nested classes

A declaration of a class/struct or union may appear in within another class. Such declaration declares a nested class

Explanation

The name of the nested class exists in the scope of the enclosing class, and name lookup from a member function of a nested class visits the scope of the enclosing class after examining the scope of the nested class. Like any member of its enclosing class, the nested class has access to all names (private, protected, etc) to which the enclosing class has access, but it is otherwise independent and has no special access to the this pointer of the enclosing class.

Declarations in a nested class can use only type names, static members, and enumerators from the enclosing class.	(until C++11)
Declarations in a nested class can use any members of the enclosing class, following the usual usage rules for the non-static members.	(since C++11)

int x,y; // globals
class enclose { // enclosing class
    int x; // note: private members
    static int s;
 public:
    struct inner { // nested class
        void f(int i) {
            x = i; // Error: can't write to non-static enclose::x without instance
            int a = sizeof x; // Error until C++11,
                              // OK in C++11: operand of sizeof is unevaluated,
                              // this use of the non-static enclose::x is allowed.
            s = i;   // OK: can assign to the static enclose::s
            ::x = i; // OK: can assign to global x
            y = i;   // OK: can assign to global y
        }
        void g(enclose* p, int i) {
            p->x = i; // OK: assign to enclose::x
        }
    };
};

Friend functions defined within a nested class have no special access to the members of the enclosing class even if lookup from the body of a member function that is defined within a nested class can find the private members of the enclosing class.

Out-of-class definitions of the members of a nested class appear in the namespace of the enclosing class:

struct enclose {
    struct inner {
        static int x;
        void f(int i);
    };
};
int enclose::inner::x = 1; // definition
void enclose::inner::f(int i) {} // definition

Nested classes can be forward-declared and later defined, either within the same enclosing class body, or outside of it:

class enclose {
    class nested1; // forward declaration
    class nested2; // forward declaration
    class nested1 {}; // definition of nested class
};
class enclose::nested2 { }; // definition of nested class

Nested class declarations obey member access specifiers, a private member class cannot be named outside the scope of the enclosing class, although objects of that class may be manipulated:

class enclose {
    struct nested { // private member
        void g() {}
    };
 public:
    static nested f() { return nested{}; }
};
 
int main()
{
    //enclose::nested n1 = e.f(); // error: 'nested' is private
 
    enclose::f().g(); // OK: does not name 'nested'
    auto n2 = enclose::f(); // OK: does not name 'nested'
    n2.g();
}

References

C++11 standard (ISO/IEC 14882:2011):

9.7 Nested class declarations [class.nest]

C++98 standard (ISO/IEC 14882:1998):

9.7 Nested class declarations [class.nest]

Language
Standard library headers
Concepts
Utilities library
Strings library
Containers library
Algorithms library
Iterators library
Numerics library
Input/output library
Localizations library
Regular expressions library (C++11)
Atomic operations library (C++11)
Thread support library (C++11)
Filesystem library (C++17)
Technical Specifications

General
overview
class/struct types
union types
Members
data members
static members
the `this` pointer
nested classes
member templates
bit fields
using-declarations
member functions
member access specifiers
constructors and member initializer lists
default member initializer(C++11)
`friend` specifier
`explicit` specifier
converting constructor
Special member functions
default constructor
copy constructor
move constructor(C++11)
copy assignment operator
move assignment operator(C++11)
destructor
Inheritance
base and derived classes
virtual member functions
override(C++11)
final(C++11)
pure virtual functions and abstract classes