QT：錯誤重載 「<<」 運營商在Qdatastream

Question

我有一個類

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

}; 

我創建了一個QHash>和< <運營商我的課。

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; 
} 

但即使是現在，我仍然得到一個錯誤：

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: error: no match for 'operator<<' in 'operator<<(((QDataStream&)((QDataStream*)out)), ((const QString&)((const QString*)it.QHash::const_iterator::key with Key = QString, T = budget))) << it.QHash::const_iterator::value with Key = QString, T = budget'

這究竟是爲什麼？顯然>>操作符似乎超載，我得到的錯誤僅適用於< <操作符。

謝謝。

Answer 1

該錯誤源自函數簽名的聲明。

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

到：從改變你的聲明

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

同樣的問題描述here

對不起，我已經錯過了一兩件事。方法也必須聲明爲朋友。所以我試了一下，這裏是工作結果;

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

要回答你關於友元函數問題，讓我指出你已經可以here