Singleton in Design Pattern

[jianma]

Hi,

I read the Singleton in the book << Design Patterns - Elements of Reusable Object-Oriented Software>>.

The Singleton class is define as :
class Singleton {
public:
static Singleton* Instance();
protect:
Singleton();
private:
static Singleton* _instance;
};

My question is why it uses protected constructor instead of private. Does it care about inheritance?

Any suggestion is welcomed.