struct base {};
 
class outer: public base
{
  private:
    enum { some=42, special=67, implementation=999, details=-13 };
 
  public:
    struct inner 
    { 
      protected: 
        void foo() { int can_usethis = special + implementation + details; } 
    };
 
    outer() : _inner() { }
 
    void call_inner(inner& i) const 
    { 
        //i.foo(); // fails (protected)
    }
 
    void call_inner() const 
    { 
        //_inner.foo(); // fails (protected)
    }
 
  private:
    inner _inner;
};
 
 
int main()
{
    outer o;
    outer::inner i;
 
    // i.foo(); // fails: protected
    o.call_inner(i);
}
 

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