Need of privatizing assignment operator in a Singleton class

Question

Can someone justify the need of privatizing the assignment operator in a Singleton class implementation?

What problem does it solve by making Singleton& operator=(Singleton const&); private?

class Singleton {
public:
  static Singleton& Instance() {
    static Singleton theSingleton;
    return theSingleton;
  }

private:
  Singleton(); // ctor hidden
  Singleton(Singleton const&); // copy ctor hidden
  Singleton& operator=(Singleton const&); // assign op. hidden
  ~Singleton(); // dtor hidden
};

Answer 1

Assignment on a singleton is simply a nonsense operation since only one object of it should ever exist.

Making the assignment operator private helps diagnose nonsense code such as the following:

Singleton& a = Singleton::Instance();
Singleton& b = Singleton::Instance();
a = b; // Oops, accidental assignment.

Answer 2

If you only want one instance, the copy constructor should be private. The assignment operator access specifier does not matter, because it will be impossible to use anyway.