C++ OOP quiz question

As by default every member of a class¹ is private, getN in the base class is declared private.

Make getN public as:

class Base {
    friend class SubClass;
    int n;
public: //<--------------------- you forgot this
    virtual int getN()
    {
        return n;
    }
};

^{1. I mean, a class defined with the keyword class. Note that class can be defined with the keyword struct and union as well, according to the C++ Standard.}

---

EDIT:

If you think because SubClass is a friend of Base, so it can access private members of Base from outside, then that is wrong. friend means member functions of SubClass can access private members of Base class.

However, if you make main() friend of Base, then your code would work:

 class Base {
        friend int main(); //make main() friend of Base
        //...
    };

Now from main(), any private members of Base can be accessed!

See this demo : http://www.ideone.com/UKkCF

Your compiler should give you some clues...

% g++ -Wall -Wextra -Wshadow -Weffc++ test.cpp
test.cpp:1: warning: ‘class Base’ has virtual functions but non-virtual destructor
test.cpp:10: warning: ‘class SubClass’ has virtual functions but non-virtual destructor
test.cpp: In constructor ‘Base::Base()’:
test.cpp:1: warning: ‘Base::n’ should be initialized in the member initialization list
test.cpp: In constructor ‘SubClass::SubClass()’:
test.cpp:12: note: synthesized method ‘Base::Base()’ first required here 
test.cpp: In copy constructor ‘SubClass::SubClass(const SubClass&)’:
test.cpp:13: warning: base class ‘class Base’ should be explicitly initialized in the copy constructor
test.cpp: At global scope:
test.cpp:13: warning: unused parameter ‘s’
test.cpp: In function ‘int main()’:
test.cpp:4: error: ‘virtual int Base::getN()’ is private
test.cpp:19: error: within this context
test.cpp:4: error: ‘virtual int Base::getN()’ is private
test.cpp:21: error: within this context
test.cpp:21: warning: unused variable ‘x’

The error message says it all: getN is implicitly private, which is the default for classes in C++.

Use

class Base {
 // ...

public:
   virtual int getN();
};

to make it a publicly accessible member. The facts that SubClass is a friend of Base and that getN is called through an instance of Subclass don't matter here - getN is simply not accessible from within main(), where it is called from. You could make main a friend of Base, however.