QT：重载 Qdatastream 中的“<<”运算符时出错

Question

我有课

class budget
{
    float transportation, grocery, food, stationery;
    QString key;
public:
//input output functions here.

};

我为我的班级创建了一个 QHash> 和

QDataStream &operator <<(QDataStream &stream, budget &myclass)
{
    stream<<myclass.getFood();
    stream<<myclass.getGrocery();
    stream<<myclass.getKey();
    stream<<myclass.getStatn();
    stream<<myclass.getTransport();

    return stream;
}

QDataStream &operator >>(QDataStream &stream, budget &myclass)
{
    float f;
    QString s;
    stream>>f;
    myclass.addFood(f);
    stream>>f;
    myclass.addGrocery(f);
    stream>>s;
    myclass.addDate(s);
    stream>>f;
    myclass.addStatn(f);
    stream>>f;
    myclass.addTransport(f);

    return stream;
}

但即使现在我仍然得到一个错误：

C:\Users\Karthik\QT\Mendrive-build-simulator-Simulator_Qt_for_MinGW_4_4__Qt_SDK__Debug........\QtSDK\Simulator\Qt\mingw\include\QtCore\qdatastream.h:381： 错误：'operator

为什么会这样？显然 >> 运算符似乎超载了我只得到

谢谢。

Answer 1

错误源于函数签名的声明。更改您的声明：

QDataStream &operator <<(QDataStream &stream, budget &myclass);
QDataStream &operator >>(QDataStream &stream, budget &myclass);

到：

QDataStream &operator <<(QDataStream &stream, const budget &myclass);
QDataStream &operator >>(QDataStream &stream, budget &myclass);

here描述了同样的问题

对不起，我错过了一件事。方法也必须声明为朋友。所以我已经尝试过了，这是工作结果；

class budget
{
  float transportation, grocery, food, stationery;
  QString key;
public:
  budget() {}

friend QDataStream &operator <<(QDataStream &stream, const budget &myclass) {

      stream<< myclass.food;
      stream<< myclass.grocery;
      stream<< myclass.key;
      stream<< myclass.stationery;
      stream<< myclass.transportation;

    return stream;

}
friend QDataStream &operator >>(QDataStream &stream, budget &myclass) {

    stream >> myclass.food;
    stream >> myclass.grocery;
    stream >> myclass.key;
    stream >> myclass.stationery;
    stream >> myclass.transportation;
    return stream;

}

更新

要回答您有关朋友功能的问题，请让我指出您已经提供的答案here