Nested Classes in C++

　　

　　A nested class is a class which is declared in another enclosing class. A nested class is a member and as such has the same access rights as any other member. The members of an enclosing class have no special access to members of a nested class; the usual access rules shall be obeyed.

　　For example, program 1 compiles without any error and program 2 fails in compilation.

　　Program1

 1 //Program 1
 2
 3 #include<iostream>
 4
 5 using namespace std;
 6
 7 /* start of Enclosing class declaration */
 8 class Enclosing
 9 {
10 public:        //if remove the keyword, may be wrong in some compiler
11     int x;
12
13     /* start of Nested class declaration */
14     class Nested
15     {
16         int y;
17         void NestedFun(Enclosing *e)
18         {
19             cout << e->x;  // works fine: nested class can access private members of Enclosing class
20         }
21     }; // declaration Nested class ends here
22 }; // declaration Enclosing class ends here
23
24 int main()
25 {
26
27 }

　　Program 2

 1 #include<iostream>
 2
 3 using namespace std;
 4
 5 /* start of Enclosing class declaration */
 6 class Enclosing
 7 {
 8
 9     int x;
10
11     /* start of Nested class declaration */
12     class Nested
13     {
14         int y;
15     }; // declaration Nested class ends here
16
17     void EnclosingFun(Nested *n)
18     {
19         cout << n->y;  // Compiler Error: y is private in Nested
20     }
21 }; // declaration Enclosing class ends here
22
23 int main()
24 {
25
26 }

　　References: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2005/n1905.pdf

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　　2013-11-27　　14:40:49