using namespace std;

class A {
public:
    int m_variable = 5;
}

include iostream>

using namespace std;

void B::method() {
    // Why do get an error stating 'invalid use of non-static data member on this line?
    int x = (A::m_variable) * 2;
    cout << x << endl;
}

class B : public A {
    // ... whatever
};

class B {
    // ... whatever
    private:
        A someVar;
};

int x = someVar.m_variable * 2;

class A {
public:
    static int m_variable;
};