Friend class keyword not working properly in C++

class base_A {
public:
    int a = 10;
    int b = 1;
    int sum();
    friend class base_B;
};
class base_B {
public:
    int mul();
};
class derive :public base_A,public base_B{
public:
    void sum_d();
};
int base_A::sum() {
    int s = a + b;
    return s;
}
int base_B::mul() {
    int m = a * b;
    return m;
}
void derive::sum_d() {
    cout << "Sum " << base_A::sum() << "\n";
};
int main()
{
    derive dobj;
    dobj.sum_d();
    dobj.mul();
}

Even though I've mentioned base_B class as a friend class of base_A. It couldn't able to access the variables in class base_A. I'm getting Error C2065 'a': undeclared identifier Public Inheritance Anyone help me on this as I am new to C++