C++ :: How To Forward Declare Inner Class?

How can I forward declare an inner class?

I currently have this:

Code:

class Enigma
{
public:
Enigma()=delete;
Enigma(char r1,char r2,char r3,char r4, char r5, char r6, char r7, char r8, char r9, char r10);
~Enigma(){delete[] R;}
Enigma(const Enigma& rhs)=delete;
Enigma& operator=(const Enigma& rhs)=delete;

[Code]...

I'd like to be able to define Rotor outside of Enigma, but the compiler complains about incomplete types.