有一个汽车类Vehicle，将它作为基类派生出小汽车类Car，卡车类Truck和公交车类Bus，定义这些类并声明一个虚函数用来显示各类信息。

好的，以下是代码实现：

#include <iostream>
using namespace std;

class Vehicle {
public:
    virtual void showInfo() {
        cout << "Vehicle: This is a general vehicle." << endl;
    }
};

class Car : public Vehicle {
public:
    void showInfo() {
        cout << "Car: This is a car." << endl;
    }
};

class Truck : public Vehicle {
public:
    void showInfo() {
        cout << "Truck: This is a truck." << endl;
    }
};

class Bus : public Vehicle {
public:
    void showInfo() {
        cout << "Bus: This is a bus." << endl;
    }
};

int main() {
    Vehicle* v1 = new Car();
    Vehicle* v2 = new Truck();
    Vehicle* v3 = new Bus();

    v1->showInfo();
    v2->showInfo();
    v3->showInfo();

    delete v1;
    delete v2;
    delete v3;

    return 0;
}

输出结果为：

Car: This is a car.
Truck: This is a truck.
Bus: This is a bus.

上面的代码中，Vehicle 是基类，Car、Truck 和 Bus 都是派生类。这些类中都定义了 showInfo() 函数，用来显示各自的信息。在 main() 函数中，我们创建了三个指向 Vehicle 类型的指针，并分别指向 Car、Truck 和 Bus 对象。然后调用它们的 showInfo() 函数，输出各自的信息。最后，我们释放了动态分配的内存。