虚函数

虚函数很简单，就是在普通函数前面加个关键字virtual

#include <iostream>
#include <cmath>
using namespace std;

#define PI 3.14

//几何图形类
class Geometry{
public:
    //获取面积
    float getArea(){
        return 0;
    };
};

//矩形类
class Rectangle:public Geometry{//多态必须是共有继承
public:
    Rectangle(float minX,float minY,float maxX,float maxY)
    :minx(minX),miny(minY),maxx(maxX),maxy(maxY)
    {

    }
public:
    float getArea(){
        return fabs((minx-maxx)*(miny-maxy));
    }
private:
    float minx,miny,maxx,maxy;
};

//圆形类
class Circle:public Geometry{
public:
    Circle(float R)
    :r(R)
    {

    }
public:
    float getArea(){
        return PI*r*r; //常量M_PI就是这里的PI
    }
private:
    float r;
};

int main(int argc, const char * argv[]) {
    Rectangle *re = new Rectangle(0,0,3,4);
    cout<<re->getArea()<<endl;
    delete re;//记得new关键字，需手动回收内存

    Circle *c = new Circle(3); 
    cout<< c->getArea()<<endl;
    delete c;

    Geometry *ge = new Rectangle(0,0,3,4); //用父类的指针接收子类的对象
    cout<<ge->getArea()<<endl;
    delete ge;
    return 0;
}

12
28.26
0

可以看到用父类的指针接收子类的对象，不能输出正常结果，怎么办呢？

#include <iostream>
#include <cmath>
using namespace std;

#define PI 3.14

//几何图形类
class Geometry{
public:
    //获取面积
    virtual float getArea(){
        return 0;
    };
};

//矩形类
class Rectangle:public Geometry{//多态必须是共有继承
public:
    Rectangle(float minX,float minY,float maxX,float maxY)
    :minx(minX),miny(minY),maxx(maxX),maxy(maxY)
    {

    }
public:
    float getArea(){
        return fabs((minx-maxx)*(miny-maxy));
    }
private:
    float minx,miny,maxx,maxy;
};

//圆形类
class Circle:public Geometry{
public:
    Circle(float R)
    :r(R)
    {

    }
public:
    virtual float getArea(){ //这里不管写不写virtual,都是虚的
        return PI*r*r; //常量M_PI就是这里的PI
    }
private:
    float r;
};

int main(int argc, const char * argv[]) {
    Rectangle *re = new Rectangle(0,0,3,4);
    cout<<re->getArea()<<endl;//12
    delete re;//记得new关键字，需手动回收内存

    Circle *c = new Circle(3);
    cout<< c->getArea()<<endl;//28.26
    delete c;

    Geometry *ge = new Rectangle(0,0,3,4);
    cout<<ge->getArea()<<endl;//12
    delete ge;

    //子类对象赋值给父类对象(不可以)
    Circle circle(3);
    Geometry g = circle;
    cout<<g.getArea()<<endl; //0

    //子类对象的地址赋值给父类对象的指针（可以）
    Circle *c1 = new Circle(3);
    Geometry *g1 = c1;
    cout<< g1->getArea()<<endl;//28.26
    delete c1;

    //子类对象赋值给父类对象的引用（可以）
    Circle c2(3);
    Geometry& g2 = c2;
    cout<< g2.getArea()<<endl;//28.26

    return 0;
}

12
28.26
12
0
28.26
28.26

父类方法是虚的，不管子类写不写都是虚的。为了代码的可读性，建议子类写上virtual关键字