c++ - Allowing a "friend" class to access only some private members

Question

Suppose I have three C++ classes FooA, FooB and FooC.

FooA has an member function named Hello, I want to call this function in class FooB, but I don't want class FooC be able to call it. The best way I can figure out to realize this is to declare FooB as a friend class of FooA. But as long as I do this, all FooA's private and protected members will be exposed which is quite unacceptable to me.

So, I wanna know if there is any mechanism in C++(03 or 11) better than friend class which can solve this dilemma.

And I assume it will be nice if the following syntax is possible:

class FooA
{
private friend class FooB:
    void Hello();
    void Hello2();
private:
    void Hello3();
    int m_iData;
};

class FooB
{
    void fun()
    {
        FooA objA;
        objA.Hello()  // right
        objA.Hello2() // right
        objA.Hello3() // compile error
        ojbA.m_iData = 0; // compile error
    }
};

class FooC
{
    void fun()
    {
        FooA objA;
        objA.Hello()  // compile error
        objA.Hello2() // compile error
        objA.Hello3() // compile error
        ojbA.m_iData = 0; // compile error
    }
};

Answer 1

There's nothing to make a class a friend of one specific function, but you can make FooB a friend of a "key" class with private constructor, and then have FooA::Hello take that class as an ignored parameter. FooC will be unable to provide the parameter and hence can't call Hello:

Is this key-oriented access-protection pattern a known idiom?