Why can friend class have access to Base class private data through Derived class

Question

This is my first time to post a question here.

class Base {
     private:
         int base;
     friend class Question;
};

class Derived : public Base{
    private:
        int super;
};

class Question{
    public:
        void test(Base& base, Derived & derived)
        {
           int value1 =base.base;  // No problem, because Question is a friend class of base
           int value2 =derived.super; // Compile error, because Question is not a friend class of base
           // Question is here
           int value3 =derived.base;  // No Compile error here, but I do not understand why.
        } 
};

The question is indicated in the last row of Class Question.

Answer 1

friend applies to all of the members of that type, whether or not the type is inherited. To stress this point: it is the members that are shared.

This means that your Question class has access to all members of Base, which is just int Base::base. Whether it accesses this member through an instance of Derived is irrelevant, because the actual member being accessed was declared on Base.