Error for static variable under private section

Hi  All

why i am getting error for static variable under private section

class Test
    {
    private :
        static int s_private_int;
    public:
        static int s_public_int;
    };

    int main()
    {
        Test::s_public_int = 145;   // OK
        Test::s_private_int = 12;   // wrong, don't touch
                                    // the private parts
        return 0;
    }

Thanks