How can I access base class's protected members through derived class? [duplicate]

Question

I'm trying several programs about inheritance, and it turned out that the following caused an error but I don't really know the rationale.

#include <iostream>

using namespace std;

class Base {
protected:
    int x = 0;
};

class Derived: public Base {

    // OK: access protected member via this
    void g() { cout<<x; } 

    // OK: access protected member of other Derived
    void h(Derived& d) { cout<<d.x; } 

    // FAIL: access Base class's protected member, why?
    void f(Base& b) { cout<<b.x; } 
};

int main() {}

I expect that the Derived class could access the ​Base class's public or protected data members and member function.

However it didn't work as what I was thinking about, could anyone help me light up my concepts?

Answer 1

There is not more to it than you already discovered. Derived instances may acces their protected members and those of other derived instances but not those of base class instances. Why? Because thats how protected works by definition.

For more details I refer you to cppreference (emphasize mine):

A protected member of a class Base can only be accessed

1) by the members and friends of Base

2) by the members and friends (until C++17) of any class derived from Base, but only when operating on an object of a type that is derived from Base (including this)

Answer 2

void f(Base& b) {cout<<b.x;}

Here you are trying to access a protected member of a different class. It does not matter that you also share the same base class. (still looking for a source)

void g() {cout<<x;}

In this example you are acccessing your own private member. (protected members of base class are inherited and protected in derived class)

void h(Derived& d) {cout<<d.x;}

Here you are accessing the private member of the same class. But for more on this look at this post: Access private elements of object of same class

Answer 3

From this documentation

A protected member of a class Base can only be accessed

1. by the members and friends of Base

   this is not your case

2. by the members and friends (until C++17) of any class derived from Base, but only when operating on an object of a type that is derived from Base (including this)

   this is your case, but the argument b is not such a derived type

The reason for protected member access is to allow a base class to define an interface for use by derived classes. That's not the same as allowing every different derived type special access to every base class object.