Conversion to Inaccessible Base Class Is Not Allowed [Duplicate]

I define a class B1 and a derived class D1 at first. Then I want to define a reference to B1 and initialize that to the D1 object I just defined. Here comes the error, saying that "conversion to inaccessible base class 'B1' is not allowed", which I don't know why.

#include "std_lib_facilities.h"

class B1 {
public:
    virtual void vf() { cout << "B1::vf()\n"; };
    void f() { cout << "B1::f()\n"; };
};

class D1 :B1 {
public:
    void vf() { cout << "D1:vf()\n"; };
    void f() { cout << "D1::f()\n"; };
};




int main()
{
    B1 b1;
    b1.vf();
    b1.f();

    D1 d1;
    d1.vf();
    d1.f();

    B1& db=d1;//error

    keep_window_open();
    return 0;
}