#include <iostream>

class S {
public:
    static void f() { std::cout << "S::f()" << std::endl; }
    static void g() { std::cout << "S::g()" << std::endl; }
    static void h() { std::cout << "S::h()" << std::endl; }
};

class A {
public:
    void f() const {
        S::f();
    }
};

class B {
public:
    void g() const {
        S::g();
    }
};

class C {
public:
    void h() const {
        S::h();
    }
};

int main() {
    A a;
    a.f();

    B b;
    b.g();

    C c;
    c.h();
}

class B;
class A 
{
private:
    void del(){}
    void mod(){}
    void ins(){}
friend class B;
};

class B : public A
{
public:
    void del()
    {
        A::del();
    }
    void mod()
    {
        A::mod();
    }
};

int main()
{
    B b;
    b.del();//可行
        b.mod();//可行
        b.ins();//报错
        return 0;
}