Nested classes in C++

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

Explanation

The name of the nested class exists in the scope of the enclosing class and name look up from a member function of a nested class visits the scope of the enclosing class after checking the scope of the nested class. As any member of its enclosing class, nested class has access to all names (private, protected, publicv etc) to which the enclosing class has accessed,otherthan that it is 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
        }
    };
};

A friend functions defined within a nested class has no special access to the members of the enclosing class even if look up from that body of a name function which 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 or outside of class body :

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, eventhough 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();
}
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