多态 virtual

#include <iostream>  using namespace std; class Fract{int n;int d;public:Fract():n(0),d(1){}Fract(int an, int ad):n(an),d(ad){reduce();}void reduce(){if (d<0){d = -d;n = -n;}if (n==0){cout << "d==0 !!" << endl;}for (int i=d; i>1; i--){if (n%i==0 && d%i==0){n/=i, d/=i;break;}}}virtual void show(){cout << n << '/' << d << endl;}virtual double value(){return (double)n/d;}};class Dai : public Fract{int i;public:Dai():i(0){}Dai(int ai , int an, int ad):i(ai), Fract(an, ad){}void show(){cout << i << '(' ;Fract::show();}double value(){return i+ Fract::value();}};int main(){Fract d1;Dai d2(2, 12, 16);Fract *p = NULL;p = &d1;p->show();cout << "d1=" << p->value() << endl;p = &d2;  //指针 实现 多态 p->show();cout << "d2=" << p->value() << endl;Fract& d = d2; //引用 实现 多态 d2.show();cout << "d2=" << d2.value() << endl;cout << "d3=" << d3.value() << endl;return 0;}