11 Classes [class]

11.4 Class members [class.mem]

11.4.11 Nested class declarations [class.nest]

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 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 in scope — end example]
Member functions and static data members of a nested class can be defined in a namespace scope enclosing the definition of their class.
[Example 2: struct enclose { struct inner { static int x; void f(int i); }; }; int enclose::inner::x = 1; void enclose::inner::f(int i) { /* ... */ } — end example]
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 3: 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]
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.