12 Classes [class]

12.2 Class members [class.mem]

12.2.5 Nested class declarations [class.nest]

1
#
A class can be declared within another class.
A class declared within another is called a nested class.
The name of a nested class is local to its enclosing class.
The nested class is in the scope of its enclosing class.
[ Note
:
See [expr.prim] for restrictions on the use of non-static data members and non-static member functions.
— end note
 ]
[ Example
: 
int x;
int y;

struct enclose {
  int x;
  static int s;

  struct inner {
    void f(int i) {
      int a = sizeof(x);        // OK: operand of sizeof is an unevaluated operand
      x = i;                    // error: assign to enclose​::​x
      s = i;                    // OK: assign to enclose​::​s
      ::x = i;                  // OK: assign to global x
      y = i;                    // OK: assign to global y
    }
    void g(enclose* p, int i) {
      p->x = i;                 // OK: assign to enclose​::​x
    }
  };
};

inner* p = 0;                   // error: inner not in scope
— end example
 ]
2
#
Member functions and static data members of a nested class can be defined in a namespace scope enclosing the definition of their class.
[ Example
: 
struct enclose {
  struct inner {
    static int x;
    void f(int i);
  };
};

int enclose::inner::x = 1;

void enclose::inner::f(int i) { /* ... */ }
— end example
 ]
3
#
If class X is defined in a namespace scope, a nested class Y may be declared in class X and later defined in the definition of class X or be later defined in a namespace scope enclosing the definition of class X.
[ Example
: 
class E {
  class I1;                     // forward declaration of nested class
  class I2;
  class I1 { };                 // definition of nested class
};
class E::I2 { };                // definition of nested class
— end example
 ]
4
#
Like a member function, a friend function ([class.friend]) defined within a nested class is in the lexical scope of that class; it obeys the same rules for name binding as a static member function of that class ([class.static]), but it has no special access rights to members of an enclosing class.