Access the protected variable from a nested class to the enclosing class - c++

Question

class A
{
public:
class B
{
public:
int changevar (int b)
{
a = b; // 5 should go in "a"
}
int get()
{
return a;
}
protected:
int a = 0; // value initialized here
};
private:
int usechangedvar(B& hello);
};
int A::usechangedvar(B& hello)
{
int final = hello.get(); // 5 should go in "final"
return final;
}
int main()
{
A::B hi;
hi.changevar(5);
A obj;
obj.usechangedvar(hi);
}
This code will not compile because it will say that I am trying to access the private method.
Regardless, I am not sure what is the correct way of doing this but finally what I want is:
Under class B there is a protected variable whose value is changed by the public method called "changevar". Once the value is changed, I need to store that value in the variable "final" inside the private method called "usechangedvar".
Note: I do not want to change the variable "a" from protected to something else.
Edit: I am open to suggestions where class B is not a nested class. I can keep it outside meaning A and B are not within one another
EDITED:
#include <stdio.h>
#include <iostream>
#include <stdint.h>
class A
{
public:
class B
{
public:
int changevar (int b) ------------- This method is called by application code(user) and whatever value user gives here should be the changed value
{
a = b; // 5 should go in "a"
}
// int get()
// {
// return a;
// }
protected:
int a = 0; // value initialized here. This could be anything private/protected
};
class C : public class B
{
};
private:
int usechangedvar();
};
int A::usechangedvar()
{
// 5 should come here which was inputted by user.
}
int main()
{
//1) I instantiate class C by doing
A::C objC;
objC.changevar(5);
// Whole purpose is to take 5 from user and then use it eventually in "usechangedvar" method.
}

Answer 1

Here is your problem reproduced with unrelated details eliminated:
class A
{
private:
int usechangedvar(int hello) { return hello; }
};
int main()
{
A obj;
obj.usechangedvar(5); //compile error, usechangedvar is private
}
Problem in your code is completely unrelated to class B as you can see here - you cannot call A::usechangedvar() from main() as it is private method of class A.
If you just need to check if your method usechangedvar() works you may add public method to A for example test(), call usechangedvar() from it and probably print the result. Then you can call obj.test() from main() and see if it works. You may want to remove that method test() if necessary after test is done.