[class.nest]
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); x = i; s = i; ::x = i; y = i; } void g(enclose* p, int i) { p->x = i; } }; }; inner* p = 0; — 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; class I2; class I1 { }; }; class E::I2 { }; — 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.