使用可选参数覆盖虚函数

Question

Why is this printing 23 as output; my expectation was 33 . Could somebody please shed some light on this.

struct A {
    virtual void f() {cout << "1";}
};

/* Private inheritance */
struct B : private A {
    void f(int x = 0) {cout << "2";}
};

struct C : B {
    void f(){cout << "3";}
};

int main() {
    C obj;
    B &ref = obj;
    ref.f();
    obj.f();
}

Answer 1

B结构中的f(int x = 0)方法不与A和C结构的f()方法共享签名。