作业3.22

<pre name="code" class="html">#include<iostream>using namespace std;class Circle{public:Circle(double r=0.0){radius=r;}double area(){return radius*radius*3.14;}void input();    private:double radius;};void Circle::input(){cout<<"please input radius:"<<endl;cin>>radius;}int main(){Circle r1;r1.input();    cout<<"the area of circle:"<<r1.area()<<endl;return 0;}