【多态】实例：几何形体程序

几何形体实例

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 Aree();
    virtual void PrintInfo();
};
class CCircle:public CShape {
public:
    int r;
    virtual double Aree();
    virtual void PrintInfo();
};
class CTriangle:public CShape {
public:
    int a,b,c;
    virtual double Aree();
    virtual void PrintInfo();
};

double CRectangle::Area() {
     return w * h;   
}
void CRectangle::PrintInfo() {
     cout << "Rectangle:" << Area() << endl;   
}
double CCircle::Area() {
     return 3.14 * r * r;   
}
void CCircle::PrintInfo() {
     cout << "Circle:" << Area() << 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:" << Area() << endl;   
}

CShape * pShapes[100];
int MyCompare(const void * s1, const void * s2);
int main() {
     int i; int n;
    CRectangle * pr; CCircle * pc; CTriangle * pt;
    cin >> n;
    for(i=0; i < n; ++i) {
         char c;
        cin >> c;
        switch(c) {
            case 'R':
                pr = new CRectangle();
                cin >> pr->w >> pr->h;
                pShapes[i] = pr;
                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<n;++i)
        pShapes[i]->PrintInfo();
    return 0;
}

int MyCompare(const void * s1, const void * s2) {
     double a1, a2;
    CShape** p1; // s1, s2是void *，不可以写“*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，元素类型是CShape*
    p2 = (CShape**)s2;
    a1 = (*p1)->Area(); // 多态
    a2 = (*p2)->Area();
    if(a1 < a2)
        return -1;
    else if(a2 < a1)
        return 1;
    else
        return 0;
}

• 如果添加一个新的几何形体，增加的语句只有这个多出来的类

• 用基类指针数组存放指向各种派生类对象的指针，然后遍历数组，就可以对派生类对象进行各种操作，这是很常用的做法

  另一个例子

  class Base {
  public: 
    void fun1() {this->fun2();} // this是基类指针，fun2是虚函数，所以是多态
                                // 所以如果不加this->，执行效果也一样
    virtual void fun2() { cout << "Base::fun2()" << endl;}
  };
  class Derived:public Base {
  public:
    virtual void fun2() {cout << "Derived::fun2()" << endl;}
  }
  main() {
     Derived d;
    Base * pBase = &d;
    pBase->fun1(); // 输出 Derived::fun2()
  }

• 在非构造函数，非析构函数的成员函数中调用虚函数，就是多态

• 否则不是多态。编译的时候即可确定，调用的是自己类或基类中定义的函数，不会等到运行的时候才决定调用自己的还是派生类的函数