C++11: How to initialize a private static const member?

Look at this demo code:

enum class Solid {gold, silver, wood};
enum class Liquid {bier, wine, oil};
enum class Gas {air, elio, argon};

class P {
private :
    struct Set {
        Solid   solid;
        Liquid  liquid;
        Gas     gas;
    };
    static const Set set[];
};

const P::set = {
        {gold, wine, air}
};

GCG tell me:

error: ‘const P::Set P::set []’ is private within this context
 const P::set = {
          ^~~

What's wrong? Yes, I would like initialize a static private class member not in the class definition.

Does anyone can explain me the how to do that? Thanks in advance!