c++继承中拷贝构造问题

class base {
public:
base(int initialvalue = 0): x(initialvalue) {}
base(const base& rhs): x(rhs.x) {}

private:
int x;
};

class derived: public base {
public:
derived(int initialvalue)
: base(initialvalue), y(initialvalue) {}

derived(const derived& rhs): base(rhs), y(rhs.y) {}
//  derived(const derived& rhs) // 错误的拷贝
//  : y(rhs.y) {} // 构造函数

private:
int y;
};