C++，如何在派生类中调用基类的重载提取运算符？

Question

所以这是我的一个大任务的一小部分，我只是不确定它的语法。

我有一个名为 Vehicle 的基类，它有以下成员：int fuelAmt 和 int fuelUsage)

（我正在使用命名空间std）

我以这种方式重载了<< 运算符：

ostream& operator<<(ostream& osObject, const Vehicle& myVehicle)
{
    cout << "Fuel Usage Rate: " << myVehicle.fuelUsage << endl
         << "Fuel Amount:     " << myVehicle.fuelAmt << endl;

    return osObject;
}

然后我这样称呼它：

cout << Vehicle;

结果是（示例）：

Fuel Usage Rate: 10;
Fuel Amount: 50;

---

我还有一个派生自Vehicle 类的Airplane 类，它引入了一个新成员：int numEngines。

如何在Airplane 类中重载<< 运算符，以便它首先调用“车辆重载运算符结果”，然后是我告诉<< 运算符从派生类...所以，这就是我的意思：

我需要它在 Airplane 类中像这样运行：

ostream& operator<<(ostream& osObject, const Airplane& myAirplane)
{
        //First print the Fuel Usage rate and Fuel amount by calling
        //the Base class overloaded << function

         //then
        cout << "Number of Engines: " << myAirplane.numEngines << endl;

    return osObject;
}

如何在这个派生类中触发输出其成员值的基类执行？

是否类似于更改标题？像这样：

ostream& operator<<(ostream& osObject, const Airplane& myAirplane): operator<<Vehicle

Answer 1

由于操作符

class Base
{
public:
    virtual std::ostream& output(std::ostream& out) const
    {
        return out << "Base";
    }
    virtual ~Base() {} //Let's not forget to have destructor virtual
};

class Derived : public Base
{
public:
    virtual std::ostream& output(std::ostream& out) const
    {
        Base::output(out); //<------------------------
        return out << "DerivedPart";
    }
    virtual ~Derived() {} //Let's not forget to have destructor virtual
};

最后，只有基类的操作符

std::ostream& operator << (std::ostream& out, const Base& b) 
{
    return b.output(out);
}

Answer 2

以下内容如何：

ostream& operator<<(ostream& osObject, const Airplane& myAirplane)
{
    osObject << static_cast<const Vehicle &>(myAirplane);
    osObject << "Number of Engines: " << myAirplane.numEngines << endl;

    return osObject;
}

Answer 3

ostream& operator<<(ostream& osObject, const Airplane& myAirplane)
{
     //First print the Fuel Usage rate and Fuel amount by calling
     //the Base class overloaded << function
     cout << (Vehicle& ) myAirplane;

     //then
     cout << "Number of Engines: " << myAirplane.numEngines << endl;

     return osObject;
}