C++ :: Cannot Define Method Of Inner Nested Class If It Is Private

it seems that I cannot define a method of an inner nested class if it is a private class. for example:

class outter {
class nested {
void foo ( void ) {} // okay - but is this inline?
} void inner::foo( void ) {} // not okay - cannot define inside another class
} void outter::inner::foo( void ) {} // not okay - 'nested' class is private!

what I want to know is, is there another way to define an inner class's method? and if not, is it eternally doomed to be inline because it has to be declared inside it's own class declaration?