建立类cylinder， cylinder的构造函数被传递了两个double值，分别表示圆柱体的半径和高度。用类cylinder计算圆柱体的体积，并存储在一个double变量中。在类cylinder中

#include<iostream>
using namespace std;
class cylinder {
public:
    cylinder(double a, double b);
    void vol();
private:
    double r, h;
    double volume;
};
cylinder::cylinder(double a, double b)
{
    r = a;
    h = b;
    volume = 3.14159 * r * r * h;
}
void cylinder::vol()
{
    cout << volume << endl;
}
int main()
{
    double x, y;
    cin >> x >> y;
    cylinder A(x, y);
    A.vol();
    return 0;
}