编写程序声明一个车(vehicle)基类，有run, stop等成员函数，由此派生出自行车(bicycle)类，汽车(motorcar)类，由bicycle和motorcar派生出摩托车(motorcycle)类，它们都有Run、Stop等成员函数。观察虚函数的作用

请看下面的 C++ 代码：

class Vehicle {
public:
    virtual void Run() {
        cout << "Vehicle is running." << endl;
    }
    virtual void Stop() {
        cout << "Vehicle is stopping." << endl;
    }
};

class Bicycle : public Vehicle {
public:
    void Run() {
        cout << "Bicycle is running." << endl;
    }
    void Stop() {
        cout << "Bicycle is stopping." << endl;
    }
};

class Motorcar : public Vehicle {
public:
    void Run() {
        cout << "Motorcar is running." << endl;
    }
    void Stop() {
        cout << "Motorcar is stopping." << endl;
    }
};

class Motorcycle : public Bicycle, public Motorcar {
public:
    void Run() {
        cout << "Motorcycle is running." << endl;
    }
    void Stop() {
        cout << "Motorcycle is stopping." << endl;
    }
};

int main() {
    Vehicle *ptrVehicle;
    Bicycle bicycle;
    Motorcar motorcar;
    Motorcycle motorcycle;

    ptrVehicle = &bicycle;
    ptrVehicle->Run(); // 输出： Bicycle is running.
    ptrVehicle->Stop(); // 输出： Bicycle is stopping.

    ptrVehicle = &motorcar;
    ptrVehicle->Run(); // 输出： Motorcar is running.
    ptrVehicle->Stop(); // 输出： Motorcar is stopping.

    ptrVehicle = &motorcycle;
    ptrVehicle->Run(); // 输出： Motorcycle is running.
    ptrVehicle->Stop(); // 输出： Motorcycle is stopping.

    return 0;
}

虚函数的作用表现在多态上，当我们使用一个指向基类的指针或引用来调用一个虚函数时，会根据它所指向的对象的类型来选择相应的实现。例如上面代码中，ptrVehicle 指针指向 Bicycle 对象时，调用 Run 和 Stop 函数时输出 Bicycle 相关信息；指向 Motorcar 时，输出 Motorcar 相关信息；指向 Motorcycle 时，输出 Motorcycle 相关信息。虚函数的特性使得多态成为可能，让我们能够非常灵活地使用继承机制编写程序。