c++ - 向 Qt 对象添加流运算符

Question

是否可以将流运算符添加到现有的 Qt 对象中？我尝试在这个示例代码中这样做(对于 QFileSystemWatcher)，最后一行不编译。 LineUser 示例已编译。

Documentation for registering stream operators .

#include <QtCore>

class LineUser {
public:
    int uId;
    QString passwd;
    qint8 statusType;
};

Q_DECLARE_METATYPE(LineUser)
QDataStream &operator<<(QDataStream &out, const LineUser &myObj) {
    out<<myObj.uId<<myObj.passwd<<myObj.statusType;
    return out;
}
QDataStream &operator>>(QDataStream &in, LineUser &myObj) {
    in>>myObj.uId>>myObj.passwd>>myObj.statusType;
    return in;
}

QDataStream &operator<<(QDataStream &out, const QFileSystemWatcher &myObj) {
    return out;
}
QDataStream &operator>>(QDataStream &in, QFileSystemWatcher &myObj) {
    return in;
}

int main() {
    qRegisterMetaTypeStreamOperators<LineUser>("LineUser");
    qRegisterMetaTypeStreamOperators<QFileSystemWatcher>("QFileSystemWatcher");
}

编译器信息很长，这里有一些错误:

error: use of deleted function ‘QFileSystemWatcher::QFileSystemWatcher(const QFileSystemWatcher&)’
                 return new (where) T(*static_cast<const T*>(t));
note: ‘QFileSystemWatcher::QFileSystemWatcher(const QFileSystemWatcher&)’ is implicitly deleted because the default definition would be ill-formed:
  class Q_CORE_EXPORT QFileSystemWatcher : public QObject error: ‘QObject::QObject(const QObject&)’ is private
         Q_DISABLE_COPY(QObject) 
error: within this context  class Q_CORE_EXPORT QFileSystemWatcher : public QObject note: declared here
         Q_DISABLE_COPY(QObject)

Answer 1

来自 here :

Any class or struct that has a public default constructor, a public copy constructor and a public destructor can be registered.

来自here :

QObject has neither a copy constructor nor an assignment operator. This is by design.

显然，有一个设计限制阻止 QObject derived 被注册为元类型(这也是有意义的，因为它不是必需的，因为默认情况下会为它们生成元信息)，因此 qRegisterMetaTypeStreamOperators 也是不行的。

虽然它应该适用于 QFileSystemWatcher *。