C++为什么要使用成员函数

 1 #include<iostream>
 2 
 3 using namespace std;
 4 
 5 class Circle
 6 {
 7 public:
 8     double pi = 3.1415926;
 9     double r;
10     double area = pi*r*r;
11 
12     double setArea()
13     {
14         return area = pi*r*r;
15     }
16 };
17 
18 int main()
19 {
20     Circle c1;
21     cout << "请输入半径:" << endl;
22     cin >> c1.r;
23 
24     cout << c1.area << endl;
25     cout << c1.setArea() << endl;
26 
27     system("pause");
28     return 0;
29 }

请输入半径:10
2.69149e+124
314.159
请按任意键继续. . .