Any solution or programming tips for Inner class?

Question

Hi, I'm having some toubt here. Hope you guys can share out some programming tips. Just curious to know whether is it a good programming practice if I do something like the code below.

class Outer {
public:
   class Inner {
   public:
      Inner() {}
   }

   Outer() {}
};

I have been doing this for structure where I only want my structure to be expose to my class instead of global. But the case is different here, I am using a class now? Have you guys facing such a situation before? Very much appreciated on any advice from you ;)

Answer 1

It's not an everyday thing, but it's not unheard of, either. You would do this if there were a class (Inner) that only makes sense to a client program when the client is using Outer.

Answer 2

If you only want a class to be exposed in a certain file, you can use an unnamed namespace within that file. Then whatever code is within that namespace is only available within that file.

namespace
{
    //stuff
}

Answer 3

I'll break the answer into two parts:

1. for cases where you only organize code, you should use namespaces instead of classes -- if the inner class isn't an entity that is only worked with from inside the class (especially only constructed in the class), then inner classes are a good idea -- another example STL function objects.
2. in C++ there is absolutely NO DIFFERENCE between structures and classes except that structures have public members by default. Hence there's no real difference when you have classes -- it's more a matter of style.

Answer 4

This is a good practice in many cases. Here's one where we implement a link list:

template <class T>
class MyLinkList {
  public:
class Node {
    public:
    Node* next;
    T data;
    Node(const T& data, Node* node) : next(node), data(data) {}
};

class Iterator {
    public:
    Node* current;
    Iterator(Node* node) : current(node) {}
    T& operator*() { return current->data; }
    void operator++(int) { current = current->next; }
    bool operator!=(int) { return current != NULL; }
};

  private:  
Node* head;

}

The above is just snippet that is not intended to be complete or compilable. The point is to show that Node and Iterator are inner classes to the MyLinkList class. The reason why this makes sense is to convey the fact that Node and Iterator are not independent to be stand alone by themselves, but they need to be qualified by MyLinkList (for instance MyLinkList::Iterator it)

Answer 5

This is purely a matter of style, however I think it is typically more common in the C++ community to use a namespace named detail for classes that are purely helpers or are purely used in the implementation of other classes. There are several advantages to using namespaces in place of inner classes, among them include: greater compatibility (how compilers resolve names in inner classes can be incredibly different between Visual C++ and GCC, for example), more encapsulation (in the inner/outer variant, the inner class has greater access to members of instances of the outer class), easier implementation (you don't have to fully qualify the helper class every single time in the implementation file, since you can put a using directive in the ".cpp" source file). If you are going to use an inner class, then you need to make the conscious decision to make that a part of your API.

Using Namespaces

namespace collection
{
     namespace detail 
     {
         class LinkedListNode
         {
             //...
         };
     }

     class LinkedList
     {
          // ...
     };
 }

Using Inner Classes

namespace collection
{
      class LinkedList
      {
           // ...
           class LinkedListNode
           {
               // ...
           };
           // ...
       };
 }