class I
{
};
class A : public I
{
};
class B : protected A
{
};
B b;
I& ri = b; // error C2243: 'type cast' : conversion from 'B *__w64 ' to 'I &' exists, but is inaccessible

class B : protected A, virtual I

I& ri = (I&)b;

Ovl>class B : protected A, virtual I
Ovl>

class A : virtual public I
{
};
class B : protected A, virtual public I
{
};

template<typename A> class I : protected A { ... };
class A { ... /* I<A> тут уже доступен! */ };
class B : public I<A> { ... };

class I
{
public:
  virtual void foo() = 0;
};

class A : public I
{
public:
  virtual void foo();
};

class B : public I
{
public:
  virtual void foo() {a.foo();}
 
protected:
  A a;
};

class B : protected A

class I
{
};
class A : public I
{
};
class B : protected A
{

   friend void DoNothing();
};

void DoNothing()
{
   B b;
   I& ri = b;
}

C>>class B : protected A
C>>

using Declaration
...
All instances of a name mentioned in a using declaration must be accessible. In particular, if a derived class uses a using declaration to access a member of a base class, the member name must be accessible. If the name is that of an overloaded member function, then all

class Derived: public MainParent, protected SomethingElse {};

class A
{
public:
    void Method() const {}
};

class B : protected A
{
public:
    using A::Method;
};

B().Method(); // OK

class I
{
};
class A
{
};
class B : protected A, public I
{
};

V>>I& ri = (I&)b;

W>class I
W>{
W>};
W>class A : public I
W>{
W>};
W>class B : protected A
W>{
W>};
W>B b;
W>I& ri = b; // error C2243: 'type cast' : conversion from 'B *__w64 ' to 'I &' exists, but is inaccessible
W>

I i = B();

struct A
{
    int foo() { return 42; }
};

class B : protected A //private здесь выдаст ошибку доступа при обращении из C к методам A
{
public :
    int bar() { return foo(); }
};

class C : public B
{
    public:
    int test() { return foo() + bar(); }
};

int main() {
    C c;

    return c.test();
}

W>class I
W>{
W>};
W>class A : public I
W>{
W>};
W>class B : protected A
W>{
W>};
W>B b;
W>I& ri = b; // error C2243: 'type cast' : conversion from 'B *__w64 ' to 'I &' exists, but is inaccessible
W>

class B: protected
{
...
I & getI() { return static_cast<I&>(*this); }
...
}