c++ - C++ 中的虚方法重写

Question

假设我们有一个名为 Vehicle 的抽象类:

class Vehicle {
    virtual bool raceWith(Vehicle *anotherVehicle) = 0;
};

我们还有它的子类 Bicycle和 Car :

//  forward declaration
class Car;

class Bicycle : public Vehicle {
    virtual bool raceWith(Vehicle *anotherVehicle) {
        throw SomeExceptionClass();
    }

    virtual bool raceWith(Car *anotherVehicle) {
        return true;
    }

    virtual bool raceWith(Bicycle *anotherVehicle) {
        return false;
    }
};

但是，这段代码抛出 SomeExceptionClass:

Vehicle *aBicycle = new Bicycle();
Vehicle *aCar = new Car();

aBicycle->raceWith(aCar);

在这里做什么？ C++ 不允许我们以这种方式使用多态方法吗？

任何帮助将不胜感激。谢谢。

编辑:用 dynamic_cast<> 提供答案也很好和 decltype变体？

Answer 1

以下将用 Vehicle 与 Bicycle 比赛:

Vehicle *aBicycle = new Bicycle();
Vehicle *aCar = new Car();

aBicycle->raceWith(aCar);

要让自行车与汽车比赛，我们需要使用双重调度。

为了使双重调度工作，我们使用 this 指针来调用下一个函数，以便第二个虚拟调用可以解析为正确的车辆类型:

class Car;
class Bicycle;

class Vehicle {
public:
    virtual bool raceWith(Vehicle& anotherVehicle) = 0;
    virtual bool raceWith(Bicycle& anotherVehicle) = 0;
    virtual bool raceWith(Car& anotherVehicle) = 0;
};


class Bicycle : public Vehicle {
public:
    virtual bool raceWith(Vehicle& anotherVehicle) override
    {
        //throw std::exception();
        return anotherVehicle.raceWith(*this);
    }

    virtual bool raceWith(Car& anotherVehicle)
    {
        return true;
    }

    virtual bool raceWith(Bicycle& anotherVehicle)
    {
        return false;
    }
};

class Car : public Vehicle {
public:
    virtual bool raceWith(Vehicle& anotherVehicle) override
    {
        return true;
    }

    virtual bool raceWith(Car& anotherVehicle) override
    {
        return true;
    }

    virtual bool raceWith(Bicycle& anotherVehicle) override
    {
        return false;
    }

};

int main()
{

    Vehicle *aBicycle = new Bicycle();
    Vehicle  *aCar = new Car();

    aBicycle->raceWith(*aCar);
}

注意 return anotherVehicle.raceWith(*this); 将执行第二个虚拟调用。

然后按以下顺序调用函数:

• main();
• Bicycle::raceWith(Vehicle& anotherVehicle);
• Car::raceWith(Bicycle& anotherVehicle);

---

以下是相同的程序，但坚持使用问题中提供的指针和异常:

#include <exception>

class Car;
class Bicycle;

class Vehicle {
public:
    virtual bool raceWith(Vehicle* anotherVehicle) = 0;
    virtual bool raceWith(Bicycle* bicycle) = 0;
    virtual bool raceWith(Car* car) = 0;
};


class Bicycle : public Vehicle {
public:
    virtual bool raceWith(Vehicle* anotherVehicle) override
    {
        //throw std::exception();
        return anotherVehicle->raceWith(this);
    }

    virtual bool raceWith(Car* car) override
    {
        return true;
    }

    virtual bool raceWith(Bicycle* bicycle) override
    {
        return false;
    }
};

class Car : public Vehicle {
public:
    virtual bool raceWith(Vehicle* anotherVehicle) override
    {
        throw std::exception();
    }

    virtual bool raceWith(Car* car) override
    {
        return true;
    }

    virtual bool raceWith(Bicycle* bicycle) override
    {
        return false;
    }

};

int main()
{

    Vehicle *aBicycle = new Bicycle();
    Vehicle  *aCar = new Car();

    aBicycle->raceWith(aCar);
}