Can I nest an externally defined class?

Can I implement a nested class which uses an externally-defined class?

class Bank {
 public:
  int encoderPos;             // active position of encoder 
  class ClickButton button;   // Button Control object (externally defined)
                              // (This is *supposed* to be a 
                              // forward-declaration kind of thing)

  Bank(int encoderPin)  
    : button(encoderPin),     // attempt to create button object calling
                              // constructor for ClickButton class
      encoderPos(0){ 
  }
}; 

This yields an error, of course:

In constructor 'Bank::Bank(int)':
warning: 'Bank::button' will be initialized after [-Wreorder]

I would expect/want the constructor for button to be called before the constructor for bank.