class noncopyable
{
    protected:
        noncopyable() {}
        ~noncopyable() {}
    private:
        noncopyable(const noncopyable &orig);
        noncopyable & operator=(const noncopyable &rhs);
};

class A : private noncopyable
{
    public:
        A & operator=(const A &rhs) {} 
};

class B : private noncopyable
{};

int main()
{
    A a1,a2;
    B b1,b2;

    a1 = a2;  // ok
    b1 = b2;  // error
}