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;
}