11 Classes [class]

11.4 Class members [class.mem]

11.4.12 Nested class declarations [class.nest]

A class can be declared within another class.
A class declared within another is called a nested class.
[Note 1: 
See [expr.prim.id] for restrictions on the use of non-static data members and non-static member functions.
— end note]
[Example 1: 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 found — end example]
[Note 2: 
Nested classes can be defined either in the enclosing class or in an enclosing namespace; member functions and static data members of a nested class can be defined either in the nested class or in an enclosing namespace scope.
[Example 2: struct enclose { struct inner { static int x; void f(int i); }; }; int enclose::inner::x = 1; void enclose::inner::f(int i) { /* ... */ } 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]
— end note]
A friend function ([class.friend]) defined within a nested class has no special access rights to members of an enclosing class.