c++構造器中成員函數初始化

#include <iostream>
using namespace std;

class Circle {
    double radius;
public:
    Circle (double r) : radius (r) {}
    double area () {return radius*radius*3.1415926;}
};

class Cylinder {
    Circle base;
    double height;
public:
    Cylinder (double r, double h) : base (r), height (h) {}
    double volume () {return base.area() * height;}
};

int main(int argc, char const *argv[])
{
    Cylinder foo (10, 2);
    cout << "foo's volume: " << foo.volume() << endl;
    return 0;
}