[C++知识库]C++学习笔记（Day15 实验八）

运算符重载实现静态多态性??? 虚函数实现动态多态性

通过基类的指针和引用访问派生类对象的成员（虚函数）

例题一

声明Point类：
有坐标_x,_y两个成员变量，对Point类重载“++”（自增）“--”（自减）运算符，实现对坐标值的改变。

使用Clion编译出现问题，暂未解决。

#include <iostream>
using namespace std;

class Point{
    int _x,_y;
public:
    Point(int x = 0, int y = 0):_x(x),_y(y){};
    //声明成员函数实现重载
    Point& operator++();//无参数是前置单目运算符
    Point operator++(int);//有参数是后置单目运算符
    Point& operator--();
    Point operator--(int);
    friend ostream& operator << (ostream& o,Point& p);
};
Point& Point::operator++(){
    _x++;
    _y++;
    return *this;
}
/* ++i在C++里面的定义最后返回的是被++的对象的引用，所以++i可以作为左值，即可以写：++i=3 */

//后缀式操作符接受一个额外的int型形参（不会使用它，仅做区分用）
Point Point::operator++(int){
    Point temp = *this;
    ++*this;//复用了前缀++的重载
    return temp;
}
Point& Point::operator--(){
    _x--;
    _y--;
    return *this;
}
Point Point::operator--(int)
{
    Point temp = *this;
    --*this;
    return temp;
}
//友元函数 返回值类型是ostream&，可以支持<<级联操作
ostream& operator << (ostream& o,Point& p){
    o << "(" << p._x << "," << p._y << ")";
    return o;
}
int main(){
    Point p(1,2);
    cout << p << endl;
    cout << ++p << endl;
    cout << p++ << endl;
    cout << --p << endl;
    cout << p-- << endl;
};

例题二

声明一个车基类，有Run、Stop等成员函数，由此派生出自行车类、汽车类、从自行车类和汽车类派生出摩托车类，它们都有Run、Stop等成员函数。

观察虚函数的作用。

没有使用虚函数

#include <iostream>
using namespace std;

class Vehicle{
public:
    int MaxSpeed;
    int Weight;
    void Run(){cout << "Vehicle Run!" << endl;};
    void Stop(){cout << "Vehicle Stop!" << endl;};
};
class Bicycle:virtual public Vehicle{
public:
    int Height;
    void Run(){cout << "bicycle run!" << endl;}
    void Stop(){cout << "bicycle stop!" << endl;}
};
class Motorcar:virtual public Vehicle{
public:
    int SeatNum;
    void Run(){cout << "motorcar run!"<<endl;};
    void Stop(){cout << "motorcar stop!"<<endl;};
};
class Motorcycle:public Bicycle,public Motorcar{
public:
    void Run(){cout << "motorcycle run!"<< endl;}
    void Stop(){cout << "motorcycle stop!"<<endl;}
};
int main(){
    Vehicle v;
    v.Run();
    v.Stop();
    Bicycle b;
    b.Run();
    b.Stop();
    Motorcar m;
    m.Run();
    m.Stop();
    Motorcycle mc;
    mc.Run();
    mc.Stop();
    Vehicle* vp = &v;
    vp->Run();
    vp->Stop();
    vp = &b;
    vp->Run();
    vp->Stop();
    vp = &m;
    vp->Run();
    vp->Stop();
    vp = &mc;
    vp->Run();
    vp->Stop();
    return 0;
}

运行结果：

Vehicle Run!
Vehicle Stop!
bicycle run!
bicycle stop!
motorcar run!
motorcar stop!
motorcycle run!
motorcycle stop!
Vehicle Run!
Vehicle Stop!
Vehicle Run!
Vehicle Stop!
Vehicle Run!
Vehicle Stop!
Vehicle Run!
Vehicle Stop!

Process finished with exit code 0

使用虚函数

#include <iostream>
using namespace std;

class Vehicle{
public:
    int MaxSpeed;
    int Weight;
    virtual void Run(){cout << "Vehicle Run!" << endl;};
    virtual void Stop(){cout << "Vehicle Stop!" << endl;};
};
class Bicycle:virtual public Vehicle{
public:
    int Height;
    void Run(){cout << "bicycle run!" << endl;}
    void Stop(){cout << "bicycle stop!" << endl;}
};
class Motorcar:virtual public Vehicle{
public:
    int SeatNum;
    void Run(){cout << "motorcar run!"<<endl;};
    void Stop(){cout << "motorcar stop!"<<endl;};
};
class Motorcycle:public Bicycle,public Motorcar{
public:
    void Run(){cout << "motorcycle run!"<< endl;}
    void Stop(){cout << "motorcycle stop!"<<endl;}
};
int main(){
    Vehicle v;
    v.Run();
    v.Stop();
    Bicycle b;
    b.Run();
    b.Stop();
    Motorcar m;
    m.Run();
    m.Stop();
    Motorcycle mc;
    mc.Run();
    mc.Stop();
    Vehicle* vp = &v;
    vp->Run();
    vp->Stop();
    vp = &b;
    vp->Run();
    vp->Stop();
    vp = &m;
    vp->Run();
    vp->Stop();
    vp = &mc;
    vp->Run();
    vp->Stop();
    return 0;
}

运行结果：

E:\Program\Test6\cmake-build-debug\Test6.exe
Vehicle Run!
Vehicle Stop!
bicycle run!
bicycle stop!
motorcar run!
motorcar stop!
motorcycle run!
motorcycle stop!
Vehicle Run!
Vehicle Stop!
bicycle run!
bicycle stop!
motorcar run!
motorcar stop!
motorcycle run!
motorcycle stop!

Process finished with exit code 0

虚基类解决的是类成员标识二义性和信息冗余问题
而虚函数是实现动态多态性的基础。

扩展练习

对实验六中的people类重载“==”运算符和“=”运算符，
“==”运算符判断两个people类对象的id属性的大小；
“=”运算符实现people类对象的赋值操作。