通過基類指針訪問子類的成員函數

Question

我正在嘗試編寫一個類，該類使用DOM將註冊列表寫入XML文件。註冊清單包含三種類型的註冊Registration，這是標準基礎註冊類別GuestRegistration和StudentRegistration，這兩種註冊均來自Registration。

該GuestRegistration類有一個獨特的成員category和學生註冊有獨特的成員qualification。

當我迭代註冊指針列表時，我只能訪問基類Registration的成員函數。有什麼方法可以訪問子類的數據成員以使用吸氣功能getCategory和getQualification？我試着創建一個GuestRegistration和StudentRegistration指針，只要類名與兩者中的任何一個匹配，但得到指針轉換錯誤。

void RegistrationListWriter::write(RegistrationList r) { 
    QList<Registration*> regList = r.getRegistrationList(); 

    for (int i = 0; i < regList.size(); ++i) { 
     QString cn = regList.at(i)->metaObject()->className(); 
     Person tempPerson = regList.at(i)->getAttendee(); 

     appendRegistrationAndType(cn); 
     appendAttendee(tempPerson); 
     //this is where my issue starts 
     if (cn == "GuestRegistration") { 
      GuestRegistration guestReg = regList.at(i); 
      appendAttendeeCatagory(guestReg.getCatagory()); 
     } 

     if (cn == "StudentRegistration") { 
      StudentRegistration* stuReg = regList.at(i); 
      appendAttendeeQualification(stuReg->getQualification()); 
     } 

     appendBookingDate(regList.at(i)->getBookingDate().toString()); 
     appendRegistrationFee(regList.at(i)->calculateFee()); 

    } 
} 

Answer 1

您可以使用dynamic_cast檢查具體的子類：

void RegistrationListWriter::write(RegistrationList r) { 
    QList<Registration*> regList = r.getRegistrationList(); 

    for (int i = 0; i < regList.size(); ++i) { 
     Registration *reg = regList.at(i); 

     appendRegistrationAndType(reg->metaObject()->className()); 
     appendAttendee(reg->getAttendee()); 

     if (GuestRegistration *guestReg = dynamic_cast<GuestRegistration*>(reg)) { 
      appendAttendeeCatagory(guestReg->getCatagory()); 
     } 
     else 
     if (StudentRegistration* stuReg = dynamic_cast<StudentRegistration*>(reg)) { 
      appendAttendeeQualification(stuReg->getQualification()); 
     } 
     // and so on ... 

     appendBookingDate(reg->getBookingDate().toString()); 
     appendRegistrationFee(reg->calculateFee());  
    } 
} 

不過，我會建議在Registration類本身實現虛擬方法，你的子類可以覆蓋註冊其他項目，如需要如：

class Registration { 
    ... 
    virtual void appendExtraAttendees(RegistrationListWriter *writer){} 
    ... 
}; 

class GuestRegistration : public Registration { 
    ... 
    virtual void appendExtraAttendees(RegistrationListWriter *writer); 
    ... 
}; 

void GuestRegistration::appendExtraAttendees(RegistrationListWriter *writer){ 
    writer->appendAttendeeCatagory(getCatagory()); 
} 

class StudentRegistration : public Registration { 
    ... 
    virtual void appendExtraAttendees(RegistrationListWriter *writer); 
    ... 
}; 

void StudentRegistration::appendExtraAttendees(RegistrationListWriter *writer){ 
    writer->appendAttendeeQualification(getQualification()); 
} 

void RegistrationListWriter::write(RegistrationList r) { 
    QList<Registration*> regList = r.getRegistrationList(); 

    for (int i = 0; i < regList.size(); ++i) { 
     Registration *reg = regList.at(i); 

     appendRegistrationAndType(reg->metaObject()->className()); 
     appendAttendee(reg->getAttendee()); 

     reg->appendExtraAttendees(this); 

     appendBookingDate(reg->getBookingDate().toString()); 
     appendRegistrationFee(reg->calculateFee());  
    } 
} 

或者：

class Registration { 
    ... 
    virtual void appendAttendees(RegistrationListWriter *writer); 
    ... 
}; 

void Registration::appendAttendees(RegistrationListWriter *writer){ 
    writer->appendAttendee(getAttendee()); 
} 

class GuestRegistration : public Registration { 
    ... 
    virtual void appendAttendees(RegistrationListWriter *writer); 
    ... 
}; 

void GuestRegistration::appendAttendees(RegistrationListWriter *writer){ 
    Registration::appendAttendees(writer); 
    writer->appendAttendeeCatagory(getCatagory()); 
} 

class StudentRegistration : public Registration { 
    ... 
    virtual void appendAttendees(RegistrationListWriter *writer); 
    ... 
}; 

void StudentRegistration::appendAttendees(RegistrationListWriter *writer){ 
    Registration::appendAttendees(writer); 
    writer->appendAttendeeQualification(getQualification()); 
} 

void RegistrationListWriter::write(RegistrationList r) { 
    QList<Registration*> regList = r.getRegistrationList(); 

    for (int i = 0; i < regList.size(); ++i) { 
     Registration *reg = regList.at(i); 

     appendRegistrationAndType(reg->metaObject()->className()); 

     reg->appendAttendees(this); 

     appendBookingDate(reg->getBookingDate().toString()); 
     appendRegistrationFee(reg->calculateFee());  
    } 
} 

Answer 2

可能您需要使方法變爲虛擬。 非虛擬方法使用編譯時使用的類的方法，而是在運行時選擇虛擬子類的方法。

Answer 3

而不是使用dynamic_cast的，你可以有基類提供的派生類用來寫自己的類特定數據的接口。

Answer 4

直接的C++工具是dynamic_cast <>（）。

總的來說，最初設計一個需要這種鑄造的項目並不是好習慣。可以考慮各種設計模式。

我看到你正在使用metaObject()，所以這意味着Registration有QObject作爲基類。在這種情況下，可以使用qobject_cast：

The qobject_cast() function behaves similarly to the standard C++ dynamic_cast(), with the advantages that it doesn't require RTTI support and it works across dynamic library boundaries.