2012年 上半年 第十三周 C++程序设计 （三十七）

【任务】阅读下面的程序
#include <iostream>
using namespace std;

class Vehicle
{
public:
void run() const { cout << "run a vehicle. "<<endl; } //(2) run()为虚函数
};

class Car: public Vehicle
{
public:
void run() const {cout << "run a car. "<<endl; 	}
};

class Airplane: public Vehicle
{
public:
void run() const {cout << "run a airplane. "<<endl;}
};

int main()
{
cout<<"(a) 直接用对象访问成员函数: "<<endl;
Vehicle v;
v.run();
Car car;
Airplane airplane;
car.run();
airplane.run();

cout<<"(b) 用指向基类的指针访问成员函数: "<<endl;
Vehicle *vp;
vp=&car;
vp->run();
vp=&airplane;
vp->run();
system("pause");
return 0;
}

（任务1）请写出程序的执行结果，并在上机时对照理解

#include <iostream>
using namespace std;

class Vehicle
{
public:
void run() const { cout << "run a vehicle. "<<endl; } //(2) run()为虚函数
};

class Car: public Vehicle
{
public:
void run() const {cout << "run a car. "<<endl; 	}
};

class Airplane: public Vehicle
{
public:
void run() const {cout << "run a airplane. "<<endl;}
};

int main()
{
cout<<"(a) 直接用对象访问成员函数: "<<endl;
Vehicle v;
v.run();
Car car;
Airplane airplane;
car.run();
airplane.run();

cout<<"(b) 用指向基类的指针访问成员函数: "<<endl;
Vehicle *vp;
vp=&car;
vp->run();
vp=&airplane;
vp->run();

system("pause");
return 0;
}

（任务1.2）如果将Vehicle类的定义修改为虚函数，其余不变，请写出程序的执行结果，并在上机时对照理解

class Vehicle

{public:

 virtual void run() const { cout << "run a vehicle. "<<endl; } //(2) run()为虚函数

};

#include <iostream>
using namespace std;

class Vehicle
{public:
virtual void run() const { cout << "run a vehicle. "<<endl; } //(2) run()为虚函数
};

class Car: public Vehicle
{
public:
void run() const {cout << "run a car. "<<endl; 	}
};

class Airplane: public Vehicle
{
public:
void run() const {cout << "run a airplane. "<<endl;}
};

int main()
{
cout<<"(a) 直接用对象访问成员函数: "<<endl;
Vehicle v;
v.run();
Car car;
Airplane airplane;
car.run();
airplane.run();

cout<<"(b) 用指向基类的指针访问成员函数: "<<endl;
Vehicle *vp;
vp=&car;
vp->run();
vp=&airplane;
vp->run();

system("pause");
return 0;
}

（任务1.3）如果将Vehicle类的定义修改为纯虚函数，找出main()函数中将使编译出错的行删除（或改为注释），请写出程序的执行结果，并在上机时对照理解

class Vehicle

{public:

 virtual void run() const = 0; //(3) run()为纯虚函数

};

#include <iostream>
using namespace std;

class Vehicle
{public:
virtual void run() const = 0; //(3) run()为纯虚函数
};

class Car: public Vehicle
{
public:
void run() const {cout << "run a car. "<<endl; 	}
};

class Airplane: public Vehicle
{
public:
void run() const {cout << "run a airplane. "<<endl;}
};

int main()
{
cout<<"(a) 直接用对象访问成员函数: "<<endl;
/*Vehicle v;
v.run();*/
Car car;
Airplane airplane;
car.run();
airplane.run();

cout<<"(b) 用指向基类的指针访问成员函数: "<<endl;
Vehicle *vp;
vp=&car;
vp->run();
vp=&airplane;
vp->run();

system("pause");
return 0;
}

 



结论：

（1）纯虚函数是不能被调用的！

（2）派生类是基类的具体化！

（3）抽象类作为共同基类！