <<list; //works fine

Trace the execution and look at the actual data written to the file. Your streaming operator for CustomClass:


QDataStream &operator<<(QDataStream &out, const CustomClass& myClass)

is not being used. The pointers are being streamed as booleans.

When you have worked out why then you can tackle the second part with some understanding.

[saravanadel]

QList<CustomClass*> list;

Is there any way to write the values of the qlist rather than streaming the pointers.

If I declare 'QList<CustomClass>' it says Object is private, since the copying is not allowed. (But I can do this with inheriting QObject which I dont want)

[wysota]

If an operator for streaming CustomClass objects looks like this:

QDataStream &operator<<(QDataStream &out, const CustomClass& myClass)
Then what will an operator for streaming CustomClass* look like?

[ChrisW67]

Perhaps use the * operator to deference each pointer


out << list.size();
foreach(CustomClass* p, list)
out << *p;

[saravanadel]

Perhaps use the * operator to deference each pointer


out << list.size();
foreach(CustomClass* p, list)
out << *p;


This is how I am using now.

[ChrisW67]

Huh :confused: Nowhere in this thread have you done anything like that.
Your code:


QList<CustomClass*> list;
QDataStream out(&file);
out <<list; //works fine


My code:


QList<CustomClass*> list;
QDataStream out(&file);
out << list.size();
foreach(CustomClass* p, list)
out << *p;


Wysota's prompting would give you something that would make your code work.
My code gives you another way to do it using the operator<<() for CustomClass that you already have.

[wysota]

Effectively the operator would do more or less the same :)

Just a word of warning -- if you use the solution provided by Chris, you have to serialize size of the list as well otherwise while deserializing you won't know how many items to read.