C++程序设计POJ》《WEEK6 多态与虚函数》

class CBase {
public:
virtual  void SomeVirtualFunction() { }
};
class CDerived:public CBase {
public :
virtual  void SomeVirtualFunction() { }
};
int main() {
CDerived ODerived;
CBase * p = & ODerived; 
p-> SomeVirtualFunction(); // 调用哪个虚函数取决于 p 指向哪种类型的对象  调用CDerived类的虚函数
return 0;
}

class CBase {
public:
virtual
void SomeVirtualFunction() { }
};
class CDerived:public CBase {
public :
virtual
void SomeVirtualFunction() { }
};
int main() {
CDerived ODerived;
CBase & r = ODerived;
r.SomeVirtualFunction(); //调用哪个虚函数取决于 r 引用哪种类型的对象
return 0;
}

/*
几何形体处理程序
Sample Input:
3
R 3 5
C 9
T 3 4 5
Sample Output
Triangle:6
Rectangle:15
Circle:254.34
39
*/
#include
#include
#includeusing namespace std;
class CShape
{
public:
    virtual double Area() = 0; // 纯虚函数
    virtual void PrintInfo() = 0;
};

class CRectangle :public CShape
{
public:
    int w, h;
    virtual double Area();
    virtual void PrintInfo();
};
class CCircle :public CShape
{
public:
    int r;
    virtual double Area();
    virtual void PrintInfo();

};
class CTriangle :public CShape
{
public:
    int a, b, c;
    virtual double Area();
    virtual void PrintInfo();
};
double CRectangle::Area()
{
    return w*h;
}
void CRectangle::PrintInfo()
{
    cout "rectanlgle:"  endl;
}

double CCircle::Area()
{
    return 3.14*r*r;
}
void CCircle::PrintInfo()
{
    cout "circle:"  endl;
}
double CTriangle::Area()
{
    double p = (a + b + c) / 2.0;
    return sqrt(p*(p - a)*(p - b)*(p - c));
}
void CTriangle::PrintInfo()
{
    cout "triangle:"  endl;
}

CShape * pShapes[100]; // 基类 指针数组
int MyCompare(const void *s1, const void *s2);

int MyCompare(const void* s1, const void *s2)
{
    double a1, a2;
    CShape** p1; // s1,s2 是 void * ，不可写 ““* s1 ”来取得 s1 指向的内容
    CShape** p2;
    p1 = (CShape**)s1;//s1,s2 指向 pShapes 数组中的元素，数组元素的类型是 CShape *
    p2 = (CShape**)s2;// 故 p1,p2 都是指向指针的指针，类型为 CShape **
    a1 = (*p1)->Area();// * p1 的类型是 Cshape * , 是基类指针，故此句为多态
    a2 = (*p2)->Area();
    if (a1  a2)
        return -1;
    else if (a2  a1)
        return 1;
    else
        return 0;
}
int main()
{
    int i, n;
    CRectangle *pr;
    CCircle *pc;
    CTriangle *pt;
    cin >> n;
    for (i = 0; i )
    {
        char c;
        cin >> c;
        switch (c)
        {
        case ‘R‘:
            pr = new CRectangle();
            cin >> pr->w >> pr->h;
            pShapes[i] = pr; // 派生类指针 赋给 基类
            break;
        case ‘C‘:
            pc = new CCircle;
            cin >> pc->r;
            pShapes[i] = pc;
            break;
        case ‘T‘:
            pt = new CTriangle();
            cin >> pt->a >> pt->b >> pt->c;
            pShapes[i] = pt;
            break;

        }
    }

    qsort(pShapes, n, sizeof(CShape*), MyCompare);
    for (i = 0; i )
        pShapes[i]->PrintInfo();
    while (1);
    return 0;
}

class myclass
public:
virtual void hello(){cout"hello from myclass"endl; };
virtual void bye(){cout"bye from myclass"endl;}
};
class son:public myclass
{
void hello(){ cout"hello from son"endl;};
son(){ hello();}
~son(){bye(); };
class grandson:public son
{
void hello(){cout"hello from grandson"endl;};
void bye() { cout "bye from grandson"endl;}
grandson(){cout"constructing grandson"endl;};
~grandson(){cout"destructing grandson"endl;};
};
int main(){
grandson gson;
son *pson;
pson=&gson;
pson->hello(); 多态
return 0;
}
/*
结果：
hello from son
constructing grandson
hello from grandson
destructing grandson
bye from myclass

*/

class Base {
private:
virtual void fun2() { cout "Base::fun2()"  endl; }
};
class Derived:public Base {
public:
virtual void fun2() { cout "Derived:fun2()"  endl; }
};
Derived d;
Base * pBase = & d;
pBase-> fun2(); // 编译出错

#includeusing namespace std;
class Base
{
public:
    virtual void fun2()
    {
        cout "base::fun2()"  endl;
    }
};
class Derived :public Base
{
private:
    virtual void fun2()
    {
        cout "Derived:fun2()"  endl;
    }
};

int main()
{
    Derived d;
    Base * pBase = &d;
    pBase->fun2(); // 神奇了，private也能访问！！！
    while (1);
    return 0;

}