c++ 友元关系与继承

class Base
{
friend class frnd;
protected:
int i;
}

// Frnd has no access to members in D1
class D1 : public Base
{
protected:
int j;
};

class Frnd
{
public:
int mem(Base b) { return b.i; } // ok: Frnd is friend to Base
int mem(D1 d) { return d.i; } // error: friendship doesn’t inherit
};

// D2 has no access to members in Base
class D2 : public Frnd
{
public:
int mem(Base b) { return b.i; } // error: friendship doesn’t inherit
};