Qt serialization, nonmember QDataStream & operator<< - c++

Question

I have a class IDocument which serve as a interface for some classes. It has some abstracts methods (virtual ... = 0).
I would like to do such all subclasses also have to implement an operator for serialization:
In addition to the overloaded stream operators documented here, any Qt classes that you might want to serialize to a QDataStream will have appropriate stream operators declared as non-member of the class:
I'm not even sure how I would make an abstract operator, but how do I define it nonmember?

Answer 1

A non-member operator is a free function, pretty much like any other free function. For QDataStream, on operator<< would look like:
QDataStream& operator<<(QDataStream& ds, SomeType const& obj)
{
// do stuff to write obj to the stream
return ds;
}
In your case, you could implement your serialization like this (this is just one way of doing it, there are others):
#include <QtCore>
class Base {
public:
Base() {};
virtual ~Base() {};
public:
// This must be overriden by descendants to do
// the actual serialization I/O
virtual void serialize(QDataStream&) const = 0;
};
class Derived: public Base {
QString member;
public:
Derived(QString const& str): member(str) {};
public:
// Do all the necessary serialization for Derived in here
void serialize(QDataStream& ds) const {
ds << member;
}
};
// This is the non-member operator<< function, valid for Base
// and its derived types, that takes advantage of the virtual
// serialize function.
QDataStream& operator<<(QDataStream& ds, Base const& b)
{
b.serialize(ds);
return ds;
}
int main()
{
Derived d("hello");
QFile file("file.out");
file.open(QIODevice::WriteOnly);
QDataStream out(&file);
out << d;
return 0;
}