c++ - Qt 序列化，非成员 QDataStream & operator<<

Question

我有一个 IDocument 类，它用作某些类的接口(interface)。它有一些抽象方法(virtual ... = 0)。

我想这样做，所有子类也必须实现一个序列化运算符:

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:

我什至不确定如何创建抽象运算符，但如何定义它为非成员？

Answer 1

非成员运算符是一个自由函数，与任何其他自由函数非常相似。对于 QDataStream , 关于 operator<<看起来像:

QDataStream& operator<<(QDataStream& ds, SomeType const& obj)
{
  // do stuff to write obj to the stream
  return ds;
}

在您的情况下，您可以像这样实现序列化(这只是一种方法，还有其他方法):

#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;
}