C++ inheritance: scoping and visibility of members

Question

Can you explain why this is not allowed,

#include <stdio.h>

class B {
private:
    int a;
public:
    int a;
};

int main() {
    return 0;
}

while this is?

#include <stdio.h>

class A {
public:
    int a;
};

class B : public A{
private:
    int a;
};

int main() {
    return 0;
}

In both the cases, we have one public and one private variable named a in class B.

---

edited now!

Answer 1

In both the cases, we have one public and one private variable named a in class B.

No, thats not true.

In the first case, you can't have two identifiers with the same name in the same scope. While in the second case, B::a hides A::a, and to access A::a you have to fully qualify the name:

b.a = 10; // Error. You can't access a private member.
b.A::a = 10; // OK.

Answer 2

Because B::a hides A::a in the second example. You can still access it, but it needs explicit qualification for the compiler to figure out you are asking for the member of parent class with the same hame.

In the first example both a's are in the same scope, while in the second example the scopes are different.